Bode’s Law and the discovery of the asteroids. 

In 1772 a German astronomer, Bode, drew attention to a curious law which seemed to connect 
together the distances of all the known  planets.  It implied that there was one missing
between Mars and Jupiter. When Neptune was discovered in 1781 the new planet’s distance 
agreed quite closely with Bode’s mathematical prediction. So where was the missing planet ?   
Surely it would have been discovered by  1790 !

When astronomers set about predicting the nature of the body they found that it would not 
approach Earth within 160 million miles, would move at a rate of  ¼ degree per day  and  be
much fainter than Mars. This would imply a diameter of about 1000 miles and  a disc of  2” arc
making it about 5th magnitude. Now there are several thousands stars between 5th and 7th magnitude,
and all would have to be watched to notice any movement. The work was undertaken by 24 astronomers 
organized by De Zach, and observations began towards the end of the 18th Century.   

Each member was given a particular portion of the Zodiac to examine but no results would be
expected for some time, because it would be necessary for each observer to become familiar 
with the star patterns.Now at the same time, Piazzi was engaged in observing the heavens with 
the view of forming a great catalogue of stars, and while comparing a previous observer’s 
catalogue of Taurus with his own, he noticed a star previously marked which was no longer visible.   
Surprised by the omission he carefully looked in the area to find it, but without success.   
On the fourth night of his search Jan 1, 1801,  he saw a small star which he did not remember 
having noticed before.  Then on the next night to his surprise the star seemed to have moved a 
little, so he carefully plotted its position.  On Jan 3rd it had moved  nearly 6’ arc from its 
previous location.  He continued to watch the star and on the 10th its movement  became stationary, 
and then it moved in the opposite direction in longitude.

On Feb 11th, Illness prevented further observations so Piazzi reported his findings to the 
Italian astronomer Oriani and the German astronomer Bode, but by the time the letters reached 
their destination the Sun interfered with continued observation.  Gauss used the observations to 
calculate an orbit and predict its position, but when the search resumed the star could not be found.   
Harding, De Zach and Olbers were more persistent in their search and on the last night of the year
1801 De Zach detected a moving star of about the seventh magnitude.  The new planet was named Ceres by 
its discoverer Piazzi.  It was found to move in an orbit whose mean distance was 2.77 while Bode’s law 
predicted 2.80

Thus the first minor planet was discovered. Olbers discovered Pallas three months later, 
and Harding discovered Juno on Sep 2nd 1804. After observing for eight years, Olbers discovered 
Vesta in Virgo at magnitude 6. This final fragment convinced Olbers that the four bodies where 
once part of a larger planet.    


Ref : Astronomy by Edmond Neison FRAS  (printed 1886)