Mercury

That Mercury has an atmosphere appears to be indicated by the varying intensity of the dark areas as observed at Meudon ; but the visibility of the markings together with the planet's low albedo, and the absence of any appearance of an atmosphere as Mercury advances on or leaves the sun's disk on the occasions of its transit, would seem to show that it is scanty. This is what one would expect from a consideration of the smallness of the planet's mass and surface gravity. Radiometric observations at Mt. Wilson show that the temperature of the sunlit side is very far above the boiling point of water at the Earth's surface, but that of the opposite side may be not greatly removed from the absolute zero.

Transits.—Since the inclination of Mercury's orbit to the ecliptic is 7° it usually happens that at the time of inferior con junction the planet passes N. or S. of the sun, but should con junction occur when it is near one of its nodes it crosses the sun's disk and is seen during transit projected on it as a small black spot. As the earth passes the line of nodes about May 7 and November 9 transits can only take place near those dates. The November transits, however, which occur with Mercury in the neighbourhood of perihelion, and consequently more remote than usual from the earth, are very much more common than those in May when the conditions are reversed, the limits of distance from the node within which the planet must be for a transit to take place being 45' and 2° 4o' respectively.

The following table gives the dates and Greenwich Mean Astronomical Times of the middle of all the transits from 1677 (the date of the first one accurately observed.) until the year 2003. The hours are in all cases reckoned from noon.

Motion of Perihelion.

What was for a long time a perplex ing problem was presented by the motion of Mercury's perihelion, the line of apsides—or the longer axis of the orbit—showing a greater advance in the direction of the planet's motion than could be accounted for by the gravitational action of all the other planets. Leverrier in 1845 concluded that the excess of motion was 35" per century—but this was increased by later investiga tions to over4o" per century. It was accordingly concluded either that Mercury is disturbed by some unknown masses of matter or that Newton's law of gravitation is not rigidly true. Leverrier attributed the excess of motion to the action of a group of planets revolving inside the orbit of Mercury, and at first this explanation seemed to be confirmed by the occasional reports of dark objects seen in transit across the sun's face. But for very many years past the sun has been photographed almost daily at some ob servatory or other, such as Greenwich or the Cape, without any such objects being detected, and photographs of the sky in the neighbourhood of the sun during total eclipses have likewise failed to disclose anything of the sort. If, then, such intra-Mer curial bodies exist, they must be extremely small. It was at one time suggested that the minute particles which produce the Zodiacal light might be invoked to explain the observed motion, but the idea was shown on theoretical grounds to be untenable. The solution of the difficulty has since been supplied by Einstein's Theory of Generalized Relativity and the matter is now regarded as satisfactorily cleared up. (T. E. R. P.)