As´tacus. See Crayfish.
Astar´te, a Syrian goddess, probably corresponding to the Ashtaroth of the Hebrews, and representing the productive power of nature. She was a moon-goddess. Some regard her as corresponding to Hera (Juno), and others identify her with Aphroditē.
Astatic needle, a magnetic needle having another needle of the same intensity fixed parallel to it, the poles being reversed, so that the needles neutralize one another, and are unaffected by the earth's magnetism; used in the astatic galvanometer.
As´ter, a genus of plants, nat. ord. Compositæ, comprehending several hundred species, scattered over Europe and Asia, but mostly natives of North America. Many are cultivated as ornamental plants. One, A. Tripolium, is native in Britain, and is found in salt marshes, having a pretty purple flower. Asters generally flower late in the season, and some are hence called Michaelmas or Christmas Daisies. The China Aster, not an aster proper (Aster or Callistĕphus chinensis), is a very showy annual, of which there are many varieties.
Asterabad´. See Astrabad.
Aste´ria, a name applied to a variety of corundum, which displays an opalescent star of six rays of light when cut with certain precautions; and also to the cat's-eye, which consists of quartz, and is found especially in Ceylon.
Aster´idæ. See Asteroidea.
As´terisk, the figure of a star, thus *, used in printing and writing, as a reference to a passage or note in the margin, or to fill the space when a name, or the like, is omitted.
Asteroi´dea, the ord. of the Echinodermata to which the star-fishes belong. See Star-fishes.
As´teroids, Planetoids, or Minor Planets, a numerous group of very small planets revolving round the sun, in the great majority of cases at mean distances, intermediate between those of Mars and Jupiter, in orbits of large eccentricity at considerable inclination to the ecliptic. The diameter of the largest is not supposed to exceed 450 miles, while most of the others are very much smaller. Over one thousand are known, and new members are being constantly discovered. The first to be discovered was Ceres, on 1st Jan., 1801, and within seven years more Pallas, Juno, and Vesta were seen. The diminutive size of these four bodies, and resemblances in their orbits, gave rise to the opinion that they were but the fragments of a planet that had formerly existed and had been brought to an end by some catastrophe. For nearly forty years investigations were carried on, but no more planets were discovered till 8th Dec., 1845, when a fifth planet in the same region of the solar system was discovered. The rapid succession of discoveries that followed was for a time taken as a corroboration of the disruption theory, but the breadth of the zone occupied makes the hypothesis of a shattered planet more than doubtful. In recent years a few have been discovered which are at times considerably within the orbit of Mars, the nearest perihelia being less than 15 million miles beyond the earth's orbit. Another group, known as the 'Trojan Planets', has been found, whose mean distances are practically identical with that of Jupiter. The total mass of the asteroids cannot exceed one-fourth that of the earth, and is probably much less. See Planets.