MERCURY, the planet of the solar sys tem which is nearest to the sun. Owing to the position of its orbit, far inside of that of the earth, it is never seen by us at any great dis tance from the sun, but seems to swing back and forth, first on one side of the central lumi nary and then to the other. (See SOLAR TiM) . Its time of revolution is a little less than three months and therefore less than one fourth that of the earth. When, starting from a point between the earth and the sun, it has completed a revolution, the earth has moved forward in its orbit, and, in consequence, nearly 30 days more are required to catch up with the earth and again come into conjunction with it. Consequently the time of one synodic or ap parent revolution is nearly four months. It follows that its greatest elongations from the sun occur at intervals of nearly 60 days, alter nately to the east and to the west. When near its greatest eastern elongations it may be seen in the west toward the close of twilight. When west of the sun it may be seen in the morning before daybreak. To the naked eye it seems to shine as a star of the first magnitude. But as it is never seen in a perfectly dark sky except when very near the horizon, it is not readily observable in high northern latitudes. It is said, in fact, that Copernicus died without ever seeing this planet.
With the aid of a telescope, Mercury may be seen the greater part of the time the afternoon when it is east of the sun; in the morning when it is west of it. But it is never seen fully illuminated unless near the farther part of its orbit, beyond the sun, when it may be lost in the effulgence of the sun's rays. When it approaches nearest to us, only a small portion of the hemisphere presented to us is illuminated. Owing to these unfavorable con ditions observations on it are extremely diffi cult, and it cannot be said that anything is certainly known of its physical constitution. The difficulty is increased by its being much the smallest of all the major planets. The result is that nothing is positively known as to the time of the rotation on its axis. About 1800, Schroe ter, a celebrated observer of the planets, thought it rotated in a little more than 24 hours. But Herschel found no foundation for this belief, and could see no evidence whatever of a rotation. About 1889 Schiaparelli, the celebrated Italian astronomer, making a very careful study of the planet, under the favoring sky of Milan, was led to the conclusion that, like the moon, Mercury's time of rotation was the same as its time of revolution in its orbit, so that it always presented the same face to the sun. A similar conclusion was reached by Lowell at the Flagstaff Observatory. But the difficulty of seeing any well-defined features on the planet is such that conservative astron omers are still in doubt on the subject, and regard the time of rotation as still unknown and not likely soon to be determined.
The most remarkable feature presented by the motion of Mercury is that the perihelion of its orbit is found to move forward considerably faster than it ought to by virtue of the aurac tion of the known bodies of the solar system. The cause of this motion has perplexed astron omers for half a century; it was at first stip posed by Leverrier to be due to the attraction of one or more unknown planets between Mercury and the sun. Another explanation was sought in the assumption that the sun's gravitation diminishes somewhat more rapidly than it would according to the law of the inverse square. If this were so, the perihelion of all the other planets ought to be effected by a similar motion, and in particular there should result disturb ances in the motion of our moon which, now that the extremely abstruse mathematical theory of that body (based upon the law of gravita tion), has been so perfected, it is certain do not exist. Similar discrepancies have been de tected in the motions of some of the other planets, notably in the node of Venus and in the perihelion of Mars. When the disturbing pull of the exceedingly tenuous, lens-shaped, cloud of particles known as the Zodiacal Light is computed and allowed for, it is found that not only these, but also the historic discrepancy in the motion of the perihelion of Mercury, disappear. It is very probable that the true explanation of them is to be found in this source.
At varying intervals the motion of Mercury in its orbit causes the planet to pass between the earth and the sun; it is then seen as an intensely black, round dot crossing the sun's disc. The next four transits will occur on 7 May 1924, 8 Nov. 1927, 10 May 1937 and 12 Nov. 1940. None of these will be visible, however, from the United States. The first transit which will be completely visible here will occur on 13 Nov. 1953 and 6 Nov. 1960. But little use is made of transits of Mercury. Attempts have been made to detect traces of an atmospheric ring about the planet during the transit, as may be done during a transit of Venus (q.v.), but these have not been certainly successful. Newcomb subjected 21 transits from 1677 to 1881 to a critical discussion to ascertain whether there might be found from them any indication that the time of rotation of the earth on its axis,— the unit of time throughout astronomy,— had changed during this interval. There was found no conclusive evidence in any appreciable change in the length of the day. It has certainly not increased or diminished by so ranch as 0.01 second in the course of the past 2,000 years.