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MERCURY

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Originally appearing in Volume V18, Page 155 of the 1911 Encyclopedia Britannica.
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MERCURY , in See also:

astronomy, the smallest See also:major See also:planet and the nearest to the See also:sun; its See also:symbol is . Its proximity to the sun makes the telescopic study of its See also:physical constitution extremely difficult. The result is that less is known on this subject than in the See also:case of any other planet. Even the See also:time of rotation on its See also:axis is uncertain. J. H. See also:Schroter inferred a See also:period of rotation of 24 h. 5 m. 30 s., which was in seeming agreement with the observations of K. L. See also:Harding. This period was generally accepted, though See also:Herschel had been unable to see any changes indicating rotation.

In 1882 G. See also:

Schiaparelli began a careful study of the See also:face of the planet with a refractor of 8 in. See also:aperture, subsequently replaced by one of 18 in. His unexpected conclusion was that the rotation of Mercury resembles that of the See also:moon, in having its period equal to that of its orbital revolution. As the moon always presents the same face to the See also:earth, so Mercury must, in this case, always See also: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 See also:plane of its See also:orbit. The rotation being See also:uniform, while the orbital See also:motion, owing to the See also:great eccentricity of the orbit, is affected by a very large inequality, it would follow that there is a See also:libration in See also:longitude of nearly 24° on each See also:side of the mean position. See also:Percival See also:Lowell in 1897 took up the question anew by combining a See also:long See also:series of measured diameters of the planet with drawings of its apparent See also:surface. The seeming constancy of the surface See also:appearance was considered to confirm the view of Schiaparelli as to the slow rotation of the planet. But there is wide See also: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 See also:round of phases is completed. When near greatest See also:elongation Mercury shines as a See also:star of the first magnitude, or brighter; but in the latitudes of central and See also:northern See also:Europe it is so near the See also:horizon soon after sunset as to be generally obscured by vapours or clouds.

The eccentricity of the orbit, 0.20, is far greater than that of any major planet, and nearly the See also:

average of that of the See also:minor See also:planets. Consequently, its distance and its greatest elongation from the sun vary widely with its position in its orbit at the time. The See also:mass of Mercury can be determined only from its See also:action upon See also:Venus; this is so small that the result is doubtful. See also:Leverrier adopted in his tables 1: 3,000,000 as the ratio of the mass of Mercury to that of the sun. S. See also:Newcomb, from the action upon Venus, reduced this to one-See also:half its amount, or 1: 6,000,000. G. W. See also:Hill, basing his conclusions on the probable See also:density of the planet, estimated the mass to be less than 1: ro,000,000 The See also: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 See also: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 See also: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 See also:line of the nodes of Mercury about the 8th of May and the loth of See also:November of each See also: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 See also:

cycle of 46 years, during which 8 transits occur in May and 6 in November. The following table shows the See also:Greenwich mean time of the See also:middle of all the transits from 1677, the date of the first one accurately observed, until the end of the present See also:century. Transits of Mercury from 1677 to 2003. h. h. 1677 Nov. 7 0 1845 May 8 8 1690 Nov. 9 18 1848 Nov. 9 2 1697 Nov. 2 18 1 861 Nov. 11 20 1707 May 5 II 1868 Nov.

4 19 1710 Nov. 6 II 1878 May 6 7 1723 Nov. 9 5 188r Nov. 7 3 1736 Nov. 10 22 1891 May 9 14 1740 May 2 11 1894 Nov. 10 7 1743 Nov. 4 22 1907 Nov. 14 o 1753 May 5 18 1914 Nov. 7 0 1756 Nov. 6 16 1924 May 7 14 1769 Nov. 9 ro 1927 Nov. 9 18 1776 Nov.

2 IO 1940 Nov. II II 1782 Nov. 12 3 1953 Nov. 14 5 1786 May 3 r8 1957 May 5 13 1789 Nov. 5 3 196o Nov. 7 5 1799 May 7 I 1970 May 8 20 1802 Nov. 8 21 1973 Nov. 9 23 1 815 Nov. I I 15 1986 Nov. I2 16 1822 Nov. 4 14 1993 Nov. 5 16 1832 May 5 0 19991 Nov.

15 9 1835 Nov. 7 8 2003 May 6 19 A perplexing problem is offered by the See also:

secular motion of the See also: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 See also:gravitation of all the other planets. This conclusion has been fully confirmed by subsequent investigations, a See also: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 See also:matter, or that the intensity of gravitation does not precisely follow See also:Newton's See also:law. The most natural explanation was proposed by Leverrier, who attributed the excess of motion to the action of a See also:group of See also:intra-See also:Mercurial planets. At first this conclusion seemed to be See also:con-firmed by the fact that occasional observations of the transit of a dark See also: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 See also:observatory and elsewhere since 1870, have never shown the existence of any such See also: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 See also: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 See also:

illumination on the sky after sunset. Such an illumination is shown by the zodiacal See also:light. But such a group of bodies, if situated in the plane of the See also:ecliptic, would produce a motion of the See also:node of Mercury equal to that of its perihelion, while the observed motion I Mercury grazes sun's See also: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 See also: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 90 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 350. The fact that the zodiacal light appears to be near the ecliptic, and the belief that, if it were composed of a See also:lens of discrete particles, their nodes would tend to scatter themselves equally around the invariable plane of the See also:solar See also:system, led him to drop these explanations as unsatisfactory, and to prefer provisionally the See also: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 See also:

principal elements of the hypothetical orbits as found by the two investigators: Newcomb. Seeliger. Intra- Ring between Zodiacal Light Mercurial Mercury and Matter. Ring. Venus. Inclination . 9° 7.5° 695° Node . . 48° 35° 40.0 Mass . — 1/37,000,000 1/2,860,000 The demonstration by E. W. See also:Brown that the motion of the moon's See also: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 See also: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 See also:

total See also:eclipse. (S.

End of Article: MERCURY

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