Mercury, the planet nearest to the sun, travelling at a mean distance from it of about 35,392,000 m. The eccentricity of the orbit of Mercury is considerable, the centre of the orbit being more than 7,000,000 m. from the centre of the sun. Thus his greatest distance from the sun amounts to about 42,669,000 m., his least to about 28,115,000 in. When nearest to the earth, Mercury's distance from us amounts to about 45,000,000 m.; but when so situated he is not visible, because, being between us and the sun, his darkened hemisphere is turned toward us. His greatest distance amounts to about 135,500,000 m. When he is at his greatest distance his illuminated face is turned directly toward the earth; but he cannot then be seen because he lies in the same direction as the sun, and is lost in the superior glory of that luminary. He is seen most favorably when nearly at his greatest elongation; that is, when two lines drawn to the sun and Mercury include their greatest angle. At such a time he is about 85,000,000 m. from us, and appears as a half disk.

As he is illuminated with great brilliancy on account of his nearness to the sun, he is a difficult object of telescopic study, the more so that when most favorably situated the illuminated portions of his disk are seen obliquely.

Therefore very little reliance can be placed on the accounts some telescopists have given of marks supposed to be seen on the planet's surface, nor can his rotation period be regarded as fully determined from features so unsatisfactorily observed. Even the estimates of his diameter can scarcely be regarded as altogether trustworthy; but it is not probable that it greatly exceeds or falls short of 3,000 miles. His volume is about 58/1000 of the earth's, and his density greater than hers in about the proportion of 10 to 9; so that his mass is about 65/1000 of the earth's. lie travels round the sun in rather less than three of our months, his mean sidereal revolution being completed in 87.9693 days. His mean synodical revolution, or the mean interval between his successive returns to inferior conjunction, amounts to 115.877 days; so that he passes through all his phases (from inferior conjunction to maximum elongation west of the sun, thence to superior conjunction through maximum elongation west of the sun, to inferior conjunction again) more than three times in the course of each year.

But as he is only visible for a short time after sunset on three or four evenings when his elongation is easterly, and for a short time before sunrise on three or four mornings when his elongation is westerly, he is not often seen. In America he might be seen oftener than in England, however, as the twilight skies are seldom free from light mist in the latter country. It is commonly said that Mercury rotates on his axis in 24h. 5m. 28s.; but great doubt rests on the determination of this element, for the reason above mentioned. - Mercury, travelling within the earth's orbit, sometimes transits (or passes across) the face of the sun. The first phenomenon of this sort ever observed took place in November, 1631, and was witnessed by Gassendi. A transit of Mercury is not so important in astronomy as a transit of Venus, because the nearness of Mercury to the sun prevents his having a measurable relative parallax; in other words, whereas Venus, seen from two distant stations on the earth during any moment of her transit, is projected at two spots measurably separated from each other on the sun's disk, this is not the case with Mercury. The observation of Mercury in transit, however, is interesting as illustrating the phenomena which occur during a transit of Venus. The formation of the "black drop," a small black ligament which at the moment of contact seems to connect the disk of the planet with the dark space outside the solar disk, is very manifest in the case of Mercury, though, owing to his disk being so much smaller than that of Venus, and his motions more rapid, the phenomena are not so readily studied.

Transits of Mercury take place at intervals of 13, 7, 10, 3, 10, and 3 years.