Kepler’s Problem, namely, that of finding the co-ordinates of a planet at a given time, which is equivalent—given the mean anomaly—to that of determining the true anomaly, was solved approximately by Kepler, and more completely by Wallis, Newton and others.
The anomalistic revolution of a planet or other heavenly body is the revolution between two consecutive passages through the pericentre. Starting from the pericentre, it is completed on the return to the pericentre. If the pericentre is fixed, this is an actual revolution; but if it moves the anomalistic revolution is greater or less than a complete circumference.
An Anomalistic year is the time (365 days, 6 hours, 13 minutes, 48 seconds) in which the earth (and similarly for any other planet) passes from perihelion to perihelion, or from any given value of the anomaly to the same again. Owing to the precession of the equinoxes it is longer than a tropical or sidereal year by 25 minutes and 2.3 seconds. An Anomalistic month is the time in which the moon passes from perigee to perigee, &c.
For the mathematics of Kepler’s problem see E.W. Brown, Lunar Theory (Cambridge 1896), or the work of Watson or of Bauschinger on Theoretical Astronomy.
ANORTHITE, an important mineral of the felspar group, being one of the end members of the plagioclase (q.v.) series. It is a calcium and aluminium silicate, CaAl2Si2O3, and crystallizes in the anorthic system. Like all the felspars, it possesses two cleavages, one perfect and the other less so, here inclined to one another at an angle of 85° 50′. The colour is white, greyish or reddish, and the crystals are transparent to translucent. The hardness is 6-6½, and the specific gravity 2·75.
Anorthite is an essential constituent of many basic igneous rocks, such as gabbro and basalt, also of some meteoric stones. The best developed crystals are those which accompany mica, augite, sanidine, &c., in the ejected blocks of metamorphosed limestone from Monte Somma, the ancient portion of Mount Vesuvius; these are perfectly colourless and transparent, and are bounded by numerous brilliant faces. Distinctly developed crystals are also met with in the basalts of Japan, but are usually rare at other localities.
The name anorthite was given to the Vesuvian mineral by G. Rose in 1823, on account of its anorthic crystallization. The species had, however, been earlier described by the comte de Bournon under the name indianite, this name being applied to a greyish or reddish granular mineral forming the matrix of corundum from the Carnatic in India. Several unimportant varieties have been distinguished.
(L. J. S.)