Supernova 1885 
A bright Moon is pretty to look at, but it fills the sky with light, overpowering the view of faint stars and galaxies.
Most of the time, that's just a nuisance. But 125 years ago tonight, it cost astronomers a chance to learn more about an important discovery.
Ernst Hartwig was looking at M31, the Andromeda galaxy, at an observatory in Estonia when he saw something amazing: a bright new star near the galaxy's core. He wanted to tell other astronomers about his finding, but the observatory's director told him not to. There was a bright Moon in the sky -- quite near where the Moon appears tonight, in fact -- and he thought the moonlight was overpowering much of M31's light, making its core appear brighter than normal. So other astronomers didn't start looking at the new star for several days.
It was a case of missed opportunity. The new star was a supernova -- a titanic stellar explosion. It was the first supernova ever seen outside our own galaxy. More observations in the days soon after the explosion would have helped astronomers learn about these stars -- and about the scale of the universe.
That's because at the time, most astronomers thought that M31 was a swirling mass of gas and dust inside the Milky Way. But the discovery of such a bright star in M31 suggested that it must be much farther than anyone had thought. So the supernova of 1885, plus others like it, helped show that the universe extends far beyond the Milky Way.
Script by Damond Benningfield, Copyright 2010