Stellar Sky

the aggregate of heavenly bodies visible at night in the sky. With good conditions it is possible to see with the unaided eye at one time in the night half of the sky about 2,500 stars (to the sixth magnitude), the majority of which are situated near the band of the Milky Way. Use of a telescope permits the observation of a significantly greater number of stars (see Table 1).

For ease of orientation, the stellar sky is divided into parts, called constellations. In each constellation the brightest stars form characteristic groups, which, after training, can be easily recognized in the sky. The division of stars into primary constellations, including zodiacal ones, dates from early antiquity. The names of the constellations are derived partly from Greek mythology (for example, Andromeda, Perseus, Delphinus) or are connected with various occupations of the ancients—agriculture, stock breeding, hunting (for example, Virgo, Bootes, Pisces, Lepus). Constellations that were distinguished much later received names connected with travel and the development of technology (for example, Sextans, Microscopium). In all, 88 constellations are recognized (see Table 2), the boundaries between which were established in 1930 according to a resolution of the International Astronomical Union. Table 2 gives the names of the constellations as well as their abbreviations. The bright stars in constellations are designated by letters of the Greek alphabet or by numbers. Certain types of stars have special designations; variables, for example, are denoted by capital Latin letters. A number of stars have proper names (see Table 3). Most stars, however, are designated by the name of a star catalog with information on the given star and the number under which the star is recorded in it (for example, Lacaille 9352).

One can also observe in the stellar sky star clusters, stellar associations, galactic nebulas, galaxies, quasars, clusters of galaxies, and other objects; bodies constituting the solar system

such as planets, asteroids, and comets; and artificial space objects, such as artificial earth satellites and space probes.

Most of these objects can be observed only with the aid of a telescope. Among those visible to the naked eye are the open clusters of the Pleiades and the Hyades in the constellation Taurus; Praesepe in the constellation Cancer; globular clusters in the constellations Tucana and Centaurus; galactic nebulas in the constellation Orion; galaxies in the constellation Andromeda; the Large and Small Magellanic Clouds; the planets Venus, Jupiter, Mars, Saturn, Mercury, and Uranus; the asteroid Vesta; comets; and the brightest artificial earth satellites.

The background of the sky is never completely black, since the sky gleams weakly because of atomic processes in the upper layers of the atmosphere. This night-sky glow from 1 square degree produces an illumination equivalent on the average to that from a star of magnitude 4.5. By day almost all celestial bodies disappear in the bright blue background of the air that is illuminated by the sun. Besides the sun, only the moon and Venus are visible to the unaided eye in the clear daytime sky.

The appearance of the stellar sky changes continuously because of the apparent diurnal rotation of the celestial sphere, which is explained by the earth’s rotation, and changes slowly because of the annual apparent movement of the sun among the stars, which occurs as a result of the earth’s revolution around the sun.