The heliacal rising of a star (or other body such as the moon or a planet) occurs when it first becomes visible above the eastern horizon at dawn, after a period when it was hidden below the horizon or when it was just above the horizon but hidden by the brightness of the sun. Each day after the heliacal rising, the star will appear to rise slightly earlier and remain in the sky longer before it is hidden by the sun (the sun appears to drift eastward relative to the stars along a path called the ecliptic). Eventually the star will no longer be visible in the sky at dawn because it has already set below the western horizon. This is called the heliacal setting. A star will reappear in the eastern sky at dawn approximately one year after its previous 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 calendars or zodiacs. The ancient Egyptians 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 Greeks also used the heliacal risings of various stars for the timing of agricultural activities. To the Maori 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.
