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Heliacal rising: Map

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The heliacal rising of a star (or other body such as the moon, a planet or a constellation) occurs when it first becomes visible above the eastern horizon for a brief moment just before sunrise, after a period of a year when it appeared to be either farther ahead of or behind the sun.

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 (one degree per day) over the dome of the pre-dawn sky, until eventually it is no longer be visible in the sky at dawn because it has already set below the western horizon. This is called the cosmical setting . 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.

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; others may never be visible at all (like the North Star in Australia).

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 Zealandmarker, the Pleiades are called Matariki 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 is called its acronychal rising.

References

  1. John Britton's & Christopher Walker's chapter 'Astronomy and Astrology in Mesopotamia' in "Astronomy before the telescope", 1969, British Museum Press, Pg 48
  2. Show Me a Dawn, or "Heliacal," Rising
  3. rising and setting of stars
  4. Archaic Astronomy and Heliacal Rising



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