Before we figured out that the universe was a very large open space with lots of stuff scattered about, the ancients had some interesting ideas on how the sky worked. Roughly speaking, celestial phenomena came in three kinds. There are the stars, which remain a constant, fixed background. There are the planets, which move about the sky in complicated but predictable patterns. And finally there were other transient phenomena – nova, supernova, comets, meteors, and the rest. These last were often seen as particularly significant by virtue of their rarity.
If you go outside tonight and look toward the western sky, you’ll see tonight’s Earth and Sky’s “Sky Tonight” object of interest: the Pleiades. It’s an open cluster of stars that many people initially mistake for the Little Dipper. It’s little, and it’s in about the same shape as the dippers. There’s six prominent stars visible, and many more dimmer stars visible in binoculars or a telescope. They are not a coincidental alignment of unrelated stars, but are in fact a gravitationally bound cluster with associated nebulosity that can be photographed with a good telescope. Though they won’t look so dramatic to the naked eye, photographed with a long exposure they look like this:
It’s more than a little odd that there’s only six bright stars. The Pleiades are the seven sisters in Greek mythology. Where’s the other one?
Nobody knows. Maybe one of the dimmer stars used to be brighter, or maybe a brighter star was obscured by some other astronomical phenomenon in the cluster. Or maybe the Greeks were just using creative license and figured that six bright stars plus some dim ones were close enough to seven to work. But if it really did change at some point, it may well be the earliest recorded instance of one of the “permanent” features of the starry sky changing. It would have been a watershed moment in the history of physics, astronomy, and cosmology. They just wouldn’t have recognized it at the time.