Heliacal Rising

Not all stars have heliacal risings: some may (depending on the Latitude of observation on the Earth ) remain permanently above the horizon, making them always visible in the sky at dawn, before they are hidden by the brightness of the sun.

Constellations containing stars that rise and set were incorporated into early Calendar s or Zodiac s. The Ancient Egypt ians based their calendar on the heliacal rising of Sirius and devised a method of telling the time at night based on the heliacal risings of 36 stars called ''decan stars'' (one for each 10° segment of the 360° circle of the zodiac/calendar). The Sumerians , the Babylonians , and the Ancient Greek s also used the heliacal risings of various stars for the timing of agricultural activities. To the Māori of New Zealand , the Pleiades are called Mataariki and their heliacal rising signifies the beginning of the new year (around June).

The corresponding rising of a celestial body above the eastern horizon at nightfall, for example, that of the full moon, is called its ''acronychal rising''.

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Heliacal Rising