MERCURY () is the smallest of the major planets, and has a diameter of only about 3,00o miles. It is also the nearest to the sun with a mean distance of 36,000,000 miles or 0.39 of the Earth's distance. But the eccentricity of its orbit, amounting to 0.206, is very much greater than that of any other major planet, and its perihelion and aphelion distances are respectively 28,500,000 and 43,350,00o miles or approximately in the ratio of 2 to 3. Its period of revolution in its orbit is 88 days, but its synodical period, i.e., the interval between, say, successive superior conjunctions with the sun, is 116 days. At its most favourable elongations it recedes only about 28° from the sun. Its orbit has a greater inclination (7°) to the ecliptic than that of any other major planet. The albedo is low, but at a mean elongation its brightness is about equal to that of a star of zero magnitude; and Surface Features and Rotation.—As the angular diameter of Mercury varies from about 5" to 13" its phases are easily shown by quite a small telescope. But under the circumstances of observation it is far from easy to define the surface features with accuracy, and even the period of the planet's axial rotation was still regarded as uncertain until recently (1927). About the beginning of the 19th century Schroeter and Harding at Lilienthal although at a favourable elongation it can reach -1.2 magnitude, the planet is frequently dimmed or obscured by the haze and mists near the horizon. It can, however, be readily seen with a telescope in the daytime when high in the sky if the atmosphere is reasonably transparent.
As Mercury has no satellite it has been necessary to determine its mass from its attraction on Venus or some other body, and the result which has been deduced (about that of the earth) is somewhat uncertain. The density of the planet is apparently between that of Mars and the Moon.
recorded an occasional blunting of the S. cusp, and from the ob servations of this phenomenon and other features Bessel deduced a period of slightly more than 24 hours. Several other observers
subsequently recorded light and dark patches and other features, but the conclusions drawn were somewhat discordant.
Between 1881 and 1889 Schiaparelli at Milan carried out a careful study of the planet, using at first a refractor of 81 inches which was eventually replaced by one of 18 inches. His con clusion was that the periods of axial rotation and orbital revolution are identical, and that the axis of rotation is approximately normal to the orbit plane. Lowell at Flagstaff some years later came to a similar conclusion.
It would, of course, follow from such conditions that the planet always presents approximately the same face to the sun, as the Moon does to the Earth, though in consequence of the large eccentricity of the orbit, the effect of libration in longitude of the surface markings as seen from the sun would cause a dis placement of nearly 24° on each side of a mean position. About 37% of the surface would be permanently sunlit; the same amount in perpetual night ; and the remainder exposed alternately to sunlight and darkness with enormous changes of temperature during each revolution of the planet.
A fresh attack on the problem of Mercury's rotation was made by E. M. Antoniadi at Meudon with the help of the great 33-inch refractor of that observatory in 1927, and the result was to establish Schiaparelli's deductions. The markings were found to be somewhat variable in intensity, but—apart from the libra tion above mentioned—constant in position relatively to the terminator, thus indicating the identity of the periods of rotation and revolution. The charts of both Schiaparelli and Antoniadi are appended and it will be seen that many of the same features are presented in both, though Schiaparelli drew them as generally linear in character, whereas Antoniadi sees them rather as diffuse spots. Some of the objects shown by the latter can be also identi fied with features recorded at various times by Denning, Barnard, Jarry-Desloges, Fournier, Danjon and other observers (see L' As tronomie, January 1928).