Bad Astronomy | 22 June 2010
Shakespeare was a decent writer, but an astronomer he wasn’t. The North Star isn’t fix’d, because the Earth’s axis wobbles slowly like a top. You wouldn’t see this by eye, since the circuit takes 26,000 years to complete, but astronomers deal with it all the time.
- "I am constant as the northern star,
Of whose true-fix’d and resting quality
There is no fellow in the firmament."
— Julius Caesar (III, i, 60 – 62)
But Shakespeare did get something right in that passage: the stars themselves do move. It’s slow, but it’s there. It’s caused by their orbital motion as they circle the center of the Milky Way. Their velocity can be hundreds of kilometers per second, but that apparent motion is dwarfed to a near standstill by their forbidding distance. Of course, that means that closer stars will appear to move faster than ones farther away, just like trees by the side of the road whiz by as you drive, but distant mountains slide along in a much more stately manner.
It takes decades, sometimes, to see that stellar movement at all — astronomers call it proper motion — but it’s not impossible. Greek amateur astronomer Anthony Ayiomamitis knew that very well, and he was able to prove it. Behold, the unfix’d heavens!
... The marked star is Barnard’s Star, a dinky, dim red bulb a mere 6 light years away — which makes it one of the closest of all the stars in the galaxy.
Barnard was a phenomenal astronomer, and inferred that since it was a red dwarf, for it to be seen at all means it must be close. He kept his eye on it over the years, and was able to measure its apparent speed across the sky. It moves a phenomenal 10 arcseconds per year, which is tiny in normal life, but pretty frakkin’ fast for a star. In 60 years since the Palomar observations, Ayiomamitis was able to capture it in the lower half of that image, where again its position is marked. Note how far it’s moved! In the intervening decades it’s traveled about 10 arcminutes, or about 1/3 the size of the Moon on the sky!
That’s fast. If every star moved that quickly, the constellations would last only a few centuries before being distorted beyond recognition. As it is, we see pretty much the same constellations ancient Sumerians did.
... back to Shakespeare: even ignoring the Earth’s wobble, he still blew it in that passage from Julius Caesar. The North Star moves too. Of course, its proper motion is pretty small because it’s a long way off, over 400 light years away. Compared to Barnard’s Star, it’s hardly moving. Given that then, I suppose, I can give Shakespeare some credit.
Perhaps the fault lies in ourselves, and not the stars.
