Asteroids

Asteroids are more properly called Minor Planets, for they orbit the Sun just as the Earth, Jupiter or the other planets do. They are, however, much smaller with the largest being only about 834 km in diameter. They are too tiny to resolve surface details with Earth-based telescopes and thus they appear as points, just like the stars. This does not, however, make them uninteresting, for they move amongst the stars with complex motions just as the major planets do. Occasionally they will even pass in front of a more-distant star.

The first asteroid was discovered by G. Piazzi from the island of Sicily on the first evening of the nineteenth century. He noticed it because over a period of several minutes it moved with respect to the stars. The object was named Ceres and found to be in an orbit about 2.8 times as large as the Earth's orbit. This placed it roughly between the orbits of Mars and Jupiter. The following year a second asteroid, Pallas, was discovered and by 1891 some 322 asteroids had been catalogued. Most orbited between Mars and Jupiter. Up to this point all discoveries had been via visual methods but then Max Wolf at Heidelberg applied the new technology of photography to the hunt and since then hundreds have been found each year. Astronomers keep track of the brightest 5000 or so objects regularly.