| In the early days, asteroids were very hard to find.
The 'asteroid hunter' had to first make a map or chart of a region of the
night sky. Then, after some time had passed, he would make another chart
of the same area. If any of the objects had moved they were asteroids.
When, in 1891, celestial photography became advanced enough, it allowed astronomers to speed up the search for asteroids. They would set up a  telescope
with a camera attached and point it at a piece of night sky. The exposure
of the photographic plate could last several hours and during that time
the telescope had to be kept pointing at exactly the same spot, keeping
a guiding star between the cross-hairs of the guiding telescope.
When the plate was developed, asteroids could clearly be seen because, unlike the stars which stayed in the one spot, the asteroids would move across the night sky and would make a streak on the photograph. |
