The heliacal rising of a star (or other body such as the moon, a planet or a constellation[1]) occurs when it first becomes visible above the eastern horizon for a brief moment just before sunrise, after a period of time when it had not been visible.[2]
Each day after the heliacal rising, the star will rise slightly earlier and remain visible for longer before the light from the rising sun makes it disappear (the sun appears to drift eastward relative to the stars by about one degree a day along a path called the ecliptic). Over the following days the star will move further and further westward (about one degree per day) over the dome of the pre-dawn sky, until eventually it is no longer visible in the sky at dawn because it has already set below the western horizon. This is called the cosmical setting.[3] The same star will reappear in the eastern sky at dawn approximately one year after its previous heliacal rising. Because the heliacal rising depends on the observation of the object, its exact timing can be dependent on weather conditions.[4]
Some stars, when viewed from a particular latitude on Earth, will not have a heliacal rising or setting. Circumpolar stars remain above the horizon throughout the whole year, making them always visible in the sky at dawn. Conversely, some stars are never seen in some locations. For example, the North Star is not visible in Australia and the Southern Cross is not seen in Europe, because they always stay below the horizon.
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 Māori of New Zealand, the Pleiades are called Matariki and their heliacal rising signifies the beginning of the new year (around June).
The Mapuche called the Pleiades Ngauponi which in the vicinity of the we tripantu (mapuche new year) will disappear by the West, lafkenmapu or ngulumapu, peering at dawn to the East, a few days before the birth of new life in nature. Heliacal rising of Ngauponi , i.e. appearance of the Pleiades by the horizon over an hour before the Sun aproximately 12 days before the winter solstice announced We tripantu
The corresponding rising of a celestial body above the eastern horizon at nightfall is called its acronychal rising.
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