On Dec. 15, 1612, the Andromeda galaxy was seen through a telescope for the first time by a German astronomer named Simon Marius. [‘On This Day in Space’ Video Series on Space.com]

The Andromeda galaxy is the closest major galaxy to the Milky Way, and it can be seen with the naked eye. Early astronomers thought it was a nebula, or a glowing cloud of space dust. Even after Simon Marius saw it through a telescope for the first time, he still couldn't tell that it was actually a galaxy filled with about a trillion stars. It took another three hundred years before Edwin Hubble came along and figured out that it was a galaxy.