STAR. 502 STAK. from the rotation of the earth on its axis. ( See Eabth.) With few exceptions the distance of the fixed stars is still unknown, and must in all cases be enormously great. Since the time of Uradlcy many attempts have been made to measure what is called the annual parallax (q.v.) of the stars, and thus determine their distances. When we consider that the motion of the earth round the sun alters our position in space a whole diameter of its orbit ( 18.5,- 000.000 miles) in six months, we should ex- pect a change in the relative positions of certain stars as seen from two opposite points of the orbit. But no such change is seen to take place, and this w'as one of the early objections to the theory of Copernicus (q.v.). The only answer that the Copernicans conld give was that the distance of the stars from us is so great that the diameter of the earth's orbit shrinks into insignificance when compared with it. The de- tection of the parallax of the fixed stars de- pended 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 at least 200,000 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 93,000,000 miles. The method now in use for measuring stellar parallaxes was first applied successfully by Bes- sel (q.v.) in 1838. He employed in his observa- tions a remarkably fine heliometer (q.v.) and adopted what is called the differential method. Having selected a star which he suspected might be near us, and therefore have a parallax large enough to measure, he proceeded to determine every clear night its position in the sky rela- tively to two neighboring very small stars. Such relative (difTerential ) determinations of posi- tions can be made with far greater accuracy than measures with the meridian circle (q.v.) ; and Bessel judged rightly that the two minute com- parison stars were really so immeasurably far away that they would themselves suffer no ap- preciable parallactic displacement. Sure enough, he found his 'parallax star,' 61 Cygni. slowly de- scribing day after day a tiny oval curve in the sky, reproducing there the earth's orbit in space. But the comparison stars did not move. The proof was complete that 61 Cygni had an appa- rent motion due to the real motion of our earth, and that it was measurable. This observation of Bessel's is one of the most famous in the annals of astronomy. When other astronomers had succeeded in re- peating Bessel's observations, and quite a number of stars came to have known parallaxes, their distances were found to be too great to be ex- pressed conveniently in miles, or any other linear imit. Therefore astronomers invented a new unit, the 'light-year.' being a distance equal to t!ie space traversed by light in one year. As light moves about 180,000 miles per second, it will be seen that the light-year is a unit of stiipendous magnitude, and will be fitted to measure the profound distances of stellar space. A few of the larger stellar parallaxes at present known are as follows: 6TAB Magnitude Parallax Distance in light-yearH 1 6.9 5.1 1 0".75 .60 (1 .40 .39 4.4 Lalande21,185 6.6 01 Cygui 8.1 Sirius 8.4 In this table the second column gives the 'magnitude' of the stars. The third column gives the parallax, or angle subtended by the earth's orbit-radius at the star. The corresponding dis- tance in light-years is in the last column. Thus we do not see the stars as they are to-day, but as they appeared so manj' years ago. The principal constellations north of the Zo- diac (q.v.) are: Ursa Major. Bootes. Ursa Minor. Corona Borealis Draco. Hercules. CeplieuB. Lyra. Cassiopeia, C.vgnus. Camelopardus. Vulpecula. Andromeda. Aquila. Perseus. Antinoiia. Auriga. Pelphinue. Leo Minor. Pegasus. CaiiPH Venatici. Ophiuehus. Coma Berenices. le principal constellations south of the are: Cetus. Canis Minor. Orion. Crux. Lepus. Argo-Navis. CentauruB. Hydra. Lupus. Crat«r. Monoeeros. Corvus. Canie Major. Eridanue, For the constellations of the Zodiac, see Zo- diac. The several stars belonging to the same constellation are usually distinguished from one another by Greek letters, beginning the alphabet with the bi-ightest : and when these are not suf- ficient by Roman letters and by numbers. Many of the most brilliant stars have special names. They are also divided according to their bright- ness into stars of the first, second, third, etc. magnitudes, a division which is necessarily some- what arbitrary. The smallest stars discernible by the unaided eye are usually called stars of tile fifth magnitude; but an unusually sharp eye can discern those of the sixth and even sev- enth magnitude. All below are telescopic stars, which are divided down to the twentieth magni- tude. The quantity of light given by a star of any magnitude is taken as 2..512 times as great as'the quantity given by a star one magnitude fainter. This number is called the 'light-ratio,' and it is so chosen that a diminution of five magnitudes corresponds to a division of stellar light by just 100 (2.512 =^^00). In other words. 100 average stars of the sixth magnitude should give as much light as one of the first. But the whole matter of stellar photometry is subject to some uncei'tainty. According to the Harvard Photometry the following are the brightest stars in the order of lucidity: Sirius, Canopus*, Arcturus, Capella. Vega, o Centauri*, Rigel, Procyon. Eridani*, j3 Centauri*, a Orionis, Altair, Aldebaran, a Crucis*. Antares, Pollux, Spica, o Piseis Austria*, Regulus. Those marked with an asterisk are not visible in our northern latitudes. No real magnitude in the proper sense of the word has yet been observed in any star. In the best and most powerfully magnifying tele- scopes, even the brightest stars of the first mag-
