Part of the heavenly dome always lay beneath the horizon. The Titan Atlas, who stood either beneath the axis of heaven in the far north (in the land of the Hyperboreans), or at heaven's western rim in by the Atlas mountains in North Africa, was said to spin the dome around upon his shoulders, causing the stars to rise and set. The Greeks imagined the heavens as a great, solid dome, which, some say, was forged of bronze, and upon which the heavenly constellations were fixed. As the constellation gradually rises earlier and earlier in the night, its place on the solar horizon at dawn is eventually replaced by the next constellation of the zodiac, namely Cancer, in mid June. So, for example, Gemini first rises above the eastern horizon in late May, appearing in the sky just prior to dawn, where its position is immediately replaced by the rising sun. They were said to rule the heavens for the period in which their position on the eastern horizon prior to dawn was replaced by the rising sun. The most important of the heliacal risings were those of the twelve constellations of the zodiac. As the months progress it is seen to rise earlier and earlier in the night, and gradually assumes a higher position in the heavens prior to dawn, until one evening, it has reached the western horizon, before dissappearing completely from view (which is known as the constellation's astronomical setting).
The first appearance of a constellation in the sky, occurs on the western horizon just prior to dawn (its so-called heliacal rising). Not all of the constellations are visible in the night sky throughout the year.
Only those few closest to the pole-namely, Ursa Major and Minor (the Bears), and Draco (the Dragon)-appear to travel at night in an eternal circle around the pole. Because of its far northern location, most of the stars are seen to rise in the east and set in the west. The constellations revolve round a central point in the northern sky known as the pole star, or heavenly axis (Greek polos). REVOLUTIONS OF THE CONSTELLATIONS PERSEUSįor those unfamiliar with the basic visible mechanics of constellar movement, what follows is a brief outline, including Greek beliefs regarding the behaviour of the stars.
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