Constellations

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Constellations are the random arrangement of the stars visible to the naked eye that has remained essentially unchanged since the time of the first written records. One of the earliest complete lists we have was compiled in about 120 BC by the Greek astronomer Hipparchus, and all the stars that he described can be found, with the same brightness and in practically the same place, in our skies today.

Traditionally, constellations represented mythical or semi-mythical beings. Over half the constellations were recognized and mentioned by Hipparchus (and by Ptolemy, whose star catalogue came down to us through the Muslim scholars as the "Almagest"). The remaining constellations, mostly in the Southern Hemisphere were not named until the sixteenth and seventeenth centuries.

Astronomers have arbitrarily divided the whole sky into eighty-eight areas, or "constellations", which differ greatly in size and shape. Traditional constellations such as the Big Dipper and Northern Cross which fall inside another astronomical constellation have been renamed asterisms.

Few of the groups of stars that form constellations look much like the objects they represent. Much imagination is needed to see the "pictures" seen by those gazing at the skies so many years ago.

As the earth moves around the sun in its yearly cycle, the sun appears to "move" through the constellations. The path is known as the ecliptic.

In antiquity the beginning of the year was reckoned from the start of spring, called the vernal equinox. The vernal equinox and the autumnal equinox are the two days each year when day and night are equal in length. The constellation through which the sun is passing at the time of vernal equinox changes slowly with the centuries, and therefore the stars associated with the season of spring also change slowly.

Constellations

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