Blogstronomy

What's the Relationship Between Stars in a Constellation?

noreply@blogger.com (T K Briggs) — Tue, 24 Jan 2012 17:00:00 +0000

"What's the relationship betweens stars in a particular constellation? Luck or something more?" - Question posed by Hannah.

Constellations are regions of the sky that are grouped around collections of stars that seem to draw pictures in the night sky. The famous ones include Ursa Major, Orion and Cygnus. From where we stand*, Ursa Major is so called because the stars that form it look a bit like a badly-conceived dot-to-dot image of a big bear; Orion looks, if you squint, a bit like a magnificent and heroic mythological hunter, and Cygnus, after a couple of Irn Brus, looks a bit like a majestic swan gliding serenely along the band of the Milky Way.

These are patterns picked out by human brains, those particularly effective pattern-spotting devices**, and these connections between the stars are, largely, there simply inside our minds alone.

Put another way, the stars in a constellation are not (usually) related to each other in any physical sense: they're much too far apart to affect each other gravitationally, and they're all sorts of distances away from us. In fact, many of the stars that make up the constellations are just as far away from each other as they are from us. Who knows, maybe our own Sun is part of some distant civilisation's perceived sky-picture?

A map of The Plough, via Wikimedia Commons

If you want to look at some specifics, then let's pick one of the most famous of our human star-pictures: The Plough (also known as the Big Dipper). The Plough is an asterism that is part of the constellation of Ursa Major, and looks like a giant saucepan in the sky.

To the right is a map of The Plough including the names of the stars that contribute to its image. The star labelled Mizar is the closest of these stars to us, at 78 light years away. Alioth, the closest of these stars to Mizar from our point of view, is 81 light years away from us- that's 3 light years further away from us than Mizar. The closest star to us (other than the Sun) is only 4 light years away, and we can barely see it from here. Dubhe is a whopping 124 light years away from us- that's 43 light years further than Mizar, or more than ten times the distance that Proxima Centauri is from our own Sun.

Here's a video that might help to get the point across. It starts off with our view of The Plough, and rotates to see the same collection of stars from different angles:

Another way to think about the way these stars, and those in the other constellations above our heads, are laid out in space is to look at one of those classic tourist photos of the leaning tower of Pisa, with some grinning plonker in the foreground pretending to use all his or her strength to hold the tower up. You can see loads of them here. These pictures work in the same way that the constellations do: your brain is fooled into thinking that because two things (grinning plonker and leaning tower) appear to be next to each other that they're actually close together. In reality the grinning plonker is much closer to the camera than Italy's most famous landmark: the connection is an apparent one, rather than a real one. The same is true with asterisms and constellations.

* Or sit, or lean, or cling desperately to a tiny ball of rock hurtling and whirling through a vast and dangerous universe. Take your pick.

** Too effective, which is why we so often see patterns where there really are none: faces of wolves in butterfly wings, asterisms in the sky, and depictions of Jesus on slices of toast.

How Close Would We Have to Be to Fall Into a Black Hole?

noreply@blogger.com (T K Briggs) — Thu, 19 Jan 2012 18:27:00 +0000

How close would we have to be to be sucked in by a black hole? - Question posed by Amy.

Any object with mass has a gravitational field. This includes you, me, trees, planets, moons and black holes (to name but a few). Things like planets, though, have a really noticeable gravitational field- you have to do a bit of work if you don't want to fall onto one of them, and the closer you get the harder it gets to avoid falling because the force that you feel is stronger the closer you are to its source.

How do Black Holes suck you in?

Black holes take this gravity thing to the extreme. Up to a point, though, you can get away from one: you just have to gun your engines and pick up enough speed to break free of its gravity, just like you do to get away from any planet or moon.

If you get too close, however, you're stuffed. This is because the speed you need to be going to break free is faster than the speed of light, and that's impossible (even light itself, the fastest thing in the universe, can't get out). So you're trapped forever inside...

You can think of this point of no return as an imaginary sphere around the black hole: if you reach the surface of this sphere, there's no going back. Ever. This boundary is known as the 'event horizon'.

How close can you get?

That depends on how big the black hole is. Or, more specifically, how much mass* it contains. A black hole's gravitational field is so strong because there is a lot of mass in a black hole. Heavier black holes have stronger gravity, which means you can feel its effects from further away. More importantly, this also means that the event horizon is further away.

Let's put some numbers on it (we can calculate it using a relatively simple formula**):

If we were to turn the Sun into a black hole right now (not sure why we'd want to, but go with it...), its event horizon would be about 3km away from its centre. That's about the size of Kettering. It's about 0.0004% of the Sun's current size- we wouldn't be able to see something that small from where we were (even if it wasn't black).

In contrast, the black hole which astronomers think is at the centre of our galaxy, with a mass approximately 4 million times that of our Sun, can be calculated to have an event horizon about 10 million kilometres away. That's getting on for the size of our entire solar system.

* Remember that 'mass' is just 'stuff': rocks and cats and planets and biscuits and socks and black holes all have 'mass' and it's measured in kilograms.
** And that formula is:

2GM / c²

Where "G" stands for the gravitational constant (this has a known value); "M" stands for the mass of the black hole (I'm using the mass of our Sun for the first example); and "c" stands for the speed of light (this also has a known value). All we do is whack in the mass we're looking at and, hey presto, we get a number for the radius of the event horizon.

How Can I Find New Planets?

noreply@blogger.com (T K Briggs) — Mon, 16 Jan 2012 22:16:00 +0000

If you've been watching the BBC's Stargazing Live you'll know the answer to this one.

Just go to the Planet Hunters website, let yourself be guided through a brief tutorial of what to do (it's easy- my cat could do it, and I haven't got a cat), and then start planet hunting!

Just in case there's any confusion, this is a real project using real data from real observations, and you can be a part of searching for new planets outside our solar system from the comfort of your own sofa, computer desk, bed or wherever else you sit and poke around on the internet.

I'm going to go and have a go now...

Tell Me About the Phases of the Moon

noreply@blogger.com (T K Briggs) — Sat, 07 Jan 2012 17:08:00 +0000

By Tomruen, via Wikimedia Commons
An animation showing the different phases of the Moon

Question posed by Dyana.

The Moon always has one half of it lit by the Sun*, and the other half in darkness**, but as it orbits us*** our viewpoint changes and we see a different portion of the lit half. The 'phases' of the Moon are our way of describing how much of the sunlit half of the Moon we can see.

We'll start the story with a phase which seems to make sense coming first:

New Moon
A new Moon is not particularly interesting to us as back-garden astronomers because we can't actually see it: the New Moon phase describes the time when the Moon is between the Sun and the Earth, so we are (in effect) viewing it from behind****, and there is no lit portion visible to us. Bear in mind that a new moon and a lunar eclipse are not the same thing (even though many people confuse them).

Waxing Crescent
As the first tiny toenail slither of the Moon starts to become visible (on the right-hand side, if you're in the Northern Hemisphere), the Moon has entered the phase known as 'waxing crescent'. As the Moon continues its orbit around us (anticlockwise, if you were hovering some way above the Earth's North Pole and looking down), this slither grows until the Moon enters the next phase, which is...

First Quarter
The first quarter is the instant when the right-hand 50% of the Moon is visible to us (again, only if you're in the Northern Hemisphere. In the Southern, it's the left-hand side because the Moon's the other way up). As the Moon continues onwards we enter the next phase.

Waxing Gibbous
Not 'gibbon', as a friend of mine has misquoted it. Gibbons are monkeys*****. 'Waxing' means that the portion we can see is still growing larger (I like to think of the Karate Kid quote "wax on..."****** to help me remember), and 'gibbous' means that the portion we can see is more than 50% of the full Moon, which is the next phase:

Full Moon
When the Moon has waxed gibbous enough, we get a full Moon. This happens when the Moon is on the opposite side of the Earth to the Sun, which means that it's half way through its orbit from where we started this post, and we can see the whole of the lit face of the Moon in all its shining glory. There's only one way to go now: back into darkness. If you're paying attention, you'll see a certain symmetry between the previous three phases and the next three.

Waning Gibbous
Remember that 'gibbous' means that we can see more than 50% of the Moon's surface, and I'll add that 'waning' means that this portion is now decreasing in size (receding to the left, for us Northern Hemispherers) until we hit the...

Last Quarter
The first quarter was when, from our point of view (N.H.-ers, again) the right-hand half of the Moon was lit, so the last quarter is when the left-hand half of the Moon is lit up (Southern Hemispherers just swap the 'left's with the 'right's, and vice-versa). As the Moon continues its orbit, we see a...

Waning Crescent
This is the phase when we see the half-Moon shrink down into a thinner and thinner cut-toenail-shaped Moon, until it disappears altogether. Then we're back to the...

New Moon
And the cycle starts all over again.

Here's a fantastic graphic that does an excellent job of clarifying some of the stuff I've said above:

The phases of the Moon, by Orion 8 via Wikimedia Commons
Note that the Earth-Moon system is not to scale (the Moon is much further away)

* Actually, this isn't always true- during a lunar eclipse some (or all) of the lit surface is shaded by the Earth.
** Though there's no such thing as 'the dark side of the Moon' (regardless of which Pink Floyd albums you listen to). Ask me about this one sometime ;-)
*** Or, if you want to get pedantic about it, we orbit each other.
**** Note that the same 'side' of the Moon always faces us, so I'm talking about the 'front' of the Moon as being whichever bit is facing the Sun at any given time.
***** As a raging pedant, I have to correct myself here: Gibbons are apes, not monkeys.
****** Conveniently forgetting the other half of that quote.

How Does the Moon Cause Tides?

noreply@blogger.com (T K Briggs) — Thu, 05 Jan 2012 21:29:00 +0000

Question posed by William.

Gravity
You're probably well aware of the fact that you'd have a different weight on different planets. On Mars, for example, you'd weigh less than you do here on Earth*, and if you were to visit Jupiter (and could find a solid surface to stand on) you'd weigh considerably more than you do here. This is because bodies (i.e. planets) with more mass have a stronger gravitational field.

Gravity has another feature, though: it gets weaker as you move away from a body that's exerting it**. That means that if you weigh yourself with a (very, incredibly accurate) set of scales in an aeroplane you'd find you weigh slightly less than you do standing in your living room. The effect is similar to that of magnetism: if you play with a magnet and something magnetic (a nail, perhaps***), you find that the magnet has less of an effect on the nail the further apart they are.

Tides

A diagram showing the direction of tidal forces on a body
(longer arrows indicate stronger force)
If the blue circle is Earth, the 'Satellite' indicated is the Moon.
From Wikimedia Commons

Now, you know that the Earth keeps the Moon in orbit with its gravitational field, but sometimes it's easy to forget that the Moon is also attracting the Earth with its gravity as well. And this is where I start answering your question...

Because of gravity losing its strength as you get further away from a massive body, the water on the surface of the Earth that's facing the Moon gets attracted towards the Moon a bit more than any of the rest of the water on Earth. This makes it bulge outwards a bit, so the water's a bit deeper there than it is elsewhere. As the Earth rotates, the Moon tugs on slightly different areas, and so the bulge appears to move across the surface of the Earth, and it's this moving bulge that makes the tides rise and fall each day.

Because of the decreasing effect of gravity as you move further away from the source, there's also a bulge on the opposite side of the Earth from the Moon (this water is being tugged on less, so it swells up). This is why we get two tides every day: one is caused by the bulge of the Moon tugging on it, the other is caused by the twin bulge on the opposite side of the planet.

Tidal Forces
What you think of as 'tides'- water going up and down the beach- are only one aspect of this gravitational phenomenon. 'Tidal forces' act not just upon the water of Earth, but on the entire planet: the rocks that make up the crust, the molten rock under the surface and even the iron-rich core are subject to tidal forces.

The effect is not limited to Earth, either. Everywhere you find two bodies orbiting each other, you find tidal forces. It is thought by some astronomers that Jupiter's moon, Europa, may be kept warm under the surface by tidal forces kneading its innards, much like blu-tak gets warm when you play with it for a while. An even more forceful example of the power of tides could be found in the history of Saturn's beautiful ring system: One possibility for their formation is that they are the result of a large icy moon orbiting too close to its parent Saturn, and being ripped apart by the stronger tidal forces experienced there.

* I'm assuming you're on Earth. If I'm wrong, then welcome to my blog. Come and visit some time.
** Everything that has mass has a gravitational field. Even you.
*** Excuse my lack of imagination: I have man flu.

When Are We Closest to the Sun?

noreply@blogger.com (T K Briggs) — Thu, 05 Jan 2012 01:00:00 +0000

I'm setting this to autopost at 1:00 am (GMT) on Friday 5th January 2012. If you're reading this at round about that time, then the answer is NOW*. Right now.

The thing is, Earth's orbit around the Sun isn't perfectly circular. Its orbit follows a path called an ellipse, which is a sort of squashed circle. This means that we're not always the same distance away from it- we get closer to and further away from the Sun on a cycle which lasts about a year, so our next closest approach (which is called 'perihelion') will be about this time next year, to within a few days.

Another interesting idea is that, due to the way celestial mechanics works, the point at which we're closest to the Sun is also the point at which we're moving fastest in our orbit, and the point at which we're furthest from the Sun (known as 'aphelion', and falling on July 5th this year) is the point at which we're moving at our slowest speed.

Here's one for your brain to chew on: here in the UK, we're at the closest point to the Sun in the middle of winter, yet furthest away from it when we're slinging on the flip-flops and reaching for the sun cream. How does that work, d'ya think?

* Here are the dates and times for the years 2000-2020 inclusive (along with some other info), straight from the NASA's mouth: http://aa.usno.navy.mil/data/docs/EarthSeasons.php

Are There Such Things as UFOs?

noreply@blogger.com (T K Briggs) — Mon, 02 Jan 2012 11:30:00 +0000

Yes.

"UFO" is an initialism that stands for

Unidentified
Flying
Object

So if you see something that is in the sky and appears to be flying, and you don't know what it is, it is most definitely a UFO.

Lots of people equate UFOs to spaceships and flying saucers, but that's not what the word (or initialism) means. It simply means that you don't know what it is. And that's boring. The interesting bits come when you manage to identify the flying object (at this point, it stops being a UFO*).

Unfortunately, no UFOs have ever been officially identified as alien spaceships. That would be very interesting and more than a little bit cool, but it hasn't happened. Most UFOs turn out to be things like satellites, meteorites, Chinese lanterns, and strange cloud formations. Some reports, however, have ended up being identified as things that you might think are obvious, such as Venus, Jupiter and (wait for it...) the Moon.

* I suppose that means it becomes an IFO, but usually when something has been identified we already have a name for it and use that one so as to avoid conclusion.

What Relevance Does New Year's Day Have in Astronomy?

noreply@blogger.com (T K Briggs) — Sun, 01 Jan 2012 19:29:00 +0000

None whatsoever.

New Year's Day represents an arbitrary point in Earth's orbit around the Sun. There's nothing special about it at all.

If we wanted to choose a date with astronomical significance to use as our new year, then we might look at the closest Earth gets to the Sun (perihelion) or the furthest away the Earth manages to get (aphelion). These occur in early January (something like the 4th or 5th, usually) and early July respectively.

In fact, today (January 1st) is only New Year's Day if you happen to be a follower of the modern Gregorian calendar, or ancient Rome's Julian calendar (which, admittedly, is most people on this planet). There are other cultures that use different calendars with different arbitrary starting points for a new year. Arguably the most famous of these is the Chinese calendar, which falls somewhere between January 20th and February 20th, but many other cultures, including the Tamils, Thailand and Hindus (mid-April), and Ethiopia (September) have different starts to their year.

So, whichever religious, cultural or geographic background you hail from, happy January 1st from Blogstronomy!

What Has the Moon Ever Done for Us?

noreply@blogger.com (T K Briggs) — Fri, 30 Dec 2011 11:32:00 +0000

How does the Moon affect the Earth? - Question posed by Priyanka.

Today...
The most noticeable effect that our Moon has on the Earth is that of the tides. Strictly speaking, tidal forces work on any matter in a system involving two bodies such as that of the Earth and the Moon. We see this effect most obviously in the seas of our planet as water around coastlines rises and falls each day. This effect (which I have blogged about in more detail here) is causing the Moon to move further away from us and, more relevant to the question asked at the beginning of this post, slowing down the Earth's rotation.

The Moon also affects the Earth's orbit around the Sun. Due to effects discussed in this post, the Earth follows a slightly wobbly path around the Sun. Also due to the gravitational interactions of the Earth, Sun and Moon, both the speed and axis of rotation of the Earth change slightly but noticeably over time which affects the seasons (although these cycles take tens of thousands of years to complete).

In the past...
The Moon may even have had an effect on life on Earth, right from its very beginnings. It may be that the Moon protected Earth from some of the asteroids and comets hurtling through the inner solar system, giving life a better chance to form and evolve.

The tides, already discussed, may have had an effect on life, too. It is well known that life probably developed in the seas of the early Earth and then made its way onto land from there. The tides provide a middle ground between the two (the 'inter-tidal zone'), and this may have helped life to gain a footing on land.

The Moon will most likely have had an affect on the evolution of certain forms of life simply by changing the light levels available at night: nocturnal creatures often have large eyes to make use of this night-time light source. If the Moon wasn't there, it may have been the case that nocturnal animals had no use for eyes at all!

Once life began to develop intelligence, maybe the Moon encouraged thinkers to look up and wonder about their place in the world. Maybe the Moon, with it's cool, easy on the eye nature and readily apparent surface features, did more to promote thinking about the wider universe than the firey, dangerous to look at Sun ever could.

In the future...
It could affect us more in the future, by acting as our first stepping stone out into space. Landing on the Moon, and maybe even living there, could teach us things we need to know if we're going to go further. Beyond that, it could be a launchpad for going into space, making the leap into the unknown cheaper and easier than it is from Earth.

Season's Greetings!

noreply@blogger.com (T K Briggs) — Thu, 22 Dec 2011 14:00:00 +0000

I would like to wish all of you, regardless of race, creed, faith (or lack thereof) all the very best of the season, and a prosperous and healthy year to come for yourself, your family and your friends. Here's your card:

I'm not sure where this originated... sorry!

It is, of course, the winter solstice today (December 22^nd) for those of us in the Northern Hemisphere of our planet. This is the name given to the point in the year at which the Earth's axial tilt conspires with its motion around the Sun to make the North Pole point further away from the Sun than it does at any other point in the year.

It is this axial tilt that causes the seasons- we have winter when the Northern Hemisphere leans away from the Sun, and summer when it leans towards it. The winter solstice - today - sees the shortest day flanked by the longest nights of the year. Since June 21^st our days have gradually shortened whilst the nights lengthen, and now they start to go back the other way, increasing until June 20^th, the longest day of 2012.

This change, this reversal of things, this crossover from a descent to darkness into an ascent to light, is precisely the reason why many people throughout human history have marked this time of year as something special. Go to this page and scroll down a bit to see the number of observances which happen (or used to happen) on or near to this date. If you thought it was just the Christian festival of Christmas you may be surprised!

Remember that, although many of these religious festivals have been going on in one form or another for timescales measured in thousands of years, the Earth has been doing its thing for over four billion of them.

What is Supersymmetry?

noreply@blogger.com (T K Briggs) — Tue, 20 Dec 2011 15:04:00 +0000

Here's a pretty cool video I came across that gives a decent introduction for one of the foremost theories in science today.

What is a Higgs Boson?

noreply@blogger.com (T K Briggs) — Sun, 18 Dec 2011 14:10:00 +0000

I just came across this great infographic over at livescience.com and thought you might like it!

Source: LiveScience

How Close Will 2005 Yu55 get?

noreply@blogger.com (T K Briggs) — Sat, 05 Nov 2011 08:55:00 +0000

2005 Yu55, assembled from radio data received by Arecibo in 2010
By NASA/Cornell/Arecibo, via Wikimedia Commons

On Tuesday, an asteroid bearing the name 2005 Yu55 will pass (in cosmic terms) within a hair's breadth of Earth: the 400 metre wide asteroid will pass within 0.85 lunar distances of us. There a lot of blog posts and news items of the "oh my [insert deity of choice], we're all nearly going to die!" variety. But these numbers don't mean a lot to most people, so in terms we can get our heads around, how close will it get?

To start with, "0.85 lunar distances" means "85% of the distance to the Moon". The Moon is about 385,000 km away, so that means that the asteroid will, at its closest approach, be around 327,250 km away from us.

That's a bit better, but it's still hard to imagine.

The Earth itself is about 12,735 km in diameter. This helps to put 2005 Yu55's closest approach into perspective: 327,250 km is more than 25 times larger than 12,735. This means that, at closest approach, you could fit 25 more Earths in the gap between Earth and 2005 Yu55.

That's better still, but how about this to get things into perspective:

As a rough guess, my head is about 20 cm in diameter. Yours will be similar. This means that if we could shrink the Earth down to the size of my (or your) head, 2005 Yu55 would be about the size of a grain of dust, but lets imagine it as a pea (because you can see one of those easily). When it passes the Earth its closest approach would be like someone throwing that pea at your head and missing by five metres.

This is, however, the closest approach of such an object that we've had advance warning of, and as such makes for interesting science - every such incident improves our understanding of close approaches, and it'll give us an opportunity to study an asteroid up-close (relatively speaking*). Also, it's worth mentioning that whilst the head-pea analogy above works well for comparing sizes, it falls down when we think about what would happen if the pea-thrower had a better aim: for the analogy to work in this case, the pea would have to leave a crater in your head the size of your eye-socket. If we considered 2005 Yu55's true relative grain-of-dust size, then the crater that would be left would be getting on for the size of a nostril - still pretty big and fairly noticeable on your face.

I'll leave you with that thought...

* Unfortunately, budding home-astronomers won't be able to see it unless they've got a fairly powerful telescope.

What is the Name of Our Moon?

noreply@blogger.com (T K Briggs) — Wed, 02 Nov 2011 17:30:00 +0000

Question posed by pudster100.

I hate to be the bearer of bad - or simply boring - news, but our moon's name is just... the Moon.

But why is it that our Moon is called 'the Moon', yet all the other planets' moons have more interesting names? Simply put, it's because ours was the first. Not the first to be formed; just the first to be noticed by us. Compared to all the other moons we now know about, and indeed pretty much everything else in the sky, the Moon is pretty flipping noticeable, and it took a fair while to realise there were any others whirling about the Solar System*.

But it hasn't always been called that. The earliest name for the Moon that I can find in the English language's lineage is 'mǣnōn', which is buried in the beginnings of the Germanic languages, from which modern languages including English, German, and the Scandinavian languages have since grown. From there, and probably through numerous intermediate steps, it developed into the Old English 'mōna' from the Middle Ages. In the 12th century, we had the much more familiar 'moone', which then morphed over time into the 'Moon' that we know and love today.

Of course, other modern languages have their own names for our Moon, all of which seem to have developed over time from arcane beginnings. There's a table of various languages' names for the Moon (and other Solar System bodies) over at nineplanets.org, and a quick scan of the list shows some pretty obvious shared-lineages for many of the world's languages: many of them have an obvious common heritage in the Latin 'Luna', for example, which is where we get our adjective 'lunar' from. A less often used Moon-based adjective is 'selenic,' from the Greek 'Selene,' with selenologists being those scientists who study selenology (the Lunar equivalent to geology).

* Incidentally, this is also probably why our solar system is just called 'the Solar System', while others have groovier names.

What is it Like to Be In an Accelerating Spacecraft?

noreply@blogger.com (T K Briggs) — Tue, 01 Nov 2011 17:30:00 +0000

The International Space Station orbits the Earth at a height approaching 400 km, but this orbit degrades over time due to atmospheric drag. Periodically, then, the Station has to be boosted back into its nominal orbit. This is done by accelerating the craft forwards (rather than upwards), so that it orbits the Earth a little bit faster, which widens its orbit. Boosts can be performed in different ways; either a craft (the ESA's Automated Transfer Vehicle, the Russian Progress resupply vehicle or, until recently, NASA's Space Shuttle) gives the Station a push when docked at the rear, or the Space Station's own engines on the Zvezda service module can be fired.

Here's what it's like to be inside the International Space Station during a boost:

Does the Moon Orbit the Earth?

noreply@blogger.com (T K Briggs) — Mon, 31 Oct 2011 17:30:00 +0000

Question posed by Jennie, after I baffled her in the pub. She'd been watching this video, and said that if Jupiter were to replace the Moon, then surely Earth would end up in orbit around Jupiter?

No.

Well, not really*. The common view of the Earth-Moon system (and of any two-body system) is that the Earth sits there on the spot (ignoring its motion around the Sun, for now) and the Moon makes its stately way around it in a big circle. The same view is generally thought of with Jupiter and the Sun, or any other situation in which an 'orbit' is involved. This isn't quite what happens...

By User:Zhatt (Own work) [Public domain], via Wikimedia Commons

Remember a time when you were younger; orbital mechanics are less of an issue and Big Uncle Bob, who's six-feet tall and built like some kind of unmentionable outhouse, picks you up and swings you around. You are very definitely flying around in a scary yet enjoyable circle, but at the same time, even though you're so light compared to him, he still has to lean back as he spins you so as not to fall over. This is pretty much what's happening with the Earth and Moon.

The Earth is a lot more massive than the Moon, so it seems like the Moon is orbiting the Earth, but in reality they're both orbiting a point between the centres of the two bodies. This common centre is actually under the surface of the Earth, but it is not at the centre of the Earth. The animation to the right gives a pretty good demonstration of this (the Earth and Moon and their distance apart are not to scale): the red cross in the centre is the point they're both orbiting around.

The fact is that they're both in orbit around each other, or more precisely, around their common centre of mass**. This point is called the 'barycentre' of a system.

In situations where one object is bigger than the other*** the centre of mass moves closer to the heavier object. If you adjust the model in your head to an extreme (Uncle Bob's morbidly obese, and you're a newborn slip of a thing), the effect is less obvious****. Can you think what might happen if both objects have the same mass?

For purposes of clarity, however, we usually say that one object (the lighter one) is 'in orbit' around the other (heavier) one: the Moon orbits Earth; Jupiter orbits the Sun. In Jennie's original comment, only the biggest pedant (me) would say that her statement was incorrect- in the animation above, just imagine the big circle is Jupiter, and the small one is Earth.

Now, you're probably thinking ill of me right now; "what an annoying, pedantic fool!" But wait! This fact is actually quite useful: the same thing happens with any two bodies that are in orbit around each other, including distant stars. This means that some stars appear to 'wobble'. For some stars, we can detect this wobble and work out that there's a planet there even though we can't see it. We can even work out things like the mass of the planet and its orbital period!

* I'm being facetious for the purposes of introducing a concept that I'd like to talk about. Of course, in the situation in which Jupiter replaced our Earth there would be significant changes in the dynamics of the system, and Jupiter would indeed dominate it.
** Or 'centre of gravity' if you're more comfortable with that phrase.
*** Strictly speaking, we should say 'more massive' rather than 'bigger' because, in this case, size doesn't matter. It's how much stuff it's made of that counts.
**** It is important to note, however, that the only way you can get the centre of mass to be in the centre of the heavier ball is if the lighter ball has zero mass (i.e. there's nothing there).

How Small Am I?

noreply@blogger.com (T K Briggs) — Sun, 30 Oct 2011 16:00:00 +0000

In my favourite series of books, The Hitchhikers' Guide to the Galaxy, Zaphod Beeblebrox is thrown into the Total Perspective Vortex; a machine created with the sole purposes of destroying your mind simply by showing you just how utterly insignificant you are.

Here's my attempt to do the same to you by showing you a video I've found that compares the sizes of various objects that exist within our universe, including our very own planet Earth:

Once you've uncurled from the inevitable foetal position that thinking about such things promotes (well, it does in me), you might like to check out a few of the other videos that I've posted.

How Much Longer Will the Sun Survive?

noreply@blogger.com (T K Briggs) — Sat, 29 Oct 2011 15:30:00 +0000

Question posed by max jon.

The lifetime of a star is determined by its mass. The more mass a star has - and this sometimes feels like it goes against intuition - the shorter its life will be.

The most massive stars in the universe, called hypergiants, have many tens of times the mass of our Sun, are in the region of a few hundred thousand times as luminous, and last for only a few million years. The largest star we know of, VY Canis Majoris (VY CMa to its mates), has somewhere between 30 and 40 times the mass of our sun, and burns 450,000 times as bright. In terms of size, if it occupied the position currently inhabited by our own Sun, its surface would be well beyond the orbit of Mars, with some astronomers calculating it to be large enough to reach as far out as Saturn's orbit. It's expected to go hypernova some time in the next 100,000 years.

It can be difficult to imagine how breathtakingly massive these things are, so this video might help:

At the other end of the scale, the smallest stars that we know of, red dwarfs, have less than half the mass of our Sun, and can burn for hundreds of billions of years- some estimates even suggest that they can last for trillions of years. There are no red dwarf stars coming to the end of their life span anywhere in the universe: there simply hasn't been enough time.

Somewhere between these two extremes is our Sun. With its 2,000,000,000,000,000,000,000,000,000,000 (that's 2 nonillion) kg of fuel packed into a volume of space 1 million km across, it is unofficially known as a yellow dwarf* star (its official classification is 'G2V', but that's less catchy) and as such has an expected life span of around 10 billion years. It's been going strong now for something like 4.5 billion years, so it's got a good 5 billion left in it yet.

If you'd like to know what'll happen to the Sun (and the Earth) when it dies, this post would be a good place to start.

* Why's it green, then?

Where Am I?

noreply@blogger.com (T K Briggs) — Fri, 28 Oct 2011 16:30:00 +0000

I get the odd question asking where certain things are: Europa, the Moon, the Voyager probes, and even some things that don't even exist. But do we have a good handle on where we are? Regular readers with a keen memory may recall this post which addresses the same question with a cool video. I'm taking it a bit further here, though.

Imagine you have a penpal. Maybe you do have a penpal; imagine this penpal is an alien, somewhere out in space, somewhere out in the vast expanse of the universe. How does he, she or it address your letters? For each of us, the Earth-based part of our address is going to be different, so I'll leave that bit off. Beyond that, how would your address look on your green and betentacled penfriend's correspondence?

He'd have to send it to the right planet. But where is Earth? You may have heard our blue-green planet referred to as the 'third rock from the Sun'; we're the third closest large body from our Sun. Mentioning our star's name is also useful because that's where solar systems get their name from: 'Sol' was a Roman Sun God, and we live in Sol's system- 'the' solar system. That would be the next line on our address, and we'd probably like to add "inner" to it, just to help the postman.

But where's the solar system?
Our solar system resides in the galaxy that we call the Milky Way, but just writing that is a bit like telling my postman that my house is somewhere in Europe. In fact, it's even less helpful than that when you think that the Milky Way measures around a hundred thousand light years across, and our solar system is somewhere in the region of a thousandth of a light year across. Add to that the fact that our Sun is only one of well over 100 billion stars, then we could at least do with a postcode.

Luckily, the Milky Way is a spiral galaxy, and we can narrow down the location of stars by talking about which of the spiral arms they're in. We're located on the inner rim of the Orion arm, about half way out from the centre of the galaxy.

But where's the Milky Way?
The massive Milky Way and Andromeda galaxies orbit each other in amongst a group of 50 smaller galaxies, all bound by gravity, known as the Local Galactic Group.

But where's the Local Galactic Group?
The LGG, resides in a volume of space 110 million light years across along with at least a hundred other galactic groups and clusters that make up the Virgo Supercluster. Superclusters are so large that they are not gravitationally bound. Consequently, this is the smallest scale that we can start to see effects due to the expansion of the universe.

But where's the Virgo Supercluster?
This is where things start to get sketchy. The Virgo Supercluster is part of a local group of superclusters known as... the local superclusters. Beyond this, there are vast, mind-bogglingly enormous structures made up of superclusters that we call 'walls' and 'filaments' that enclose even more mind-bogglingly vast voids. These are what make up what is known as the 'Observable Universe' (mainly because it's the bits of the universe that we are able to observe.

So in brief, here's that address:

[Insert your Earth-based address here],
The Inner Solar System,
The Orion Arm (Inner Rim),
The Milky Way Galaxy (nr. Andromeda),
The Local Galactic Group,
The Virgo Supercluster,
The Local Superclusters,
The Universe.

Here is an excellent set of maps that you can send your extraterrestrial correspondee so that they can find their way here (GPS is only useful for the last few thousand kilometres):

By Azcolvin429 (Own work), via Wikimedia Commons

You can click on the image to go to the original and read all the info on it, but be warned that, as befits the scales upon which we are working, the image is BIG.

What is a Gamma Ray Burst?

noreply@blogger.com (T K Briggs) — Thu, 27 Oct 2011 16:30:00 +0000

Simply speaking, a gamma ray burst (known in the trade as a 'GRB') is a burst of gamma rays; that is, a relatively short-lived blast of intense gamma radiation*. There is nothing in the universe more powerful than a GRB. There are two types of gamma ray burst, with differing causes:

Long duration gamma ray bursts

Some GRB host galaxy shots from Hubble

These are bursts that last in the region of 20-40 seconds, but contain an energy equivalent to that put out by our Sun over its entire lifetime. These are thought to originate from the high-energy event that is a hypernova (like a supernova but much more violent).

Short duration gamma ray bursts
These are bursts that last up to two seconds and contain significantly less energy than the long-duration bursts (although they still contain a lot of energy compared to other events). These are thought to be produced by collisions of neutron stars with each other, or with black holes.

That sounds dangerous...

Without trying to be alarmist, it is. Even when a gamma ray burst happens as much as 10 billion light years away and is pointing in our direction, it's a pretty noticeable event for anything looking in the right direction with the right kind of telescope. All of the gamma ray bursts detected so far have originated outside of the Milky Way, and have been harmless to Earth.

If a gamma ray burst originated in our own galaxy, however, and were to be shot in our direction, it'd be very bad news indeed: for a gamma ray burst originating about 50,000 light years away (that's about half the width of the Milky Way), most of the life on the side of the planet facing the burst at the time would probably be wiped out fairly quickly, and the rest would have their life span significantly shortened by the effects of radiation damage. Added to this, the burst would cause significant ozone depletion in the Earth's atmosphere: the ozone layer is one of our most important shields against solar radiation.

Around 450 million years ago there was an extinction event** (known as the Ordovician-Silurian extinction event), and it is thought that this was caused by a gamma ray burst: it's possible that this has happened before, and it may happen again. And if it does, there's not a lot we could do about it.

There's some good news though, right?

Yeah; there's always good news! In this case, the best news is that a gamma ray burst doesn't blast out from a hypernova in all directions. Instead, for complicated reasons, a burst is directed out in two relatively narrow beams along the axis of rotation*** of whatever it is that's producing the GRB. This means that it's fairly unlikely that we'll be lying directly in the path of any given GRB, even if it originates close by.

The other good news is that GRBs themselves are pretty rare: on average, it seems that there are only a handful of GRBs expected in any particular galaxy in any particular million-year period.

So don't lose any sleep over it!

If, however, you'd like to lose some sleep, you could try this post which talks about various ways in which the human race could find itself being killed off. Don't have nightmares!

* Gamma rays are a form of radiation, like x-rays, infrared radiation, microwaves and even visible light. These forms of radiation differ from each other in terms of their wavelength and frequency- gamma radiation has a very short wavelength and correspondingly high frequency.
** An 'extinction event' is when a significant number of Earth's species become extinct in a relatively short period of time.
*** That is, it's blasting out from the poles.

Is Earth the Fifth Smallest Planet?

noreply@blogger.com (T K Briggs) — Wed, 26 Oct 2011 16:30:00 +0000

Question posed by Marvin.

No. It used to be, but since Pluto was re-classified as a dwarf planet, every other planet in the list has shifted down a place. Even if we included Pluto, we'd have to also include Eris (at least), which means that Earth still wouldn't be the fifth smallest planet.

So where's Earth on the list of planets in our solar system, if we put them in size order?

Comparing the sizes of the planets and the Sun. (Origin unknown)

Earth comes in at fourth place. Here's the full list, from smallest to largest, and a rather nifty graphic comparing them all:

Mercury (Average diameter: 4879.4 km, or about 2/5 the size of Earth)
Mars (Average diameter: 6772 km, or about 1/2 the size of Earth)
Venus (Average diameter: 12,104 km, or about 9/10 the size of Earth)
Earth (Average diameter: 12,735 km)
Neptune (Average diameter: 49,105 km, or about 3.8 times the size of Earth)
Uranus (Average diameter: 50,532 km, or about 4.0 times the size of Earth)
Saturn (Average diameter: 114,630 km, or about 9.0 times the size of Earth)
Jupiter (Average diameter: 138,350 km, or about 10.8 times the size of Earth)

What Would We See if the Moon Was Replaced by a Planet?

noreply@blogger.com (T K Briggs) — Tue, 25 Oct 2011 16:30:00 +0000

Video brought to my attention by Geeks Are Sexy.

What would our view of the sky look like if our rocky companion, the Moon, found itself popping out of existence and being replaced by another body from our solar system, maybe Mars, or even Jupiter? Scratch your bonce no longer, because this fab video will give you a pretty good idea!

Yes, it leaves out some objects that could be pretty interesting (Saturn, for instance: imagine the rings!), but it certainly helps to put things in perspective.

If you like this video, then I'm pretty sure you'll love this one too.

What's the Difference Between a Comet and a Meteor?

noreply@blogger.com (T K Briggs) — Mon, 24 Oct 2011 16:30:00 +0000

Question inspired by this tweet by @VirtualAstro.

The best way to answer this question is to look at two slightly different questions:

What is a comet?

A comet is a relatively* small body (we're talking tens of kilometres, rather than thousands) made largely of ice (including water ice as well as frozen forms of substances such as methane, ammonia, carbon monoxide and carbon dioxide) mixed up with rocky material and a variety of organic compounds.

Comet Hale-Bopp, March 29th, 1997.
By Philipp Salzgeber, via Wikimedia Commons

In the outer solar system, comets are often described as 'dirty snowballs': small, dark frozen lumps of stuff that are difficult to detect due to these characteristics. These rocky bodies are known as the 'nucleus' of a comet. When they're on orbits that draw them towards the inner solar system, however, they're much more impressive: radiation from the Sun causes the volatile chemicals to vaporise, forming an atmosphere (called a 'coma') around the comet nucleus.

As parts of the comet vaporise, some of the solid matter (dust and rocky bits) that makes up the comet is carried away too, and some of this is left behind in the comet's trail. As well as this, the Sun's radiation pressure causes some of the vaporised material to form a tail, which is illuminated by the Sun.

Sometimes, if the solar system lines itself up in the right way, a comet comes close enough to the Earth, and is in the right place, for us to see the comet and its tail in the night sky. I remember, as a fifteen-year-old in 1997, being able to view comet Hale-Bopp with my unaided eye for a period of time measured in months - the planet as a whole was able to view Hale-Bopp without the aid of telescopes or binoculars for a little longer than a year and a half.

Comets, especially ones that we can see in all their glory from our vantage point on Earth, are quite rare.

What is a meteor?

A Perseid meteor (the streak to the right of the middle)
By Mila Zinkova (Own work), via Wikimedia Commons

The International Astronomical Union (IAU) defines a meteoroid as "a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom"; basically anything from a dust particle up to a lump of rock about 50 metres in diameter.

If the orbit of a meteoroid coincides with that of the Earth, it will enter the Earth's atmosphere and, due to friction, start to burn up. This leaves a trail across the sky. At this point, the meteoroid becomes a meteor.

Meteoroids collide with the Earth's atmosphere (hence becoming meteors) quite often- meteors of about 20 metres in diameter hit around once per century; those with a diameter up to four metres hit around once per year; meteors in the region of 40 centimetres across collide with us about once per day. Meteors of smaller sizes enter the atmosphere over progressively shorter timescales. A meteor's visible journey can last a fraction of a second, and is easy to miss!

What's the difference?

Comets are bigger and (hopefully**) further away. Their bright tales are caused by volatile gases being vaporised due to the comet getting closer to the Sun. If a comet were to enter our atmosphere it would probably be a bad thing. Comets don't turn up all that often (not ones that we can see with our unaided eyes, anyway). When they do turn up, they can remain visible in the sky for days, weeks, or even months.

Meteors are smaller than comets and aren't classed as meteors until they enter the Earth's atmosphere. Their bright tracks are caused by friction with the atmosphere causing their outer layers to burn up. Meteors enter the Earth's atmosphere all the time, and it's not unusual to see one on a clear night, with low light pollution and a bit of luck. When you see one, it's gone in a flash.

* Relative to stars and planets and things.
** If a comet were to enter our atmosphere that would probably be a bad thing. In 1908, something exploded over Tunguska, Siberia. The explosion demolished an area of woodland that was roughly equivalent to the area covered by London within the M25 today. Many astronomers suspect that this event was in fact a small comet entering the Earth's atmosphere and exploding before it hit the ground.

In Which Year Was the First Picture of Earth Taken From Space?

noreply@blogger.com (T K Briggs) — Sun, 23 Oct 2011 15:00:00 +0000

The Blue Marble, snapped by Apollo 17 astronauts.
By Latitude0116 at en.wikipedia, from Wikimedia Commons
(Originally by NASA)

What year was the first pic of Earth from space taken? - Question posed by @janshs.

On December 7th, 1972, the crew of Apollo 17 snapped what was to become an iconic image of the Earth. Known as 'the Blue Marble', the image (shown to the right, showing its original upside-down view of our planet) is one of not very many at all that show the entire daylit side of Earth in one shot*. The image is often seen inverted to show our planet the 'right way up'.

But this wasn't the first.

By Frank Borman, via Wikimedia Commons

In December 1968, another iconic shot of the Earth had already been snapped by the crew of Apollo 8 as it orbited the Moon. Coming out from the far-side of the Moon, the crew noticed for the first time the Earth rising in the Moon's sky, much like we see the Moon rising in the Earth's sky. This was the first time such an event was seen by human eyes, and became known as 'Earthrise'. The most famous shot to be taken at this time was in colour, but the first photograph to be taken as the astronauts realised what they were witnessing was in black and white. This is the image that I have included here, as it is less often seen.

But this wasn't the first either.

On August 23rd 1966 (at 16:35 GMT, if you're interested!), the unmanned probe Lunar Orbiter 1 took a similar Earthrise photograph, also in black and white, using its onboard camera. The spacecraft was on its 16th orbit, and about to pass behind the Moon. There is a thoroughly reprocessed version that was released in 1998 and shows a much clearer view of a small section of the original photograph, but here I am including a copy of the original 1966 image:

By NASA, via Wikimedia Commons

But, you guessed it, that wasn't the first one either.

In 1960 the experimental Tiros-1 weather satellite took and transmitted the first television image of the Earth from space. The satellite was operational for only 78 days, but provided evidence that satellites could be useful for observing weather conditions from space.

Again, however, this wasn't the first image taken of Earth from space.

This one was...

As far back as 1946, before even Sputnik achieved its fame, the German-designed V-2 rocket had its uses in astronomy. Originally designed as a ballistic missile and fired at London and Antwerp during the Second World War, its purpose was re-imagined and became humanity's first known successful sub-orbital space flight vehicle. The V-2s were unmanned, but were loaded with instruments to test the air and measure cosmic radiation at different altitudes. On October 24th 1946 a group of US scientists launched a V-2 rocket that included a camera designed to take a picture every 1.5 seconds, and the very first image of Earth as seen from space was retrieved from the reinforced steel cassette after the V-2 slammed into the ground at White Sands. Here it is:

By U.S. Army, via Wikimedia Commons

The V-2 rockets took photographs of the Earth from altitudes of over 100km - around five times higher than the 22km record set by the Explorer II scientific research balloon in 1935, which was high enough to make out the curvature of the Earth.

* Many images that appear to show the entire Earth (or at least half of it) in daylight are actually composite images.

Faster Than the Speed of Light?

noreply@blogger.com (T K Briggs) — Sun, 23 Oct 2011 11:02:00 +0000

Last month scientists released findings which claim to have recorded particles travelling faster than the speed of light. If accurate, these findings will rock the world of science, shaking the foundations of much of what we think we understand. Now, scientists around the world are searching for answers: some are going through the experiment's processes, instruments and data with a fine-toothed comb, trying to find what is almost inevitably an error. A few are exploring the possibility that these particles can actually travel faster than the speed of light, how that might happen, and what that would mean for our current understanding of the universe.

Marcus DuSautoy is mathematics' answer to Brian Cox, and presents Faster Than the Speed of Light? for the BBC, which puts forward the important aspects of the situation in a way which is accessible to anybody with a basic grasp of science and, more importantly, an interest in the deeper workings of life, the universe and everything.

You can watch it via the BBC's iPlayer here (available until 31^st October):

Faster Than the Speed of Light?

Or, for probably a limited time, here is the program split into four parts and hosted at YouTube (just in case you can't use iPlayer- could someone outside the UK please let me know if these are viewable?):

Part 1

Part 2

Part 3

Part 4