James Challis - The Search For The Eighth Planet

The Search For The Eighth Planet

In 1846, Airy finally persuaded a reluctant Challis to join in the search for an eighth planet in the solar system. Adams had predicted the location of such a planet as early as 1844, based on irregularities in the orbit of Uranus. Adams failed to promote his prediction successfully and there was little enthusiasm for a systematic search of the heavens until Airy's intervention. Challis finally began his, somewhat reluctant, search in July 1846, unaware that Frenchman Urbain Le Verrier had independently made an identical prediction. German astronomer Johann Gottfried Galle, assisted by Heinrich Louis d'Arrest, finally confirmed Le Verrier's prediction on 23 September. The planet was named "Neptune". It soon became apparent from Challis's notebooks that he had observed Neptune twice, a month earlier, failing to make the identification through lack of diligence.

Challis was full of remorse but blamed his neglect on the pressing business of catching up on the backlog of astronomical observations from the observatory. As he reflected in a letter to Airy of 12 October 1846:

I have been greatly mortified to find that my observations would have shewn me the planet in the early part of August if I had only discussed them. ... I delayed doing this ... chiefly because I was making a grand effort to reduce the vast numbers of comet observations which I have accumulated and this occupied the whole of my time.