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Bitesize Astronomy

Week of February 22, 1999

On occasion, I will take a few weeks to explore a theme in astronomy on these pages. In 1997 that theme was the solar system. This time, for the next few weeks, I invite you to take a look at the Universe In Motion.

We think of the skies as static, unchanging. The only motions we see easily are the rising and setting of the Sun, Moon and stars, but that is a reflection of our own Earth's rotation. The stars themselves don't seem to move at all among themselves, and it takes a keen eye to discern the motion of the planets, which takes days or weeks to become obvious.

Yet objects in the sky are in constant motion, and some move with an incredible intrinsic velocity. Usually, these objects are so far away that the distance itself shrinks the apparent motion, the way distant mountains hardly seem to move at all even though you may be driving past them at 100 kilometers an hour. Sometimes it does take many years to perceive the movement of heavenly bodies, and sometimes it happens in the blink of an eye. Every Monday, we'll take a look at some of these celestial travelers.


Historically, and even in modern times when one wants to be poetic, people have called the sky the ``firmament''. That word has connotations of solidity, of permanence. It implies that the sky (or should I say the ``heavens'') is unchanging, and most of all that the stars are fixed to it, like gems on black velvet. The idea of stars being unchanging and unmoving is so ingrained in us that we give a different name to the stars that move: we call them ``planets'', after the Greek word for ``wanderer''.

But is it really true? Are the stars really fixed? Those of you that know me already know the answer to that. It's no. But regular readers also probably already know that no answer is that simple in astronomy, certainly when it's the topic of this week's Snack, but especially when we're still talking about the Universe in Motion!

The stars do move. Slowly, to be sure, but we have plenty of time. It takes a lot of time to notice that movement, and even then the unaided eye would not see it. We need instruments, and good ones, to see the motion. It's not surprising that the ancient thinkers believed the Earth to be the one that was fixed in the heavens; Aristotle even ``proved'' it by showing that if the Earth moved, we would see that motion reflected in the stars (just like you can imagine that a tree looks like it is passing you when you walk past it). Since the stars didn't move, the Earth mustn't either. The great astronomer Tycho Brahe also looked for such movement but didn't see it. Small wonder; even for such a keen eyed observer as Tycho, without a telescope looking for stellar motion is hopeless.

But why should we expect to see this motion? The answer is right at the tips of your fingers. Literally. Try this: hold your thumb up about 20 or 30 centimeters from your face. Make sure that beyond your thumb is some distant object, like a tree seen through a window. Close your left eye and note where your thumb is with respect to that tree. Now open your left eye and close your right. Did you see your thumb apparently shift position? That's because your right and left eyes have a small separation, and view the thumb from slightly different angles. The tree, being much farther away, suffers this perspective effect much less. Your thumb seems to jump back and forth as you alternately open and close your eyes. Even better, if you know the distance between your eyes, you can measure how much your thumb appears to move and, by applying a little bit of trigonometry, calculate its distance. This effect is called ``parallax'', and 150 years ago it changed the face of astronomy.

The Earth orbits the Sun once a year. In half that time it moves halfway around the Sun (of course!). The Earth's orbit is roughly 150 million kilometers in radius, so in six months it is twice that distance or roughly 300 million kilometers from where it started (across the diameter of the Earth's nearly circular orbit). That's a pretty big distance, a lot more than the distance between your eyes! You'd think any nearby stars would jump back and forth over a six month stretch just like your thumb did!

Alas, stars are a bit farther away than your thumb, even when compared to the size of Earth's orbit. It wasn't until 1838 that Frank Bessel (who later went on to invent Bessel functions, which in turn went on to torture me in calculus class) measured the first parallactic shift for a star. It was the unassumingly dim star 61 Cygni, and he found the distance to be three parsecs (about 10 light years).

As we saw with the tree behind your thumb, the farther something is away from you, the smaller it seems to shift. This puts an upper limit to the distance we can apply this method due to inaccuracies inherent in our telescopes. Until recently, this limit was about 100 parsecs (about 320 light years). However, the Hipparcos satellite recently pushed this limit much farther out due to its amazingly accurate measurements, and we now have good distance for stars out to many hundreds of parsecs. This is extremely important because it's really the best and only direct way to measure the distances to stars, and we use those distances to find the distances to even farther objects! Your measurements are only as good as your ruler, and luckily our modern ruler is very accurate.

If you want to see diagrams of all this and have more info about it, I have another page that discusses parallax and has links to other pages at the bottom.

Incidentally, the first part of this Snack mentioned that stellar motion is not as simple as any one source of movement. There is another source, and that will be the topic of next week's Snack! I promise, it will be peculiar.



©2008 Phil Plait. All Rights Reserved.

This page last modified Saturday, 05-Mar-2011 18:03:22 UTC
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