MERCURY, PLANET, in astronomy, the smallest major planet and the nearest to the Sun; its symbol is . Its proximity to the Sun makes the telescopic study of its physical constitution extremely difficult. The result is that less is known on this subject than in the case of any other planet. Even the time of rotation on its axis is uncertain. J. H. SchrSter inferred a period of rotation of 24 h. 5 m. 30 s., which was in seeming agreement with the observations of K. L. Harding. This period was generally accepted, though Herschel had been unable to see any changes indicating rotation. In 1882 G. Schiaparelli began a careful study of the face of the planet with a refractor of 8 in. aperture, subsequently replaced by one of 18 in. His unexpected conclusion was that the rotation of Mercury resembles that of the Moon, in having its period equal to that of its orbital revolution. As the Moon always presents the same face to the earth, so Mercury must, in this case, always present very nearly the same face to the Sun. Schiaparelli also announced that the axis of rotation of the planet is nearly perpendicular to the plane of its orbit. The rotation being uniform, while the orbital motion, owing to the great eccentricity of the orbit, is affected by a very large inequality, it would follow that there is a libration in longitude of nearly 24 on each side of the mean position. Percival Lowell in 1897 took up the question anew by combining a long series of measured diameters of the planet with drawings of its apparent surface. The seeming constancy of the surface appearance was considered to confirm the view of Schiaparelli as to the slow rotation of the planet. But there is wide room for doubt on the question.
The period of orbital revolution of Mercury is nearly 88 days, or somewhat less than three months. Consequently, the period of synodic revolution is less than four months, during which the entire round of phases is completed. When near greatest elongation Mercury shines as a star of the first magnitude, or brighter; but in the latitudes of central and northern Europe it is so near the horizon soon after sunset as to be generally obscured by vapours or cloudy.
The eccentricity of the orbit, o- 20, is far greater than that of any major planet, and nearly the average of that of the minor planets. Consequently, its distance and its greatest elongation from the Sun vary widely with its position in its orbit at the time.
The mass of Mercury can be determined only from its action upon Venus; this is so small that the result is doubtful. Leverrier adopted in his tables i: 3,000,000 as the ratio of the mass of Mercury to that of the Sun. S. Newcomb, from the action upon Venus, reduced this to one-half its amount, or i : 6,000,000.
G. W. Hill, basing his conclusions on the probable density of the planet, estimated the mass to be less than i : 10,000,000. The adoption of a mass even as large as that of Newcomb implies a greater density than that of the earth, but it is not possible to estimate the probability that such is the case.
The most interesting phenomenon connected with Mercury is that of its occasional transit over the disk of the Sun at inferior conjunction. These occur only when the planet is near one of its nodes at the time. The earth, in its orbital revolution, passes through the line of the nodes of Mercury about the 8th of May and the loth of November of each year. It is only near one of these times that a transit can occur. The periodic times of Mercury and the earth are such that the transits are generally repeated in a cycle of 46 years, during which 8 transits occur in May and 6 in November. The following table shows the Greenwich mean time of the middle of all the transits from 1677, the date of the first one accurately observed, until the end of the present century.
A perplexing problem is offered by the secular motion of the perihelion of Mercury. In 1845 Leverrier found that this motion, as derived from observation of the transits, was greater by 35" per century than it should be from the gravitation of all the other planets. This conclusion has been fully confirmed by subsequent investigations, a recent discussion showing the excess of motion to be 43* per century. It follows from this either that Mercury is acted upon by. some unknown masses of matter, or that the intensity of gravitation does not precisely follow Newton's law. The most natural explanation was proposed by Leverrier, -who attributed the excess of motion to the action of a group of intraMercurial planets. At first this conclusion seemed to be confirmed by the fact that occasional observations of the transit of a dark object over the Sun had been observed. But no such observation was ever made by an experienced astronomer, and the frequent photographs of the Sun, which have been taken at the Greenwich observatory and elsewhere since 1870, have^never shown the existence of any such body. We may therefore regard it as certain that, if a group of intra-Mercurial planets exists, its members are too small to be seen when projected on the sun's disk. During the eclipses of 1900 and 1905 the astronomers of the Harvard and Lick Observatories photographed the sky in the neighbourhood of the Sun so fully that the stars down to the 7th or 8th magnitude were imprinted on the plates. Careful examination failed to show the existence of any unknown body. It follows that if the group exists the members must be so small as to be entirely invisible. But in this case they must be so numerous that they should be visible as a diffused illumination on the sky after sunset. Such an illumination is shown by the zodiacal light. But such a group of bodies, if situated in the plane of the ecliptic, would produce a motion of the node of Mercury equal to that of its perihelion, while the observed motion 1 Mercury grazes sun's limb.
of the node of Mercury is somewhat less than that computed from the gravitation of the known planets. The same is true of the node of Venus, which might also be affected by the same attraction. To produce the observed result, the inclination of the ring would have to be greater than that of the orbit of either Mercury or Venus. In 1895 Newcomb showed that the observed motions, both of the perihelion of Mercury and of the nodes of Mercury and Venus, could be approximately represented by the attraction of a ring of inter-mercurial bodies having a mean inclination of 9 and the mean node in 48 longitude. He also showed that if the ring was placed between the orbits of Mercury and Venus, the inclination would be 7-5 and the longitude of the node 35. The fact that the zodiacal light appears to be near the ecliptic, and the belief that, if it were composed of a lens of discrete particles, their nodes would tend to scatter themselves equally around the invariable plane of the solar system, led him to drop these explanations as unsatisfactory, and to prefer provisionally the hypothesis that the sun's gravitation is not exactly as the inverse square. (See GRAVITATION.)
In 1896 H. H. Seeliger made a more thorough investigation than his predecessor had done of the attraction of the matter producing the zodiacal light, assuming it to be formed of a series of ellipsoids. He showed that the motions of the nodes and perihelion could be satisfactorily represented in this way. The following are the three principal elements of the hypothetical orbits as found by the two investigators:
Ring between Mercury and Venus.
Zodiacal Light Matter.
Inclination . Node . . Mass , 35 1/37,000,000 6-95 40-o s 1/2,860,000 The demonstration by E. W. Brown that the motion of the moon's perigee is exactly accordant with the Newtonian law of gravitation, seems to preclude the possibility of any deviation from that law, and renders the hypothesis of Seeliger the most probable one in the present state of knowledge. But the question is still an open one whether the zodiacal light has an inclination of the ecliptic as great as that computed by Seeliger. This is a difficult one because the action on Mercury is produced by the inner portions of the matter producing the zodiacal light. These are so near the Sun that they cannot be observed, unless possibly during a total eclipse. (S. N.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)