Mercury
Collier's New Encyclopedia (1921)
in astronomy, the planet nearest the sun . Its stationary points are from 15 to 20 degrees of longitude from the sun , hence it rises and sets not The light of the sun and the haze of the far from the time when the sun does so. horizon combine to render observation of the planet difficult . It varies in brightness from 15" to 12" of the celestial circle or vault . Hence it is sometimes telescopic, and at other times visible to the naked eye, being as bright as a star of the second magnitude. It was known to the ancients . Its diameter is about 3,- 200 miles; its mass about 1-19 that of the earth; its sidereal period is 88 terrestrial days. It is seen at its greatest brightness as an evening star, at average intervals of about 116 days. Its average distance from the sun is 36,000,000 miles. The orbit of Mercury is remarkable for its extreme eccentricity, the distance from the sun varying periodically from about 28,500,000 to 48,500,000 miles. In Chemistry.- A diatomic metallic element; symbol, Hg; at. wt., 200; sp. gr., Mto Grepair any eb abave 久 10 MERCURY 13.59; boiling point, 357.25; known from the earliest historical times, and the only liquid metal at ordinary temperatures. It is found most frequently in the form of mercuric sulphide, or cinnabar, an ore found in Spain, Austria, and other parts of the world, from which it is extracted by roasting the ore in a furnace, and conducting the vapors into a chamber where the mercury is condensed, while the sulphurous acid is allowed to escape. It possesses a luster like that of polished silver, and solidifies at-39.5° to a tinwhite malleable mass, contracting at the moment of solidification. In Classical Mythology. A Roman deity, identified with the Greek Hermes. He was the son of Jupiter and Maia. He was the messenger and herald of the gods, and as such he was represented as a youth, lightly clad, with the petasus or winged hat, and wings on his heels, bearing in his hand the caduceus or emblem of his office as a herald, a rod with two serpents twined round about it.
What Has Changed in the Last Century
Collier's in 1921 could give little more than an orbit and a diameter, because the planet is so hard to observe near the sun; its figure of 3,200 miles is close to the true 3,032 miles (4,879 km). Radar observations in 1965 overturned the long assumption that Mercury turned once per orbit, showing instead a 59-day spin locked in a 3:2 resonance with its 88-day year, and spacecraft have since mapped it: Mariner 10 flew past in 1974-75, MESSENGER orbited from 2011 to 2015 and confirmed water ice in permanently shadowed polar craters, and BepiColombo is on its way. The "extreme eccentricity" and the slow turning of the orbit that goes with it had just been explained by Einstein's general relativity in 1915, one of that theory's first triumphs. On the chemical side the change is chiefly in reputation: the liquid metal Collier's describes so neutrally is now treated as a serious poison, its industrial use restricted worldwide by the 2013 Minamata Convention and its familiar thermometers largely withdrawn.
Written for this edition, 2026.