STARS are distinguished from planets by remaining apparently immovable with re spect to one another, and hence they were early called fixed stars, a name which they still retain, although their perfect fixity has been completely disproved in numerous cases, and is no longer believed in regard to any. Twinkling, or scintillation (q.v.), is another mark which distinguishes stars from planets.
The first thing that strikes the observer is the apparent ciaily motions of the stars. The greater part appear to rise iu the e.. describe smaller or greater arcs in the heavens, and set in the west; while others describe complete circles around a point n. of the zenith, that described by the so-called polar star being the smallest visible to the naked eye. These apparent motions arise from the rotation of the earth on its axis. . Had the earth only this rotatory motion, the aspect of the starry heavens at any spot on the ei•th's surface would be the same at the same hour of the night all the year round; which is keown not to he the case. In consequence of the earth's motion round the sun, or the arryireto advauca of the sun among the stars, the aspect of the heavens at a par ti•ular hour is always eleingincr. The position of the stars recurs four minutes earlier each night. and only al die same time after the lapse of a year.
With few exceptions. the crsfaara of the fixed stars is still. unknown. and mi:st in all be enormously great. Since tile time of Bradley. many attempts have been made to measure what is called the yearly parallax of the stars, and thus determine their distances. When we consider that the motion of the earth round the sou brings us at one time a whole diameter of its orbit (184 millions of miles) nearer to a particular region of the heavens than we were six months before, we should expect a change in the relative dis tances of the stars as seen from the two points—that as we approach them they should seem to separate. But no such change is seen to take place; and this was one of the early objections to the theory of Copernicus. The only answer that the Copernicans could give was, that the distance of the stars from its is so great that the diameter of the earth's orbit is as a point compared with it. The detection of the parallax of the_ fixed stars depended upon the perfection of instruments. The parallax of a star is the minute angle contained by two lines drawn from it, the one to the sun, the other to the earth.
If that angle amounted to a second, the distance of the star would be 206,000 times that of the sun; and when the measurement of angles came to be„ reliable to a second, and still no parallax was discernible, astronomers could say that the distance of the nearest stars must be more than 206,000 times that of the sun—i.e., 206,000 times 92 millions of miles, or about 20 billions of miles. It is only since between 1832and 1838 that anything like positive determinations of parallax have been made, chiefly by Hen derson, Bessel, and Peters. The first published (Dee., 1838) was that of the double star 61 in the constellation of the Swan, by Besse], who made the parallax 0".37, giving a distance over 550,000 times that of the sun, or 52 billions of miles. so that the light of this star is about 81 years in reaching the earth. The nearest of all the stars yet measured is a Centauri, the finest double star in the southern heavens, whose parallax was deter mined by Henderson and Maelear at the cape of Good Hope to be 0.9128" ( the observa tions were made in 1832-33; the result read before the Astronomical society, Jan., 1839), or as subsequently corrected, 0.976", corresponding to a distance of about 20 billions of miles, and requiring 31- ye&rs for its light to reach us. To Sirius, the brightest of the stars, a parallax of 0.15", has been assigned, implying a distance six times that of cc Cen tauri. "It has been considered probable, from recondite investigations, that the average distance of a star of the first magnitude from the earth is 9S6,000 radii of our annual orbit, a distance which light would require 15 years to traverse; and further, that the average distance of a star of the sixth magnitude (the smallest distinctly seen without a telescope) is 7,600,000 times the same unit—to traverse which, light, with its prodigious velocity, would occupy more than 120 years. If, then, the distances of the majority of stars visible to the naked eye are, so enormously great, how are we to estimate our dis tance from those minute points of light discernible only in powerful telescopes? The conclusion is forced upon us that we do not see them at they appeared within a few years, or even the lifetime of man, hut with the rays which proceeded from them several thousands of years ago!"—Hind's Astronomy.