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Date: March 31, 2012

Title: Encore : What’s in a Name? – The Story of Parsecs

Podcasters: Olaf Davis

Links: Cosmic Web

This podcast originally aired on May 29th 2010
http://365daysofastronomy.org/2010/05/29/may-29th-whats-in-a-name-the-story-of-parsecs/

Description: Sometimes the units we use for certain measurements contain clues about what people were measuring – and how – when they were first used. Taking the example of the ‘parsec’, equal to just over three light-years, I’ll explore what its strange name and even stranger definition can tell us about the history of astronomical measurements in our Solar System and beyond.

Bio: Olaf is a second-year Astrophysics PhD student in Oxford. His research involves mathematical models and computer simulations of various astronomical phenomena, including the behaviour of energetic particles around black holes and the large-scale distribution of galaxies across the Universe. His blog, the Cosmic Web, is about astronomy and aimed at the layman.

Sponsor: This episode of the “365 days of Astronomy” is sponsored by —

Transcript:

Hello. My name’s Olaf Davis and I’m a PhD student in the astrophysics group at the University of Oxford. In this podcast I’m going to talk a bit about a unit called a parsec, which is a unit of length which astronomers use to measure distances to far off objects. But it’s a bit of a funny unit; the definition of it in particular when you first hear it sounds quite strange, and you might wonder why anyone would ever choose to measure things using such a strange unit. So I’m going to talk about why the units is defined the way it is, and what that can tell us about the history of its use.

I think it can often be quite interesting to look at different units that we use for measurement, because they often contain clues about the history of how they were used. One everyday example is the foot – twelve inches; although it’s a lot longer than the average length of a human foot, the name comes from a time when people used to pace out distances using their feet, if they were measuring the size of a house or of a field or whatever. So we have that name that we still use now that’s a reminder of that historical fact. Another example is the use of seconds and minutes: sixty seconds in a minute and sixty minutes in an hour, which is because our system for measuring time comes down to us from the ancient Babylonians, who used to count in a system based on the number sixty, in the same way that our number system is based on ten. So in this case it’s not the name of the unit but it’s the value – the definition of a second and a minute – which tell us something about the ancient history of where this unit came from.

As I said I think the parsec is a unit that contains some interesting history about how it was used – and still is used now – in astronomy. A parsec is a unit of length – like a mile, or a kilometer or a light-year – so you can use it to measure any sort of length you want. But it was first coined in the context of measuring distances of objects from Earth, and specifically using a method called parallax. Now parallax is quite a simple physical effect which I’m sure you’ll be familiar with, and I’ll just go through a little demonstration to tell you what it involves.

Hold your finger up at arm’s length in front of a distant background like a picture on the other side of the room. Close one eye and look at your finger through the other, then switch back and forth between your two eyes, and you should see the finger seem to move relative to the background. Now the reason for that’s quite simple – because your two eyes are in slightly different places, they see the light from the finger at slightly different angles, causing it to line up with different parts of the background.

Now, if you do the same thing again but this time holding your finger just a few inches in front of your face, you’ll see it ‘move’ much further. And that’s because the difference in angle is much greater when your finger’s close to the two eyes.

That effect of the finger moving is called parallax. But what does it have to do with measuring distances? Well, because fingers at different distances will appear to move through different angles, you can imagine using that to work out how far away your finger is if you don’t know already: you could say “oh, it only seems to move a little way compared to the picture so I know my finger must be a long way in front of me.” Now of course the idea of doing that to measure how far off your own finger is is pretty ridiculous, but if you scale everything up it becomes a very useful technique: replace the finger with, say, a planet that you want to measure the distance to – such as Mars – and the painting on the wall with a distant background of stars. If you can measure how much Mars is shifting across that background of stars when you switch points of view, it will tell you how far away it is.

Now the problem here is that closing and opening your two eyes wouldn’t make Mars shift by any noticeable distance at all – to get a good measurement you’d need your eyes to be thousands of miles apart! So of course in real life you’d replace the two ‘eyes’ with two observers in different parts of the world.

And that’s what was done historically – the first accurate measurement of the distance to Mars was performed in 1672 by the astronomer Cassini, who took measurments at a telescope in Paris while his assistant went to French Guyana in South America to act as the second observer. The distance they measured was quite close to the correct one, which is usually a couple of hundred million kilometers, though it varies as they move around their orbits. Now having this measurement performed accurately was a really big step for our astronomical knowledge, because it gave us a measure of the scale of the Solar System. All the relative distances were already known – Mars is about one and half times as far from the Sun as Earth is, Jupiter is five times as far and so on – but a single measurement in actual numbers was needed to fix the scale of everything. Measuring the Earth-Mars distance gave us that.

So, we had an idea about the size of the Solar System and the relative distances of planets. But the next big step up from there is to ask – how far away are other stars? Are they just a bit further than the planets, or are they a million times further away, or what? That’s the problem that people turned to over the next century or so.

Now to answer that question, even two observers on opposite sides of the Earth weren’t good enough – even the nearest stars would shift such a tiny amount that you couldn’t measure it without a modern telescope more accurate than any that were available at the time. But in fact parallax can help us, without having to leave the Earth’s surface. Because even though our planet is only a few thousand miles across, it’s constantly moving through space as it circles the Sun – the position of the Earth now and its position six months from now are almost two hundred million miles apart! That’s plenty of distance to do a parallax measurement on nearby stars.

Now in the 1830s there were three astronomers working on getting a good parallax measurement for stars, using the most powerful telescopes available at the time. They were Bessel, Henderson and von Struve. The Englishman Henderson actually got the first measurement, but German Bessel was able to publish it first and so got the credit. He’d measured the distance to the star 61 Cygni, which he found to be almost a million billion kilometers away. Incidentally, it was Bessel who first coined the term ‘light year’, for the simple reason that talking about ‘millions of billions of kilometers’ is pretty unwieldy! So he proposed ‘light year’ and reported that 61 Cygni was 9.8 light years away. We’ve actually since improved the measurements, and know that it’s just over 11.

So, that’s parallax and how it gives us distance measurements. But what about the parsec? Well that term wasn’t actually used until the beginning of the twentieth century. Although a light year has a simple, obvious definition – the distance light travels in a year – it’s not the most natural unit to use when measuring distances with parallax. What you actually measure is an angle, which you then convert to a distance. So the English astronomer Frank Watson Dyson suggested a system of units related to the angles being measured – you might invent a unit of distance corresponding to a parallax of one degree, so it’s simple to convert your angle measurement to the distance.

In fact a degree is actually a very big angle in this context. Even using the whole Earth’s orbit, the closest stars only shift apparent position by less than a thousandth of a degree! So the unit chosen corresponds instead to one three-thousand-six-hundredth of a degree. Why that fraction? Well, I spoke at the beginning about hours being divided into minutes and seconds. And actually degrees are also divided up in the same way: 360 degrees in a full circle, 60 minutes in a degree and 60 seconds in a minute. And again, that’s because our system of angles comes to us from the Babylonian mathematicians. The units are often called ‘arc-minutes’ and ‘arc-seconds’ to distinguish them from the more familiar time units we’re used to. So one three-thousand-six-hundredth of a degree is one arc second. And the name parsec is short for ‘parallax of one arc second’: it’s the distance at which you’d see a parallax – a shift – of one second of arc. The name itself was suggested by an astronomer named Turner – Dyson himself had actually suggested ‘astron’. I think on the whole I’m glad that ‘parsec’ was the one that stuck.

Now the name parsec caught on quite quickly in the astronomical community because it was so convenient, and to this day it’s the usual unit that’s used: if you read a research paper talking about distances to objects it will probably quote them in parsecs – or kiloparsecs, or megaparsecs. But in popular astronomy writing you always see distances in light years, because the definition is so much simpler and doesn’t involve parallax and angles and so on. But if you do read a distance somewhere given in parsecs, you’ll now know what it means – a parsec is this funny distance at which the Earth’s orbit creates an angle of one three-thousand-six-hundredth of a degree. And its value is a little over three light years, so for example the 11 light years to 61 Cygni is three and a bit parsecs, making it one of the closest stars to the Sun.
Finally, I want to end by mentioning something a bit more recent – because parallax is still the best method for getting distances to stars in our own galaxy.

There’s a spacecraft called Gaia which will be launching in 2012, and will follow the Earth on its orbit around the Sun and take parallax measurements of stars. Not only will it measure them with an amazing accuracy, giving very precise positions for the closer stars, but it will measure the distances to a total of one billion stars, giving us an unprecedented map of our region of the galaxy and stretching all the way to the galactic centre. So in a way this is the next step up, giving us a detailed picture of what’s going on in a whole chunk of the Milky Way, using the same method of parallax that first put a scale on our Solar System.

Ok, I’ll wrap up there. I hope you’ve learned something interesting about parallax and the parsec. Thanks for listening, and if you feel like it come and look over my blog, the Cosmic Web. Bye!

End of podcast:

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