A little ramble through 19th century astronomy - Uranus and Neptune in 1875

 Academy, Saturday 16^th October 1875

The Satellites of Uranus and Neptune.— Since the completion of the great Washington refractor of twenty-six inches’ aperture, it has been devoted to the observation of these objects, which are too faint for any but the very largest telescopes. Professor Newcomb, who made the micrometer measures, has discussed them in an appendix to the Washington Observations, arriving at the conclusion that the orbits of these satellites are all sensibly circular (being certainly less eccentric than those of the planets of our system), and that those of the Uranian system lie in one plane.

Only one satellite to Neptune has been detected, though a second was repeatedly looked for, and no trace of any satellites exterior to the four known members of the Uranian system has been seen, though Sir W. Herschel concluded there were six. From the excessive faintness of the four satellites of Uranus, Professor Newcomb considers that their masses can hardly exceed 1/15,000  of that of the planet, so that their mutual perturbations would be insensible, while the sun’s disturbing  effect is exceedingly small, and thus the problem of determining their motions becomes comparatively simple.

The chief importance of this problem is that it gives the masses of Uranus and Neptune more accurately than any other method, all that is required for this purpose being to know the greatest apparent distance of one of the satellites from its primary and the period of revolution. In this way Professor Newcomb finds the mass of Uranus to be 1/ 22,600  of that of the sun, a result which is probably true within 1/200 part , while the mass of Neptune is 1/19,400 .One element of uncertainty in these results is the chromatic aberration of the eye-piece used n 1873, the webs being illuminated by red light, while that of the satellites is greenish yellow, so that the web is not really on the satellite when it appears to be. The effect of this is that there is a difficulty in converting the measured distance into seconds of arc. Professor Newcomb has not been able to detect any markings on Uranus and can therefore form no conclusion as to the period of rotation, which is a matter of some interest.

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