1801-2001: the bicentenary of Ceres' discovery

Ceres was the first asteroid to be discovered in 1801, exactly 200 years ago. Before that moment the existence of asteroids had not been foreseen...


The astronomer G. Piazzi
The night Piazzi discovered Ceres by Nanni Riccobono

On January 1, 1801 Father Giuseppe Piazzi was observing for a new star catalogue a collection of the precise location of all stars visible in the sky. While observing stars in the constellation Taurus, he saw a small, starlike object that was not listed on any of his star maps. He carefully recorded its location, but on the next night, he noticed that the object had moved slightly to the east. Stars don't do such a thing. Over the next six weeks he recorded its motion, and discovered that it was moving relative to the background stars. Its rapid motion indicated that the object was not a star, but rather an object in the solar system.
The first asteroid was discovered. Piazzi had discovered Ceres, the largest known asteroid (roughly one-third the size of the moon). Three more asteroids were discovered just years after--Pallas in 1802, Juno in 1804, and Vesta in 1807. Asteroid discoveries were rare from then until 1891, when photographic search methods were first introduced.
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Ceres: the missing planet? by Livia Giacomini
In the eighteen century, scientists tried to find a mathematical formula to describe the solar system and in particular, the distances from the planets to the Sun. Johannes Titus and Johann Elert Bode came up with a law (today called Titus Bode's law) that at, that time, seemed to fit the reality of the solar system.
This empirical law predicted pretty well the distances between the Sun and all the planets known at that epoch (Neptune and Pluto, the farthest ones, which don't respect it at all, have been discovered only afterwards).


where :

is the distance (in A.U.)
of each planet from the Sun
N=0,1,2,4 doubles for every
successive planet of
the solar system
planet N predicted distance real distance
Mercury 0 0.4 0.39
Venus 1 0.7 0.72
Earth 2 1.0 1.00
Mars 4 1.6 1.52
Gap 8 2.8 2.77
Jupiter 16 5.2 5.20
Saturn 32 10.0 9.54
Uranus 64 19.6 19.19
Neptune 128 38.8 30.07
Pluto 256 77.2 39.53

From this law, however, there seemed to be a gap at 2.8 A.U., where a missing planet should have existed. At the end of the year 1800, a search for this missing planet was to be organized, when the first asteroid was found by Giuseppe Piazzi and named after the patron goddess of Siciliy, Ceres. Ceres was first believed to be the missing planet that could confirm Titus Bode's law. But very soon other bodies were discovered in the same region (Pallas in 1802, Juno in 1804 and Vesta in 1807). It soon became clear that Titus-Bode's law had to be refused, and between Mars and Jupiter, not just one, but many minor planets had to exist (in what is today called the asteroid's main belt).
As soon as it was discovered, it became necessary to compute the orbit of this new object, so that it could be possible to observe it again in the future. Here, again, Ceres was an innovation in the asteroid's science: Piazzi wasn't able to observe it long enough to compute it's orbit with the methods available. It was only thanks to a great mathematician, Carl Friedrich Gauss, who invented in that occasion a new method for the calculation of orbits, that Ceres didn't become lost. Using this new method, Wilhelm Olbers predicted its position, and Ceres, which in the meanwhile had become unobservable, was found again on december 31, 1801. The new method of calculus soon became a new milestone of mathematical astronomy, being still used today with some simple modifications.

 


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