II. Account of Observations at the Cambridge Observatory for detecting the Planet exterior to Uranus

Abstract

bridge, placed in my hands a paper containing the results of computations he had made to account for irregularities in the motion of Uranus, on the hypothesis of disturbances caused by a more distant planet. These results embraced the mass, mean distance, mean longitude at a given epoch, longitude of perihelion, and excentricity, of the orbit of the disturbing body, with its probable geocentric longitude at the end of September." The time was unfavourable for looking after the planet, on account of its angular distance from opposition; but the main reason which deterred the Professor from the search was, that it was " so novel a thing to undertake observations in reliance upon merely theoretical deductions, and that while much labour was certain, success appeared very doubtful."'rThe publication of M. Le Verrier's first memoir, and the close agreement of his deductions with those of Mr. Adams, together with the recommendation of the Astronomer Royal, induced Professor Challis to undertake the search of the predicted planet.1The observations were commenced July 29, 1846, three weeks before the expected Opposition of the planet.In suggesting a plan for finding the planet, Mr. Airy advised the close scrutiny of a zodiacal zone, having its centre in the ecliptic at 325°of longitude, (the point marked out by Mr.Adams and M. Le Verrier) extending each way 150 of longitude and 5°of latitude. A power not lower than 120 was advised, with which a zone of 15' would be well:seen ; also that the transits should be taken at one wire; and the north polar distances estimated in parts of intervals of 3', marked by horizontal wires. It was calculated that 80 sweeps, each averaging one hour, repeated three times, would occupy about 300 hours of observing weather. In this mode of observing, the telescope is supposed to be " fixed " during the whole, or at least during the consecutive portions of each sweep.* This paper was presented on the evening of Nov. 13, 1846. 'T M. Le Verrier published his first determination of the probable longitude of the exterior planet in June 1846, yet it does not appear that any systematic attempt was made elsewhere to detect it by observation for nearly four months, nor until he had published his second determination. I Mr. Airy's suggestion was made in a note dated July 9, 1846.

The three-body problem

“…The deviation reveals the existence of additional bodies, which in principle is a similar method that was used in the discovery of Neptune through variations in the orbit of Uranus [Adams, 1846, Airy, 1846, Challis, 1846, Galle, 1846. Using the times of transit and the planetary period of a transiting planet, a fitting is made to a linear function to predict the subsequent eclipses (t n = t o + nP ) of that planet.…”

The three-body problem

“…The method of TTVs relies on the observable deviation from Keplerian dynamics. The deviation reveals the existence of additional bodies, which in principle is a similar method that was used in the discovery of Neptune through variations in the orbit of Uranus [Adams, 1846, Airy, 1846, Challis, 1846, Galle, 1846. Using the times of transit and the planetary period of a transiting planet, a fitting is made to a linear function to predict the subsequent eclipses (t n = t o + nP ) of that planet.…”