Until recently, astronomers did not even know what they were.

And the astronomers knew I'd been away from the field for two years.

Astronomers have known about this movement for a very long time.

Astronomers didn't know a lot in general about the universe so it was easy to make theories.

Astronomers know that the Sun is a rather "run of the mill" star.

Without this information, astronomers would not know where to look for the object at future dates.

Astronomers do not know what the great attractor is made of.

Astronomers thus knew where to look for any object in the region that could be the source of the blast.

Our astronomers don't know anything about the surface of Perelandra at all.

How do astronomers know for sure that stars are really as far away as they state?