UNIVERSE

Milky Way

noreply@blogger.com (Universalanatomi) — Thu, 2 Jun 2016 16:11:00 -0700

The Milky Way is by far the largest object in the night sky, which it occupies an important fraction. It is observable preferably through a night without moon and especially without light pollution. The Milky Way appears as a long white trail that runs through the night sky 360 degrees. This is due to the very large number of stars that make up the Milky Way, their lights superimposed to give this whitish appearance. It is by observing with a telescope, Galileo was able to resolve individual stars. All the stars we can see with the naked eye are part of the Milky Way.

The solar system is part of the Milky Way. Our Galaxy is dominated by his record, a huge "plate". Being inside the disk we see it from the side, like a long trail of stars, the Milky Way. It contains from 150 to 250 billion stars, gas in different phases (hot and molecular) and dust that absorbs visible light, which explains the dark streaks in the most central parts of the Galaxy.

The structure of our Galaxy

Our Galaxy is a type of galaxy "spiral". It is dominated by a disc with the stars revolve around the center, which consists of a bulb and a bar. Between 70% to 80% of stars are in the disc which has a diameter of 90,000 light years to a thickness of just 2,500 light years. The disc consists of several spiral arms in which the majority of stars concentrated. It is believed that the arms are formed naturally by gravitational instabilities induced during rotation of the disk. The two spiral arms dominate the galaxy are those of Scutum-Centaurus and Perseus, the Sun lying in a secondary arm called the Orion Arm.

Our position in the Galaxy

The Sun is one of the many stars of the galaxy disk. It is almost exactly in the plane of the disc, with the Galactic Centre distance of 28,000 light years. The Sun orbits the Galactic center with a speed of 230 km / s.

To know exactly our position in the Galaxy, it is necessary to determine the distances of stars. The distance to nearby stars is determined by the effect called parallax. They use their changes in position on the sky when viewed from Earth at two different positions around the sun. The GAIA satellite to be launched by ESA in 2013 can observe the position of almost a billion stars! The distance to the most distant stars is determined by the specific properties of certain variable stars, Cepheids, whose variability period depends on their intrinsic luminosities.

Halley's Comet

noreply@blogger.com (Universalanatomi) — Thu, 2 Jun 2016 15:55:00 -0700

The appearance in the sky of this beautiful comet is listed for centuries. But it is only since his last visit in 1986 that knows more about this exotic solar system.
Halley's Comet is one of the known comets of the public. His spectacular pass in 1910 has long marked the spirits.

History of comet

In 1705, the English astronomer Edmond Halley announced that comets that had appeared in 1531, 1607 and 1682 were in fact one and the same comet. He predicted, thanks to the laws of Newton, his return in 1759. The comet actually came back in 1758 with a few months in advance, 16 years after his death. The comet was named in his honor, even if now, comets are baptized in the name of their discoverer.

This triumph of the late astronomer allowed to permanently sit laws of Newtonian mechanics.

famous passages

The most famous appearance of Halley's comet was undoubtedly at the Battle of Hastings in 1066, which marked the invasion of England by the Normans. This famous event is represented on the 32nd stage of the famous Bayeux Tapestry. This appearance on the eve of the battle was interpreted as a bad omen. The English defeat that ensued gave them ample reason.

More recently, last appearances dating from 1910, which was an impressive sight and winter 1986 when several probes went visits to the comet. The Giotto probe afforded some impressive photos of the comet nucleus and learn more about these strange bodies of the solar system.

The accurate prediction of the next passage of the comet is difficult because it undergoes gravitational influence of the giant of the solar system that may cause it to radically change its trajectory. However, with the data collected, astronomers estimate that the comet will return in 2061 near the Earth.

Specifications

Orbit of the comet: The comet's orbital period varies around 76 years. With perihelion located 88 million kilometers from the Sun and an aphelion located 5240 million kilometers, its orbit is highly elongated. The comet orbits in retrograde direction (east to west) and is inclined 18 ° to the plane of the ecliptic. Only when the comet is less than 200 million kilometers that we can begin to see on Earth.

With all these features, astronomers think it would come from the large tank, located in the outer solar system called the Oort Cloud. However, comets coming from this reservoir has a period greater than 200 years. Now Halley's Comet to a period of 76 years. This anomaly could be explained by the gravitational perturbations of the giant that would have shortened its orbit.

A dark core: With an albedo of only 3%, Comet Halley is even darker than coal. With the Giotto probe, astronomers have seen surprised by the darkness of the sun so bright when it first appeared. This albedo because of Comet Halley one of the darkest planets of the solar system.

Low density: The density of the comet nucleus of Halley's comet is much lower than previously thought: 0.1 mg / cm3. We must therefore imagine a porous foam made of water ice, dry ice and hydrocarbons rather than a block of ice. Astronomers have in fact estimated the mass of this star range only between 20 and 100 billion tons. The comet, with each pass, losing about 0.1% of its mass which represents some 100 billion kilos! It was estimated that the icy geysers of Halley's Comet expelled at their maximum activity around 30 000 liters of water per second!

Saturn and mythology

noreply@blogger.com (Universalanatomi) — Sun, 29 May 2016 17:08:00 -0700

Saturn (alchemy and astrology) .- The planet Saturn has under its patronage lead, among Greek alchemists and those of the Middle Ages. The same sign designates the star and metal. The dark color of one was compared to that of amber, and the slow apparent motion of the planet likened to gravity (density) of lead.

Saturnus (Roman Religion). - One of the most ancient gods of Latium and central Italy. A tradition collected by Pompey Trogus and Justin represented as a king of the Aborigines, ie d. primitive populations. The ancient, eg Varro and Augustine after him, had attached the word Saturnus at the root sat contained in satum (verb sero, sow) sator. This is an etymology that seems always admissible. Saturn was therefore, at least originally, the god who presided over seeding, which protected the seeds entrusted to the earth; but he did not remain confined to these special powers. As far as we can judge by the Greek additions later modified the physiognomy of the god, Saturn, husband of Ops was for the inhabitants of Latium agricultural deity par excellence. His main attribute was a sickle, or rather a curved knife, which he used to cut the harvest, pruning trees, to prune vines

Finally, under the epithet Stercutus or Sterculius, he was the god of fertilizers that increase soil fertility. His character was clearly chthonic; in him the ancient Romans worshiped the subterranean forces that germinate seeds buried in the ground; it seems to have been a close relationship between Rome and Saturn Dis Pater. Macrobius says that the altar of Saturn adjoined a chapel Dispater (Saturnalia, I, 10, § 48).

The month of December was dedicated to Saturn, because the sowing were then completed, and because it was the time of year beginning in the womb of the earth, the work of germination, distant prelude to the future harvest. Saturn was very formerly worshiped on the same Rome location. There was, in historical times, on the southern slope of the Capitol, while a quarter of the city which preserved the memory of this cult: there was the temple of Saturn and Ops, which were deposited the Treasury, public and the state archives; not far opened in the enclosure called Servius Tullius, a door Saturn, Porta Saturnia; Varro adds that the Capitoline hill was originally called mons of Saturn, and that before the founding of Rome there was this height a town that bore the name Saturnia.

Black hole

noreply@blogger.com (Universalanatomi) — Sun, 29 May 2016 16:52:00 -0700

In astrophysics, a black hole is a massive object whose gravitational field is so intense that it prevents any form of matter or radiation to escape. Such objects do not emit light and are then black. Black holes are described by the theory of general relativity.
They are not directly observable, but several techniques of indirect observation in different wavelengths have been developed and used to study phenomena that they induce on their environment. In particular, the material that is caught by a black hole is heated to significant temperatures before being swallowed and sends thus a large amount of X-rays Thus, even if a black hole does not emit itself radiation, it may still be detectable by its action on the environment. The existence of black holes is a certainty for almost all of the relevant scientific community (astrophysics and theoretical physicists).

A black hole has a given mass concentrated at a point called gravitational singularity. This mass to define a sphere called horizon of the black hole, centered on singularity and whose radius is a limit below which the black hole prevents radiation from escaping. This sphere is something of the spatial extension of the black hole. For a black hole of mass equal to the mass of the Sun, its radius is about 3 kilometers away. In an interstellar distance (million kilometers), a black hole does not exert more attraction than any other body of the same mass; so it is not a "vacuum cleaner" irresistible. For example, if the Sun was replaced by a black hole of the same mass, the orbits of its planets would remain unchanged.

It is impossible to directly observe a black hole. However, it is possible to infer its presence by its gravitational action on the environment, especially within microquasars and active galactic nuclei, where the nearby material falling into the black hole will be greatly heated and emit strong radiation X . observations and can detect the presence of massive objects and very small. The only objects that these observations imply and are consistent within the framework of general relativity are black holes.

properties

A black hole is an astrophysical object as another. It is characterized by the fact that it is very difficult to observe directly, and the central region can not be described satisfactorily by physical theories in their state of the early twenty-first century as it boasts a gravitational singularity. The latter can only be described in the context of a theory of quantum gravity, missing today. By cons, we know perfectly describe the physical conditions in the immediate vicinity, as well as its influence on the environment, which can detect through various indirect methods.

Moreover, black holes are amazing in that they are described by a very small number of parameters. Indeed, their description in the universe we live in, is up to three parameters: mass, electric charge and angular momentum. All other parameters of the black hole (eg size or shape) are fixed by those. By comparison, the description of a planet involves hundreds of parameters (chemical composition, differentiation of its elements, convection, atmosphere, etc.). The reason that a black hole is described by these three parameters has been known since 1967: it is the no-hair theorem demonstrated by Werner Israel. He explains that the only long-range interactions are fundamental gravitation and electromagnetism, the only measurable properties of black holes are given by the parameters describing these interactions, namely mass, angular momentum and electric charge.

For a black hole mass and electric charge are the usual properties that describes classical physics (i.e. non-relativistic) the black hole has a gravitational field proportional to its mass and a proportional electric field dependent. The influence of angular momentum is against specific to general relativity. This one stipulates that a rotating body will tend to "train" space-time in its vicinity. This phenomenon not yet observed at present in the solar system because of its extreme weakness for non compact stars, is known as the Lense-Thirring effect (also called frame dragging, in English). It takes a considerable amplitude near a rotating black hole, to the point that an observer located in the immediate vicinity would inevitably be driven in the direction of rotation of the black hole. The area where this occurs is called ergorégion.

Pluto

noreply@blogger.com (Universalanatomi) — Sun, 29 May 2016 16:19:00 -0700

The planet Pluto was discovered by Clyde Tombaugh in 1930 at Lowell Observatory in Flagstaff, Arizona. Long regarded as the last planet of the solar system, she was demoted to the status of dwarf planet in 2006.

Its orbit is highly eccentric and its distance from the sun varies a lot between 30 and 49 astronomical units. Thus, during certain periods, Pluto is closer to the Sun than Neptune, which was the case for example between 1979 and 1999. The orbit is also highly inclined at an angle of 17 degrees to the ecliptic plane.

Spectroscopic observations revealed that the surface of Pluto is covered with ice nitrogen and methane. A very thin atmosphere is present with a pressure 100,000 times weaker than the earth pressure. The average temperature is -220 degrees Celsius.

Charon and the other satellites

Because of its remoteness, Pluto is very difficult to study. It was so until 1978 that the astronomer James W. Christy of the US Naval Observatory discovers that the planet has a satellite, Charon, at a distance of 19 600 km.

Thanks to a very rare phenomenon, the passage from Earth in the orbital plane of the system between 1985 and 1990, astronomers have observed a series of mutual eclipses of the two main bodies. They were able to deduce their size (from Selected by the New Horizons probe): 2370 km in diameter Pluto and Charon in 1208. The separation between the two bodies and represents that almost eight times the diameter of Pluto, a very rare scenario.

The rotation period of the two bodies is the same and equal to the period of mutual orbit, a little over six Earth days. This is an exceptional phenomenon: the two bodies always have the same face and appear stationary in the sky to the other, synchronous rotation generated by mutual tidal forces.

Unlike Pluto, Charon has no atmosphere and its surface would be covered with water ice. The density of Charon suggests that the satellite contains a proportion of ice much larger than Pluto, which could be explained if Charon was formed in a collision from Pluto mantle materials.

In Roman mythology, Pluto was the god of the Underworld, the equivalent of the Greek god Hades. Charon was the ferryman of Hades who with his boat helped the deceased to cross the Styx.

A composite view of Pluto satellites

It was not until the new millennium that other smaller satellites are detected, all by the Hubble Space Telescope: Nix and Hydra in June 2005, Kerberos in June 2011 and finally Styx in June 2012.

Styx is the closest of the four small satellites, about 42,000 km of the central couple. It is also smaller, with a diameter between 10 and 25 km. In Greek mythology, the River Styx was that separated the underworld the world of the living.

Nix is 49,000 kilometers of torque and has a very elongated shape, with 56 kilometers long and 25 kilometers wide, which gives the satellite an irregular and chaotic rotation. Nix was the goddess of the night and the mother of Charon.

Kerberos has a diameter of between 13 and 34 kilometers, and is located in an orbit at 59,000 kilometers of the couple Pluto and Charon. The Kerberos name comes from the famous Cerberus, the three-headed dog who stood guard at the entrance to the underworld.

Finally, Hydra was detected at a distance of 65,000 kilometers. As Nix, Hydra has formed an elongated, 58 by 34 km, and a chaotic rotation. Its name comes from the Hydra killed by Hercules.

In order of increasing distance to the couple Pluto-Charon orbital periods of these satellites is 20 Earth days to Styx, Nix 25, 32 and 38 for Kerberos Hydra.

Neptune

noreply@blogger.com (Universalanatomi) — Sun, 29 May 2016 10:53:00 -0700

Neptune the planet completes an orbit to 4.495 billion kilometers, or 30 astronomical units from the Sun, making it the most distant of the planets of the solar system (Pluto is no longer competing since she was demoted to the rank of world dwarf). Because of this distance, the planet has a long orbital period: 165 Earth years.

Neptune diameter of about 49 500 km, slightly smaller than Uranus, but even four times that of Earth. Neptune has against a top Uranus mass about 17 times the mass of Earth.

The atmosphere of Neptune

The planet consists essentially of hydrogen and helium. It contains a huge rock core liquid, water, ammonia and methane which represents two thirds of the diameter. The outer third is composed of hydrogen, helium, water and methane.

The outer surface of Neptune consists of thick layers of cloud and appear blue due to the presence of methane. These clouds move at speeds that can range up to 2000 kilometers per hour, a speed record for the solar system.

Breaking the monotony, there are also long white clouds composed of methane ice crystals, and large blue spots due to hurricanes similar to those of Jupiter

Neptune satellites and rings

Neptune has at least 13 satellites. The most interesting of these satellites is Triton, the only one massive enough to have achieved a spherical shape. It has a diameter of 2700 km and is surrounded by a thin atmosphere rich in nitrogen with traces of methane.

Voyager 2 are measured in 1989 the lowest temperature ever recorded on a solar system body: -236 degrees Celsius. Its surface covered with nitrogen ice is very complex, with some formations that look like geysers and may reject nitrogen to an altitude of 8 kilometers.

Triton moves in a retrograde orbit, that is to say in the opposite direction to the rotation of Neptune, which is very unusual. The most plausible explanation is that Triton was not formed at the same location as Neptune, but was probably captured by the gravitational pull of the planet in the distant past.

The observations also show that the satellite is approaching inexorably Neptune under the effect of tidal forces. It should eventually explode and give birth to a beautiful ring around the planet.

Note also that Neptune is surrounded by a series of thin rings probably composed of dust. These rings are irregular and have denser parts that give them the appearance of arcs. The first were detected from Earth during stellar occultations in the 1980s, others were discovered by Voyager 2 in 1989.

Uranus

noreply@blogger.com (Universalanatomi) — Sun, 29 May 2016 10:43:00 -0700

The planet Uranus is 19 astronomical units from the Sun is 2.87 billion kilometers. At that distance, it takes 84 Earth years to make one revolution around our star. It is the third largest planet in the solar system with a diameter of 51 800 km.

The atmosphere of Uranus

Uranus is mainly composed of hydrogen and helium, with also some methane and trace amounts of other compounds.

When flying over Voyager 2 in 1986, the planet appeared uniformly blue green, with no visible detail. Because of its mass, Uranus has less internal energy release than Jupiter and Saturn and convection in the atmosphere is more limited, resulting in a lack bands and a much more uniform appearance.

Note however that more recent observations by the Space Telescope revealed a more pronounced banded structure, possibly due to climate change during the revolution of the planet around the sun.

The blue-green color of Uranus comes from the methane in the upper atmosphere. The sun's rays are reflected from the highest clouds in the world but must cross the methane layer that lies above. But mostly methane absorbs red fraction of the sunlight and the blue portion is the only one able to escape.

The rotation of the planet is relatively fast, with a slightly higher 17 hours period.

Uranus is distinguished from other bodies in the solar system that its rotation axis is almost in the plane of the orbit. Thus, when flying by Voyager 2 in 1986, the South Pole of the planet facing the sun. In 2007, after a quarter of an orbit, it will be the equator of the planet that will be the most lit area.

These variations could cause slow climate changes observed on the planet. Note again that the unusual position of the axis of rotation is likely the result of a collision with another protoplanet in the distant past.

Satellites and rings of Uranus

Uranus has at least 27 satellites, of which the top five are, in order of increasing distance to the planet Miranda, Ariel, Umbriel, Titania and Oberon. The largest of these satellites is Titania, with a diameter of 1580 kilometers, less than half that of our Moon.

The planet is also surrounded by several thin rings discovered from Earth during the observation of a star occultation in 1977. Their existence was then confirmed when flying by Voyager 2 and other rings yet later discovered, particularly by the Hubble space telescope.

These rings are formed of dark ice balls and little reflective, the largest up to several meters in diameter.

The discovery of Uranus

Uranus was observed repeatedly in the seventeenth and eighteenth centuries, but these early observations mingled with the star. It was not until 1781 that English astronomer William Herschel recognized it for what it was, a planet, the first not to have been identified in antiquity.

Various names were proposed, especially Georgium Sidus in honor of King George III or Herschel to greet its discoverer. But it was the German Johann Elert Bode who finally proposed the winning name, Uranus, inspired by Uranus, the Greek god of the sky.

The two largest satellites, Titania and Oberon, were discovered by William Herschel in 1787. Ariel and Umbriel were discovered by the English in 1851 and William Lassel Miranda by American Gerard Kuiper in 1948. Their names come from characters in parts William Shakespeare.

Saturne

noreply@blogger.com (Universalanatomi) — Sun, 29 May 2016 04:04:00 -0700

Saturn orbits the Sun at an average distance of 9.5 astronomical units, or 1.4 billion kilometers. Second planet in size with a diameter of 121,000 kilometers, it is best known for its beautiful rings.

Like Jupiter, it rotates very fast on itself in about ten hours, and consists mainly of hydrogen and helium, as evidenced by its low density of only 0.69 times that of water .

The planet has been studied in detail by four probes: Pioneer 11 in 1979, Voyager 1 in 1980 and Voyager 2 in 1981 and especially since 2004 Cassini-Huygens.

These have revealed the complexity of the rings and have provided high-resolution images of the visible surface of Saturn. The latter has proved far less colorful than that of Jupiter, albeit with parallel yellow stripes at the equator and some white spots.

The internal structure of Saturn

Saturn has a similar internal structure of Jupiter. A stronger flattening of the world suggests that the rocky core is larger. The weaker magnetic field indicates that meanwhile the metallic hydrogen layer is to be thinner.

The upper part of the atmosphere is also similar to that of Jupiter, with the same three layers (NH3, and H2O NH4SH) and a parallel band structure at the equator.

The lack of color and contrast is due to the lower gravity of Saturn, which makes the three layers are spread over several hundred kilometers, instead of several tens of kilometers to Jupiter. The deep layers are thus masked by hundreds of kilometers of haze.

Like its neighbor, Saturn emits more energy than it receives, ie 2.5 times. This is probably not due to the energy accumulated during the training phase, but rather to helium falls inward of the planet, similar to our rainfall. These movements are transforming gravitational energy into heat and can also explain the low helium concentration in the outer layers.

Saturn's rings

Saturn's rings were observed for the first time by Galileo in 1610, which took the two satellites of the planet. Christian Huygens described the ring as a single in 1655. They were not identified as many rings as a few years later, in particular by Cassini in 1675 which identified the discontinuity that bears his name.

The rings have an external diameter of about 600 000 km and a thickness of just 2 km. The images taken by the sensors show that the relatively wide rings that can be seen from the Earth are in fact formed of a multitude of very fine and very close rings.

These thin rings are themselves composed of a myriad of small independent solid bodies in orbit around the planet faster and even turn they are close to Saturn. These bodies consist essentially of ice or ice-covered rock. Their size varies between one millimeter and several tens of meters.

Jupiter

noreply@blogger.com (Universalanatomi) — Sat, 28 May 2016 16:47:00 -0700

Jupiter is the largest representative of our solar system. gas giant, its atmosphere is the seat of violent events, the most emblematic event is the Great Red Spot, a jewel of the cyclone outer solar system. Nevertheless, it has a solid core probably bigger than our planet. Jupiter could be a star, but for lack of material it has never lit ... The fact remains that this planet has a retinue of satellites that are real worlds, many of which would be likely to create conditions favorable to the development of organic matter.

Size and weight

Fifth planet of the solar system and first gaseous planet, is the largest band (142 984km in diameter). It is also the heaviest, its mass being 2 times the total of all other planets in the system (1,900e27 kg)! A comparison of the Earth, Jupiter is a giant: its radius is 12 times greater than that of Earth, its mass is almost 320 times more ... well it would land in 1300 to fill Jupiter!

Observations and discoveries

Due to its impressive size, it is the fourth brightest star of the terrestrial sky behind the Sun, the Moon and Venus (which is much smaller but much closer). Depending on the distance that separates us, its magnitude is situeentre -2.6 and 1.6. Thus, it was discovered there was already a long time, during prehistoric times! In 1610, Galileo discovered even four of the 40 satellites of the giant, using his telescope: Io, Europa, Callisto and Ganymede. This was of importance since it could highlight the non-centrality of the Earth within the solar system, which strongly supported the heliocentric theory of Copernicus. Unfortunately, after having supported this theory, Galileo Galilei, was arrested by the Inquisition: It had to recant his beliefs and was imprisoned for the rest of his life.

Different layers, Different colors

Given the huge amounts of gas, the atmosphere of Jupiter is obviously composed of several layers ... Between the core and the outer layer, lies the bulk of Jupiter, a liquid metallic hydrogen layer. This materialization of hydrogen is possible only a titanic pressure: 4 million bars! Liquid hydrogen is a good conductor, resulting in a very strong magnetic field. The outer layer of the atmosphere, that which is directly observed, is probably composed of ammonium hydrosulfide, and a mixture of ice and water. When we observe Jupiter from Earth, we see yellow orange This is precisely due to the sulfide ammonium and phosphorus from its upper atmosphere. When approaching (as the Galileo probe), there are a range of much wider color corresponding to the height of the cloud into the atmosphere. The colors of the clouds deeper to those on the surface: blue, brown, white and red. Of course, as in the terrestrial sky, in the case of a clear sky on the surface, we see the deeper cloud layers. The bright colors of the clouds are certainly due to chemical reactions between sulfur and other compounds, in fact, the sulfur compounds are spread in a wide variety of colors.

Mars

noreply@blogger.com (Universalanatomi) — Sat, 28 May 2016 08:11:00 -0700

After Earth, we find Mars at an average distance of 1.5 astronomical units from the Sun, or 230 million kilometers. Unlike other planets, Mars has a very similar rotation period of the Earth, the alternation between day and night is therefore in line on our planet.

The inclination of the axis of rotation relative to the orbital plane also has a similar value, which leads the planet to be subjected to a similar cycle of seasons to that of the Earth, slightly slower since the year Mars is longer than ours.

View of the Earth, the planet appears generally reddish, with some dark areas and whitish polar regions. With the changing seasons, its appearance changes a lot. In summer, the polar regions are shrinking and the dark areas are expanding. In winter, the white cover on the poles is very marked and the dark areas are more discreet.

These dark areas are darker rocks regions and their appearance change is probably due to a layer of variable thickness dust with the seasons. The red appearance of the planet is in turn due to the presence of iron oxide.

The atmosphere of Mars

Mars has a diameter of 6800 km. It has a very thin atmosphere with a pressure less than a percent of the land value. The atmosphere consists of carbon dioxide greater than 95 percent, a small amount of nitrogen, argon and oxygen, and trace amounts of other gases. There are also some water vapor, sufficient to give birth to ice clouds or fog.

Images taken from the surface show that the sky appears orange, which is likely due to fine dust particles in the atmosphere.

The temperature at the surface of Mars varies between a minimum of about -140 degrees Celsius at night and daytime maximum of 0 degrees in winter and 20 degrees in summer.

The atmosphere of Mars is sometimes animated formidable storms that cover the entire planet and can last several months. The surface is then completely hidden by dust raised by the wind. This is for example the product early in the Mariner 9 probe had been unable to observe the surface for several weeks.

The surface of Mars

The probes revealed a fascinating and rich surface different types of formations: extinct volcanoes, craters, canyons and dry river beds. These curricula are not uniformly distributed over the planet, but rather consolidated in a given hemisphere.

The northern hemisphere of Mars is dominated by volcanic formations. It includes in particular two areas where many volcanoes are concentrated: the dome of Tharsis, with 3 volcanoes whose height exceeds 20 kilometers, and on the other side of the planet, Elysium Planitia.

Tharsis the dome is near Olympus Mons, the largest volcano in the solar system that culminates at 26 km altitude with a base 600 km in diameter.

Martian volcanoes are the same type as the volcanoes of Hawaii, with flanks very gentle slope. They are due to the presence of a hot spot in the coat which ejects the washer outwardly through the crust.

Their sheer size is probably related to the lack of plate tectonics. On Earth, because of tectonics, the crust moves relative to the hot spot, producing a succession of small volcanoes. On Mars, the crust is set and lava accumulation in a point form gradually huge volcanoes.

From a study of the number of these craters in volcanic areas, it has been possible to date them approximate: the Tharsis dome is such relatively young with only a few hundred million years.

The southern hemisphere is very different, dominated by old impact craters several billion years. Unlike lunar craters whose appearance does not change with time, the Martian craters are subject to erosion from the atmosphere, which alters the shape, rounded edges and covers them inside a thick layer of dust.

Un panorama de la surface martienne pris lors de la mission Pathfinder en 1997

The satellites of Mars

Let's end this quick overview of Mars, noting that the planet has two small satellites, Phobos and Deimos, with a dimension of about 10 kilometers. These satellites appear very irregular and are covered with craters. Due to the proximity of the asteroid belt and their irregular appearance, astronomers suspect them to be asteroids captured by Mars.

Earth

noreply@blogger.com (Universalanatomi) — Sat, 28 May 2016 07:53:00 -0700

The Earth formed there are about 4.54 billion years and life appeared less than a billion years later. The planet is home to millions of living species, including humans. The biosphere of the Earth has greatly altered the atmosphere and other abiotic characteristics of the planet, enabling the proliferation of aerobic organisms as well as the formation of ozone, which associated with the Earth's magnetic field, blocks part solar radiation, enabling life on Earth. The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist during this period and the Earth should be able to sustain life (as we know it) for at least another 500 million years.

The Earth's crust is divided into several rigid segments called tectonic plates that move over millions of years. Approximately 71% of the land surface is covered by salt water oceans that form the hydrosphere with other water sources such as lakes, rivers or aquifers. The geographic poles of the Earth are mostly covered with ice (Antarctic ice sheet) or floes. The interior of the planet remains active with a thick mantle composed of silicate rocks (usually solid, but locally melted), a liquid outer core of iron that generates a magnetic field, and an inner core of solid iron.

Earth interacts with other objects in space, especially the Sun and the Moon. At present, Earth orbits the Sun in 365.26 solar days, or a year sidéraleNote 1. The rotation axis of the Earth is tilted by 23.44 ° from perpendicular to the plane of the ecliptic, which produces seasonal variations on the surface of the planet with a period of one tropical year (365.2422 solar days). The only known natural satellite of the Earth's moon orbiter that began some 4.5 billion years. This causes the tides, stabilizes the axial tilt and slowly slows the Earth's rotation. There are about 3.8 billion years ago, during the Late Heavy Bombardment, numerous asteroid impacts caused significant changes then its surface.

Earth has the distinction, from the perspective of the human being, being the only place in the universe to harbor life as we know it, such as wildlife (including among others the human species) and flora. Human cultures have developed many representations of the planet, including personification as a deity, a belief in a flat earth, the Earth as the center of the universe and the modern perspective of the world as a global system requiring reasonable management.

The science that studies the Earth's geology. Given the influence of life on the composition of the atmosphere, oceans and sedimentary rocks, geology borrows from biology part of its chronology and its vocabulary.

Venus

noreply@blogger.com (Universalanatomi) — Sat, 28 May 2016 07:28:00 -0700

After Mercury, we come to Venus at a distance of about 0.72 astronomical units from the Sun (108 million kilometers). View from the earth, Venus is never far away from the Sun, with an angular separation of a maximum of 45 degrees.

Venus is one of the most interesting objects to observe because, due to its revolution around the Sun, the planet has a moon as the visible phases of cycle using simple binoculars. In addition, when the revolution brings relatively close to Earth, Venus is the brightest object in the sky after the Sun and Moon.

the orbit of Venus

The last transit of Venus this century occurred between 5 and June 6, 2012. This image shows the transit of Venus across the Sun as the recorded satellite SDO (Solar Dynamics Observatory). The space observatory SDO was launched in 2010 by NASA to study the Sun, but has also been involved for this very special occasion. The images were taken in the far ultraviolet, usually used to study the solar corona. For patients and optimistic people, the next transit of Venus will occur in 2117.

Venus is similar to Earth from the perspective of the size (12 100 kilometers in diameter), mass and chemical composition. The most apparent difference is its external appearance. In contrast to our planet, Venus has a completely opaque atmosphere that prevents us to observe its surface.

This barrier was a major obstacle in the study of the planet. Thus, the rotation period was unknown until the early 1960s, when astronomers made use of a radar to measure it. They then discovered that Venus differs from other planets by a reverse rotation of the normal and a very long period of about 243 Earth days.

Venus really began to be studied with the advent of the space age. She was the first planet in the solar system to be flown by a probe in this case Mariner 2 in 1962. A whole armada of probes followed, first other US missions Mariner, who flew over the planet, and several Soviet probes Venera and US Pioneer Venus Multiprobe which plunged into the atmosphere and landed on the surface.

Finally arrived Pioneer Venus Orbiter, other Venera probes, Magellan and the American mission, which began orbiting the planet and were able to map its surface with radar.

The surface of Venus

Venus topography was unveiled we mainly probes that are placed in orbit around the planet and have explored using radars. First it was Pioneer Venus Orbiter, and several Venera probes, and finally the Magellan mission to meticulously mapped 98 percent of the surface of the planet in several years, with a resolution of about 100 meters.

The probes revealed that the surface of Venus is globally dominated by vast plains. The monotony is broken, however, by two huge highland areas the size of a continent, called Aphrodite Terra and Ishtar Terra.

The soil of Venus photographed by the Soviet probe Venera 13 in 1982

The Venusian landscape is in turn dominated by volcanic formations. Many volcanoes are visible, especially with Maxwell Montes, which rises to 11 km altitude.

There are also strange dome-shaped pancake, probably due to a very viscous lava, and many traces of ancient lava flows.

Everything seems to show that in fact the planet Venus has been very active volcanic view until very recently, perhaps only 10 million years BC. The probes have revealed, however, no activity at present and did not detect any trace of plate tectonics like Earth.

In addition to the volcanic formations, another important feature is the presence of many craters, all of diameter greater than several kilometers, evidence that the atmosphere dense with shattered all meteorites low size.

Finally, the appearance of the surface was revealed by some photographs by Venera probes which are laid gently onto the ground. These images show deserts and rocky landscapes, appearing orange because of the atmosphere of Venus.

Mercury

noreply@blogger.com (Universalanatomi) — Sat, 28 May 2016 07:11:00 -0700

Mercury is the closest planet to our star, the Sun, but also the smallest of our Solar System. It is a terrestrial planet type (ie with a rocky surface, and an internal structure with different concentric layers) and it has no natural satellite. It is a planet difficult to observe since very close to the Sun, it appears only shortly after sunset or before sunrise.

Mercury has virtually no atmosphere because of heat reigning on its surface and the low gravity of the planet. This thin atmosphere is composed mainly of potassium (31%), sodium (25%) and oxygen (9.5%).

Due to its proximity to the Sun, Mercury is a very hot planet, with an average surface temperature of 179 ° C, but it can range from -183 ° C to the side in the shade, at 427 ° C for the light side. On the surface, the Sun appears 4 times larger than Earth, and its light is 8.9 times more intense.

Mercury's surface is covered with craters, and so closely resembles in appearance to the Moon, our natural satellite. These craters are ancient and tell the history of our Solar System when he was just born and the planetesimals collided with the planets to merge with them. These craters have remained intact because the planet has no internal activity a priori, which leaves no chance to the surface to renew.

Mercury has a relatively large metal core (volume occupies 42% of global volume, against 17% for the Earth) and as regards its internal composition, the planet is made of 70% metals (mainly located in the nuclei) and 30% silicate (coat). Proportionally, Mercury is the planet of the Solar System that has a quantity of iron twice as important as any other object in our solar system, which earned him the nickname "Metal planet."

Moon: History of discoveries

noreply@blogger.com (Universalanatomi) — Fri, 27 May 2016 16:56:00 -0700

After the Sun, the Moon is certainly the star who most needed his presence in the skies of Earth Our satellite also has something "more": the complexity of its cycles .This gave him a special significance both in mythologies that in the oldest astronomy.

As was the case for all the stars, from the seventeenth century, the use of raw glasses, and the advent of Newtonian mechanics, a new way of viewing the moon is made daily. The complexity of its movement only became more evident, and its study has now found new tools provided by celestial mechanics. But above all, its surface has now been studied in detail. The first maps of the moon were drawn and she was born selenography, which runs from mid-nineteenth century through photography.

Along the way, astronomers have watched on the surface of our satellite variations, real or imagined. This questions is found intimately linked with that of the possibility of this geologic activity on the Moon. In particular, the question arose whether the craters and lunar seas were volcanic or whether to invoke other causes. If astronomers have finally opted for the meteorite hypothesis

picture

It took them to wait for the exploration of the Moon from 1960 to be able to base this conclusion on a solid argument.

Key dates

640 BC. AD: Thales recognizes the lunar phases a result of the reflection of sunlight.

1609: First observations of the Moon by Galileo using a telescope.

1830-1837: Publication by Beer and Maedler their map of the Moon.

1833 To calculate the inequalities of the Moon, Fish provides a process that is incomplete for a theory of our satellite, but which could be applied with advantage in the case of the planets.

1787 Herschel believes observe a volcanic eruption on our satellite (Aristarchus crater).

1840: First photographs of the Moon.

1894 -1910: Photographic Atlas Publication of the Moon Loewy and Puiseux.

1959: First shot from the opposite side of the Moon by the probe Luna 3.

1969-1972: Twelve human trample on the moon in the Apollo program.

The phases of the Moon.

(According Hevelius)

The moon before telescope

Moon played a big role in all mythologies. Its cycles, often put in relation with the idea of fertility, were all closely monitored. From the seventh century BC, the Ionian philosophers also began to speculate on the nature of this star. Is this a fiery or earthy body? A perfect crystal sphere or another habitable earth? These issues will be discussed, in different variations, until the appearance of the first telescopes in the seventeenth century.

Steps selenography

As its etymology suggests, selenography is to the moon, what geography is to the Earth. The principles are the same, but measurements, remained for a long indirect, made this term synonymous primarily lunar mapping. The observation in 1609 by Galileo and his contemporaries of the Moon's surface with the first glasses revealed the presence of mountains associated with clear our satellite regions. But the question that immediately arose was whether the dark regions, in contrast, were not seas. We quickly abandoned this idea. The current nomenclature of the Moon, the first steps back to the cards Hevelius and Riccioli, however, still bears traces of these questions. Following these seminal works, it should be noted cartographic efforts Cassini, Tobias Mayer, Schroeter, to Lohrmann and Schmidt, and especially Beer and Maedler whose card, completed in 1837, represents the last great completion of selenography before the use of the photograph.

At the age of photography

The first photographs of the Moon date back to 1840. Although this new technique is not able to compete with the direct observation with regard to the fine detail that can be detected, it brings a certain objectivity, allowing its use for achieving new maps of our satellite. Photographic Atlas of the Moon, directed by Loewy and Puiseux between 1894 and 1910 is probably the culmination of this approach. But photography has other early recognized benefits. And Will it be used for photometric studies, or to explore the beaches of the electromagnetic spectrum located beyond the visible range. Today, traditional photography is hardly used by astronomers. However, it has known a last moment of glory during the exploration of our satellite program in the 1960s, during which we could take pictures of the Moon "on site".

The variations on the Moon?

Observers such as William Herschel, Schroeter Gruithuisen, Littrow, believed distinguish our satellite traces of constructions "made by human hands." Kepler himself in the Dream says that lunar craters are too regularly trained for nature is responsible. A closer look obviously every time then proved that these constructions (walls, trenches, canals and roads supposedly) were not artificial but purely natural formation. However, these detailed observations, if misinterpreted, also brought to light the possibility of changes to the surface of the Moon, also called transient lunar phenomena (LTP). Episodic lights, craters that change shape. This showed it to the existence of current geological activity on our satellite? Could they claim the existence of a volcanic earthquakes or the pest on the Moon? Nobody believes today. But astronomers of the past had their reasons to think otherwise.

The origin of the craters and seas

The generations of astronomers who have followed in an attempt to detect changes on the surface of the Moon often had in mind the possibility that the Moon is the seat of volcanic eruptions. Indeed, such a type of geologic activity has long been privileged to explain these formations called craters. It showed craters of volcanoes, extinct in the main, but some of which could still give some signs of activity. It was the volcanic hypothesis of the origin of lunar craters and seas. But another option began to be seen around 1895 and eventually prevailed from the 1960s is the meteorite hypothesis. Thus, astronomers now recognize that the craters of the moon have nothing volcanic, but instead were formed during a large meteorite bombardment that took place in the early days of the Solar System

Space exploration

Long destination imaginary journeys, the moon has begun to be considered seriously as a goal reached by few engineers of the early decades of the twentieth century engaged in the development of the first rocket engines (Robert Goddard, Robert Esnault-Pelterie, etc.) . But it was not until after the Second World War, to the availability of technologies developed for military purposes (mainly the German V1 and V2 rockets), added to the context of the Cold War place in 1953 to priority programs access to space. Reach the moon being then very quickly perceived as a sign of technological expertise required to launch intercontinental missiles, and the most obvious symbol of the victory of one or the other side in this new conflict so strongly tinted propaganda.

The first firing of rockets towards the moon date from 1958, and the following year, the first automatic probes effectively managed until our satellite. Quickly, these test shots called the emergence of human spaceflight programs, an American on the Soviet side. It is essentially to prepare for the arrival of the first man on the moon that will be scheduled the launch frenzy that constantly occur over the following decade. launch. Finally, three years after the first soft landing of a Soviet probe, the first American crew of the Apollo program in 1969 set foot on the Sea of Tranquility

Since then, one is tempted to say that the whole thing summed up in one sentence: we walked on the moon and came back. A total of sixty trips to the moon will take place. Nearly 400 kilograms of moon rocks will have been reported to Earth to be studied there. Only eight trips (all belonging to the Apollo program) have been inhabited, six moon landings, which have allowed twelve humans to set foot on our satellite. Yet even as the Cold War that justified all this no longer appears as a spasm of our history, the first lunar exploration program have fundamentally changed the vision we can have today world. They marked a turning point not only in the history of knowledge, but in human history.

The dust of the Whirlpool Galaxy in infrared

noreply@blogger.com (Universalanatomi) — Fri, 27 May 2016 06:28:00 -0700

How are spiral galaxies? To find out, the Hubble Space Telescope observed the near infrared and photogenic M51 spiral so reveal the dust itself allows to track the most dense able to form stars gas. To better distinguish dust, most of the visible light from the stars was removed digitally. The resulting image shows large-scale swirling patterns, while many brilliant concentrations visible smaller scale correspond to open previously hidden star clusters. To see the image in the visible here. A simple pair of binoculars is sufficient to guess the sketch of the Whirlpool Galaxy in the constellation of the Hunting Dogs (Canes Venatici). M51 is about 30 million light years from us, and this image covers an area of about 15 000 light-years from the bottom up. Astronomers believe that the spiral structure of M51 is primarily due to its gravitational interaction with a smaller neighboring galaxy.

Coronal hole

noreply@blogger.com (Universalanatomi) — Wed, 25 May 2016 16:10:00 -0700

This sinister and dark shape spanning the active Sun is a coronal hole: a low density region extending above the surface where the solar magnetic field opens freely into interplanetary space. Studied extensively in ultraviolet and X-rays from space from the 60s, coronal holes are known sources of fast solar wind electrons and atoms flowing into the space along open magnetic field lines. During periods of low activity, coronal holes typically cover regions located above the Sun's poles. But this coronal hole, one of the largest ever observed in the current cycle of solar activity, extends south pole (bottom) to the northern hemisphere. Coronal holes such that the latter may last a few solar rotations, until the magnetic fields are shifted and changing the configuration. false color visible, this image of the Sun was January 8, extreme ultraviolet by the EIT instrument onboard the SOHO space observatory.

The supergiant Gamma Cygni

noreply@blogger.com (Universalanatomi) — Tue, 24 May 2016 01:13:00 -0700

The supergiant Gamma Cygni

The supergiant star Gamma Cygni lies at the center of the Northern Cross, famous asterism in the constellation Cygnus. Known by the proper name Sadr, the bright star also occupies the center of this beautiful celestial landscape formed a complex stars, dust clouds, and bright nebulae distributed along the plane of our galaxy, the Milky Way . The field covers more than 3 degrees (the equivalent of 6 full moons) on the sky and includes the open cluster NGC 6910 and the emission nebula IC 1318. To the left of Gamma Cyg and adopting the form of two luminous wings cosmic divided by a long dark dust vein, one can easily understand why IC 1318 is colloquially called Butterfly nebula. In the top left of Gamma Cyg are still young and tightly grouped stars of NGC 6910. Some distance estimates for Gamma Cyg place it at around 750 light years from us, while those for IC 1318 and NGC 6910 range from 2000 5 000 light-years.

The Cat's Eye

noreply@blogger.com (Universalanatomi) — Tue, 24 May 2016 01:06:00 -0700

The Cat's Eye

Fixing interstellar space, the beautiful nebula Cat's Eye is located 3,000 light-years from Earth. Classic planetary nebula, the Cat's Eye (NGC 6543) represents the brief yet glorious final phase in the life of a solar-type star. The dying central star of the nebula may have produced the simple exterior pattern composed of concentric layers of dust ejecting the outer layers in a series of regular convulsions. But the formation of the beautiful, more complex inner structures is not well understood. In this image from the Hubble Space Telescope, the true measure cosmic eye over half a light-year in diameter. Contemplating the Cat's Eye, astronomers can see the destiny of the sun, destined to enter its own phase of evolution ... in planetary nebula in about 5 billion years.

Solar system

noreply@blogger.com (Universalanatomi) — Mon, 23 May 2016 18:26:00 -0700

The new definition of the name of a desired planet by the executive of the International Astronomical Union (IAU) officially our Solar System has 8 planets, downgrading Pluto to the status of dwarf planet. This new definition was adopted by members of the Planet Definition Committee, a committee set up by the IAU to reflect on what a planet in light of recent discoveries both within our Solar System as around d other stars.

The main celestial body of the Solar System is the Sun, a yellow dwarf star of the main sequence of type G2 that contains 99.86% of the known mass of the solar system and gravitationnellement14 dominates. Jupiter and Saturn, the two most massive objects orbiting the Sun, together account for over 90% of the remaining mass.

Most large objects in orbit around the Sun are in a plane close to that of Earth's orbit, called the ecliptic. Typically, the orbital plane of the planets is very close to the ecliptic while comets and Kuiper belt have mostly an orbit that form a significantly greater angles to it.

All the planets and most other objects orbit in the same direction as the rotation of the sun, that is to say anticlockwise from an observer standpoint located above the solar north pole. Some objects orbit in a retrograde direction, as Halley's Comet.

The trajectories of objects orbiting around the Sun following Kepler's laws. They are approximately ellipses with one focus is the sun. The orbits of the planets are nearly circular. Those smaller bodies have various eccentricities and can be highly elliptical. This is particularly the case of some comets and other small bodies, some dwarf planets and more generally of trans-Neptunian objects including the Kuiper belt and the Oort cloud.

The distance of a body from the Sun varies during its rotation around the sun. Called the closest point to the Sun to the orbit of a body perihelion, the farthest being its aphelion.

Informally, the Solar System is often divided into separate zones. The inner Solar System includes the four terrestrial planets and the asteroid belt. The rest of the system may be regarded merely as externe15 Solar System; other separate the region beyond Neptune of four giant gazeuses16.

The majority of planets in the solar system have their own school system. Planetary rotating body around a planet are called natural satellites or moons. Most of the largest natural satellites fly in a synchronous orbit, always presenting the same face to the planet around which they revolve. The four largest planets also have a planetary ring.

Astronomy in Islam

noreply@blogger.com (Universalanatomi) — Mon, 23 May 2016 17:50:00 -0700

Between the time of Ptolemy and that of Copernicus, a period of over a thousand years, astronomy knows no notable development in Europe. In the Islamic world, by cons, important progress will occur between the ninth and eleventh centuries, both in the mathematical tools of astronomy in observing the sky.

This golden age of Muslim astronomy will begin in the reign of Caliph Al-Rashid and his son al-Mamun, both focusing on promoting scientific and cultural work in their empire. During his reign in Baghdad between 813 and 833, the Caliph Al-Mamun and will establish the largest library from that of Alexandria, the House of Wisdom, and set in 829 the first permanent astronomical observatory in the world.

Al-Khwarizmi

The most notable scholar of the ninth century Persian Al-Khwarizmi. He wrote the first book on algebra, al-Jabr Hisab w'al-muqabala and founded the same time this discipline. He introduced and spread the use of numbers we use today (they are called for Arab although they are actually from India). Its main direct contribution to the astronomy book will Sindhind zij, based on Hindu astronomy, in which he sets the tables on the position of the Sun, Moon and planets, and studying a variety of subjects such as eclipses or visibility of the moon.

Al-Farghani

Around the same time, the Persian Al-Farghani writes Astronomy Elements (Kitab fi al-Harakat al-Samawiya Jawami wa al-Ilm Nujum), a book based on the Ptolemaic astronomy. It also introduces new ideas, for example that the precession must affect the apparent position of the planets, not just the stars. This book will play a significant role in Western Europe when it was translated into Latin in the twelfth century.

Al-Battani

Around the late ninth century, the dominant figure is the Arab astronomer Al-Battani will observe the sky from Syria and make measurements of outstanding accuracy for its time. It will thus determine the length of the solar year, the value of the precession of the equinoxes and the obliquity of the ecliptic. He would also like to establish a catalog of 489 stars.

From a theoretical point of view, his main work, Kitab al-Zij is of fundamental importance as it introduces for the first time trigonometry in the study of the celestial sphere. This new approach will prove more powerful than the geometrical method of Ptolemy. This book will be translated into Latin in the twelfth century and influence many major European figures of the sixteenth and seventeenth centuries.

Al-Khujandi

In 994, the astronomer Al-Khujandi, from the current Tajikistan, built a huge mural sextant to Ray observatory near Tehran, the first instrument for more accurate measurements than the arc minute. He uses it in particular to determine a more precise value of the obliquity of the ecliptic.

Al-Biruni

Around the same time appears another scholar, al-Biruni, from near the Aral Sea. Like its predecessors, it is interested in many subjects such as mathematics and geography. In astronomy, he distinguished himself by his observations of lunar and solar eclipses, but also by a more modern approach to the experimental method, especially when analyzing errors which undermine its actions and those of Al-Khujandi.

Omar Khayyam

In the eleventh century Persian Omar Khayyam, now best known for his poetry, is interested also in various subjects, particularly algebra and astronomy. It creates new astronomical tables, but is especially remarkable in determining the length of the solar year with extreme precision for the time.

This golden age of Islamic astronomy will end in the twelfth century. The works of this prosperous period will gradually be translated into Latin, particularly in Toledo, Spain, and spread in Europe. It will be through these translations that European scholars of the late Middle Ages rediscover the theories of Ptolemy and will take note of progress made in the Muslim world.

The Greek astronomy

noreply@blogger.com (Universalanatomi) — Mon, 23 May 2016 17:30:00 -0700

From a historical point of view, Greek astronomy was dominated by two characters, Aristotle and Ptolemy, who introduced ideas and incorrect models that would dominate scientific thought for almost two millennia.

Aristote

The key figure is Aristotle, a philosopher of the fourth century BC, who leaned on the ideas of one of his predecessors, Plato. According to him, the world must have a spherical shape and movement of all celestial bodies must be circular and uniform, ie at constant speed.

In the system of Aristotle, as in that of Pythagoras, the Earth was motionless at the center of the world and surrounded by a series of crystalline spheres. The problem of the Pythagorean model was the fact that each planet was associated with a single sphere, which could not explain the irregularities of the apparent motions.

Aristotle overcame this problem by creating a more complex system containing 55 spheres nested into each other. Each planet was then associated with a group of spheres whose movement is superimposed. Combining different rotations allowed to give each planet a complex movement that could be adjusted to match the one that was observed in the sky.

With a combination of 55 spheres, Aristotle came relatively well to reproduce the apparent motions of the planets. His system was nonetheless a major flaw: it was unable to explain the apparent changes in brightness of the planets.

We now know that these changes are due to the change in distance between the Earth and each planet. But in the system of Aristotle planets were at a fixed distance from the Earth and the brightness changes remained unexplained.

Aristotle also introduced a more philosophical concept that came to be accepted until the sixteenth century: the distinction between the Earth and the heavens. For him, the interior of the lunar orbit, which included the Earth and its atmosphere, represented the realm of imperfection and change. Beyond the Moon, was the kingdom of perfection and immutability.

Ptolemy

The main flaw of Aristotle's system was its inability to explain the brightness variations of the planets. For this reason, an Alexandrian astronomer Claudius Ptolemy, changed the system in the second century AD, but without undermining the principles enunciated by Plato and Aristotle.

For Ptolemy, the heavenly bodies were not related to crystalline spheres centered on the Earth. Indeed, each planet was moving on a small circle, called epicycle, the center itself was moving in a great circle centered on the Earth, called deferent.

By adjusting the size and position of all circles involved, Ptolemy got a system that can accurately reproduce the apparent motions of the heavenly bodies. He was more able to explain the variations in brightness of the planets, since they now saw their distance from Earth varies.

This double success explains that the Ptolemaic system, which improved the Aristotelian in form but not the spirit, was accepted until the sixteenth century.

Heracleides

Despite the dominance of Aristotle and Ptolemy, two other Greek philosophers proposed much more realistic systems. At the time of Aristotle, Heraclitus advanced that the Earth was not stationary, but actually turned on itself. The apparent rotation of the sky 24 hours then explained much more natural way. The explanation was good, but it was not accepted.

Later, to explain the specific movements of Mercury and Venus, which seemed to hover around the Sun, Heracleides forward these two planets do not turn around the earth but around the sun. He thus obtained a more realistic description, even if he thought the rest of the heavenly bodies, including the Sun, still revolved around the Earth.

The astronomy of ancient Egypt

noreply@blogger.com (Universalanatomi) — Mon, 23 May 2016 17:15:00 -0700

If only the Library of Alexandria had withstood wars and conquests! Unfortunately, our knowledge of astronomy of ancient Egypt are very limited and we just few papyrus and a few inscriptions on tombs or temples.

In Egyptian mythology, the goddess Nut represents the sky, the air Shou and Geb the earth. This image shows part of the Greenfield Papyrus (1025 BC) where we see the body of Nut, held in place by Shou, and that of Geb, the ground.

A solar year of 365 days

At the time of ancient Egypt, The Nile flood occurred every year around July 19. Coincidentally, it is also at this time that the brightest star in the sky, Sirius, called Sothis in Greek and Egyptian Sopdet had its helical rising and thus made his first appearance of the year. As the Nile flood would fertilize the land and feed the people, observing the helical rising of Sirius, and more generally of the night sky, became a key element of the Egyptian civilization.

Based on their time measuring the apparent movement of the Sun, rather than the cycles of the moon, the Egyptians invented the solar calendar. As the helical rising of Sirius occurred approximately every 365 days and nights, they divided the year into 365 days. As the cycle of the moon lasted about 30 days and nights, they divided the year into 12 months of 30 days each month is still divided into three decades of 10 days.

Finally, to reach a total of 365, they added five additional days, called epagomenal days, which became day celebration of the gods Osiris, Seth, Isis, Nephthys and Horus.

As the astronomical year does not last exactly 365 days, the Egyptian calendar gently drifting over the cycle of the sky, about one day every four years. The Nile flood therefore coincided with the official start of the year that every 1,460 years, a length of time we named the Sothic period.

It was not until Julius Caesar established the Julian calendar and leap year in 45 BC that the timeframe be better aligned with the stars.

A 24-hour day

Egyptians also invented division of the day into 24 hours. To better get into the sky and measure the passage of time, they cut the sky in small groups recognizable stars who got up one after the other during the night. To coincide with the decades of 10 days, each group of stars was chosen so that its helical rising is separated from the previous 10 days. So we had 36 groups of stars, which named the decans.

Since the length of the night depends on the season, the number of observable decanates overnight varies. But at the beginning of the summer, at the time of the heliacal rising of Sirius, the night lasts about 8 hours and only 12 decans are observable. This number was taken - in somewhat arbitrary - as the basis of the new system. The principle was extended to the day itself divided into 12 hours. Thus the Egyptians established the 24-hour day that we still use.

Egyptian myths

The Egyptians had a very rich and often associated with celestial phenomena mythology. In some texts, the world was a big rectangular box whose north and south sides were longer. This box was a flat ceiling supported by four pillars. These were connected by a mountain range and a celestial river flowed quietly in a projection along this chain. Boats plying the celestial river and carried the Moon, Sun and planets.

In another interpretation, it was the body of the goddess Nut, deployed on top of the world, arms and legs spread, which formed the heavens. Her uterus engendered every morning the sun is at its mouth and swallowed the sunset to the west. Geb, god of the Earth, was lying under Nout, his wife and sister.

All celestial bodies were usually associated with deities. The Sun representing different gods depending on its position in the sky, Khepri at dawn, noon and Atum Re at night. Moon represented herself as several deities, Aah, Thoth and Khonsu.

The constellation of Orion was of particular importance and evoked Osiris, the first child of Nut and Geb, god of death and renewal. The death of Osiris and his rebirth were powerful symbols of annual drought Egypt always followed the Nile River and fertilization of the land.