Introduction about constellations

Division of the Sky
      The sky (both northern and southern hemispheres) is divided into 88 constellations. A constellation is a grouping of stars usually resembling a mythical figure from Greek or Arabic folklore. Many constellations are easily recognisable in the sky e.g. the Big Dipper (the plough). These constellations act as guideposts to the heavens enabling astronomers, professional and amateur alike, to find their way around the night sky. The constellations also serve a particular function, rather than just as superficial patterns, every region of the sky is designated to a particular constellation, this facilitates the current, though rather old and peculiar, method of naming stars. In astronomy the stars are named according to their brightness (magnitude) and given Greek letters. For instance, the brightest star in the constellation Cygnus (the Swan) is given the name Alpha Cygni, second brightest Beta Cygni and so on, until one runs out of Greek letters that is, then its on to Roman Numerals. Many of the stars also have common names as well e.g. Alpha Cygni is better known as Deneb. Here are a few of the more famous constellations: Ursa Major (the Bear / the Big Dipper), Orion (the Hunter), Cassiopeia (the W shape), and the Southern Cross.

Celestial co-ordinate system:

      From the Earth, the constellations seem to be struck on the inside of a hollow sphere known as the celestial sphere. This sphere appears to rotate around the Earth in an east-west direction every 24 hours. A grid of lines known as right ascension and declination help astronomers locate stars on the celestial sphere, and star maps are a projection of the imaginary sphere onto a flat surface,

Right Ascension (R.A.)
      R.A. is one of the co-ordinates used to locate positions on the celestial sphere. Lines run from the North to the South celestial pole and are similar to the Earth's lines of longitude, except that they are measured in units of time.

Declination (DEC.)
      DEC. is one of the co-ordinates used to locate positions on the celestial sphere. Lines run from East to West and are linked to the Earth's lines of latitude: stars of 0° Dec., for example, lie in the same plane as the Earth's equator.

Magnitude:

Apparent Magnitude is a measure of apparent brightness, which is the visible-light brightness of a celestial object observed from Earth, depending on both the distance of the object and its actual or true brightness.

Absolute Magnitude is the magnitude (visible-light brightness) that a celestial object would have if it were observed at a standard distance of 32.6 light years (10 parsecs). Absolute magnitude differs from apparent magnitude, which is a measure of how bright an object looks to an observer on the Earth.

      Astronomers here on Earth use apparent magnitude. The brighter a star is the lower its magnitude, i.e. a star with a magnitude of 1.2 is brighter than one with mag. 3.0, and a star mag. -0.7 is brighter than one of mag. -0.1. During the 18^th century the ratio between magnitudes was fixed at 2.5 (2.5118865 to be exact). This means that a star of a given magnitude is 2.5 times brighter than a star one magnitude dimmer.