MERCURY

, the smallest of the inferior planets, and the nearest to the sun, about which it is carried with a very rapid motion. Hence it was, that the Greeks called this planet after the name of the nimble messenger of the Gods, and represented it by the figure of a youth with wings at his head and feet; from whence is derived <*>, the character in present use for this planet.

The mean distance of Mercury from the sun, is to that of the earth from the sun, as 387 to 1000, and therefore his distance is about 36 millions of miles, or little more than one-third of the earth's distance from| the sun. Hence the sun's diameter will appear at Mercury, near 3 times as large as at the earth; and hence also the sun's light and heat received there is about 7 times those at the earth; a degree of heat sufficient to make water boil. Such a degree of heat therefore must render Mercury not habitable to creatures of our constitution: and if bodies on its surface be not inflamed, and set on fire, it must be because their degree of density is proportionably greater than that of such bodies is with us.

The diameter of Mercury is also nearly one-third of the diameter of the earth, or about 2600 miles. Hence the surface of Mercury is nearly 1-9th, and his magnitude or bulk 1-27th of that of the earth.

The inclination of his orbit to the plane of the ecliptic, is 6° 54′; his period of revolution round the sun, 87days 23hours; his greatest elongation from the sun 28°; the excentricity of his orbit 1/5 of his mean distance, which is far greater than that of any of the other planets; and he moves in his orbit about the sun at the amazing rate of 95000 miles an hour.

The place of his aphelion is <*> 23° 8′; place of ascending node <*> 14° 43′, and consequently that of the descending node <*> 14° 43′.

His Length of day, or rotation on his axis, Inclination of axis to his orbit, Gravity on his surface, Density, and Quantity of matter, are all unknown.

Mercury changes his phases, like the moon, according to his various positions with regard to the earth and sun; except only, that he never appears quite full, because his enlightened side is never turned directly towards us, unless when he is so near the sun as to be lost to our sight in his beams. And as his enlightened side is always towards the sun, it is plain that he shines not by any light of his own; for if he did, he would constantly appear round.

The best observations of this planet are those made when it is seen on the sun's disc, called its transit; for in its lower conjunction, it sometimes passes before the sun like a little spot, eclipsing a small part of the sun's body, only observable with a telescope. That node from which Mercury ascends northward above the ecliptic, is in the 15th degree of Taurus, and the opposite in the 15th degree of Scorpio. The earth is in those parts on the 6th of November, and 4th of May, new style; and when Mercury comes to either of his nodes at his inferior conjunction about these times, he will appear in this manner to pass over the disc of the sun. But in all other parts of his orbit, his conjunctions are invisible, because he goes either above or below the sun. The first observation of this kind was made by Gassendi, in November 1631. Several following observations of the like transits are collected in Du Hamel's Hist. of the Royal Acad. of Sciences, pa. 470, ed. 2. And Mr. Whiston has given a list of several periods at which Mercury may be seen on the sun's disc, viz, in 1782, Nov. 12, at 3h 44m afternoon; in 1786, May 4th, at 6h 57m in the forenoon; in 1789, Dec. 6th, at 3h 55m afternoon; and in 1799, May 7th, at 2h 34m afternoon. There are also several intermediate transits, but none of them visible at London. See Dr. Halley's account of the Transits of Mercury and Venus, in the Philos. Trans. n°. 193.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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MENISCUS
MENSTRUUM
MENSURABILITY
MENSURATION
MERCATOR (Gerard)
* MERCURY
MERIDIAN
MERLON
MERSENNE (Martin)
MESOLABE
METO