ABERRATION OF THE PLANETS

ABERRATION OF THE PLANETS. This is quite of the same nature with that of the stars, only that its amount and direction are greatly affected by the motion of the planet itself combining itself with that of the earth, and producing on the whole a more complex result. When the planet is stationary, the aberration disappears altogether, because the light itself, participating of the motion of the planet, strikes the earth not only with its usual direct motion, but also with a lateral motion exactly the same as that of the earth itself. The eye of the spectator, therefore, and the light have the same motion laterally; and thus the effect is quite the same as if they had relatively no lateral motion at all. It is the same as if both the earth and the planet were at rest, and therefore there cannot be any aberration. In every other case, the aberration is determined by combining the motion of light not only with the earth's, but with the planet's motion also; and doing this it is found, that in every case the aberration is equal to the motion of the planet about the earth or its geocentric motion, during the interval that

Aberration. light employs in passing from the planet to the earth. Thus, in the sun, the aberration in longitude is constantly 20", that being the space moved by the sun, or, what is the same thing, by the earth, in 8 7", the time in which light passes from the sun to the earth. In like manner, knowing the distance of any planet from the earth, by proportion it will be, as the distance of the sun is to the distance of the planet, so is 8 7" to the time of light passing from the planet to the earth: then computing the planet's geocentric motion in this time, that will be the aberration of the planet, whether it be in longitude, latitude, right ascension, or declination.

Since the motion of the planets affects so much the aberration, ought not the motion of the fixed stars relatively to our system, if they have any, as some have suspected, be rendered sensible in this manner? Their prodigious distance has hitherto rendered these motions, if they do exist, almost insensible. But this would not affect the motion of light. This element flies through the remotest parts of the system: and if it be really material, the motion with which it is propelled from one point must continue for ever afterwards to affect it, unless opposed or modified by extraneous influence. If the stars, therefore, have any motion laterally in respect of the earth, so will the light which issues from them, and which, preserving undiminished its original impulse, must strike the eye of a spectator on the earth not only with a direct motion, but also with one to the right or left similar to that of the star; and this ought to affect the aberration just in the same manner as if the star were no farther off than any of the planets. The same thing would be observed if the earth, along with the whole solar system, as the late Dr Herschel and other astronomers have attempted to prove, be advancing forward among the stars. Since, however, no such effect has ever been noticed, it would seem to follow, that the stars, as well as the sun, are really at rest; or if they have any motion, it is but a slow one, even compared with that of the earth or the planets round the sun. This is certainly a curious speculation, which we have never seen discussed by astronomers. See ASTRONOMY. (G.B.)