NEPTUNE, W in astronomy, is the outermost of the known planets, its mean distance from the Sun being 2,793,500,00o miles; and the time occupied in completing a revolution is 164.8 years.
It is noteworthy that "Bode's law" (see PLANET) which expresses with considerable accuracy the relative distances of the other planets from the Sun breaks down badly at Neptune, giving the value 38.8 units, whereas the actual distance is but 30.1 units. The light of Neptune being only equal to that of a star of the eighth magnitude, the planet is, of course, invisible without optical aid, but a telescope of moderate aperture will show that its image, unlike that of a star, possesses a sensible disc. This is of a green ish hue and about 2.5" in diameter, which, at the planet's distance, corresponds to some 33,00o miles. It has not been possible, how ever, to detect any features on its surface from which its rotation period could be derived, but from a comparison of its brightness with that of neighbouring stars Maxwell Hall in 1883, and again in 1915, found temporary periodic fluctuations suggesting rotation is about 7h 50t or a little more. Moore and Menzel have found by spectroscopic methods a period of h, and, since it has been shown by Jackson and others that a period of 7h5om is inconsistent with the shift in the mode of the satellite, it seems probable that the photometric value is half the true period. The spectroscopic results also indicate that the rotation of the planet is direct or in a direction contrary to that of the orbital motion of the satellite.
As regards its physical condition it is known that Neptune is similar to the other members of the outer group of planets in possessing a low density. Its mass, which is derived from the motion of its satellite, is about 16.9, and its density 0.24, the corresponding values for the earth being taken as unity.
As is the case with the other major planets Neptune is en veloped in a dense atmosphere, and its spectrum shows a number of strong absorption bands, the origin of which is not yet known. These bands show a well marked progression in intensity from Jupiter outwards through Saturn and Uranus to Neptune (see Plate, Planets).
The detection of Neptune through its action upon Uranus before its existence had been made known by observation is a striking example of the precision reached by the theory of the celestial motions. So many agencies were concerned in the final discovery
that the whole forms one of the most interesting chapters in the history of astronomy. The planet Uranus, before its actual dis covery by Sir William Herschel in 1781, had been observed as a fixed star on at least 17 other occasions, beginning with Flam steed in 169o. In 182o Alexis Bouvard of Paris constructed tables of the motion of Jupiter, Saturn and Uranus, based upon a dis cussion of observations up to that year. Using the mutual per turbations of these planets as developed by Laplace in the me canique Celeste, he was enabled satisfactorily to represent the observed positions of Jupiter and Saturn ; but the case was entirely different with Uranus. It was found impossible to represent all the observations within admissible limits of error, the outstanding differences between theory and observation exceeding I'. In these circumstances one of two courses had to be adopted, either to obtain the best general representation of all the observations, which would result in the tables being certainly erroneous, or to reject the older observations which might be affected with errors, and base the tables only on those made since the discovery by Herschel. A few years of observation showed that Uranus was deviating from the new tables to an extent greater than could be attributed to legitimate errors of theory of observation, and the question of the cause thus became of growing interest. Among the investigators of the question was F. W. Bessel, who tried to reconcile the difficulty by an increase of the mass of Saturn, but found that he could do so only by assigning a mass not otherwise admissible. Although the idea that the deviations were probably due to the action of an ultra-Uranian planet was entertained by Bouvard, Bessel and doubtless• others, it would seem that the first clear statement of a conviction that such was the case, and that it was advisable to reach some conclusion as to the position of the disturbing body, was expressed by the Rev. T. J. Hussey, an English amateur astronomer. In a letter to Sir George B. Airy in 1834 he inquired Airy's views of the subject, and offered to search for the planet with his own equatorial if the required estimate of its position could be supplied. Airy expressed himself as not fully satisfied that the deviation might not arise from errors in the perturbations. He therefore was not certain of any ex traneous action; but even if there was, he doubted the possibility of determining the place of a planet which might produce it. In 1837 Bouvard, in conjunction with his nephew Eugene, was again working on the problem ; but they appear not to have gone farther than to collect observations and to compare the results with bouvard's tables.