Astronomy Jargon 101: Parallax – Universe At present
In this series, we explore the weird and wonderful world of astronomy jargon! You probably don’t know how close you are to today’s topic: parallax!
How do you measure the distance to a star? The question has frustrated astronomers for centuries. The stars are obviously far away, but beyond that it’s hard.
Fortunately, there is a trick. And you can do it at home.
Put your finger to your nose. Close one eye. Notice the position of your finger relative to something far in the background. Now switch your eyes. If you got it right, your finger should appear to be wiggling relative to this background.
Now keep your finger at arm’s length. Repeat the exercise. Your finger was probably still wiggling, but hopefully just a little.
Now look at something far away. Do the same alternation of eyes. You probably won’t see any wobble at all.
This is parallax: the apparent change in position of an object from different perspectives. If you know the distance between the two viewpoints (such as the distance between your eyes), you can measure the angle of wobble and use high school trigonometry to calculate the distance to the object.
Tycho Brahe is known to use parallax to argue against the heliocentric model of the universe at the end of the 16th century. If the sun were at the center of the solar system and the earth orbited it, then the stars should appear to move back and forth in our orbit over the course of the year from our different angles. He didn’t see any squats, so the earth was in focus (and therefore no parallax).
The solar system (not to scale) shows how the apparent position of a star in the sky can shift over the course of a year.
It’s a shame Brahe didn’t have a telescope. The stars are so far away that you need powerful optics to see the parallax. It was not until 1838 that the German astronomer Friedrich Bessel successfully measured the parallax to a star: 61 Cygni, about 10 light years away (he also coined the term light year, but that is a different technical term).
Parallax is still the basis of distance measures. Recently, the Gaia satellite has published parallax distances to over a billion stars.
That’s a lot of wobbling.