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Heliacal rising


The heliacal rising (/hɪˈləkəl/, hi-LY-ə-kəl) of a star occurs annually 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. Historically, the most important such rising is that of Sirius, which was an important feature of the Egyptian calendar and astronomical development.

Relative to the stars, the sun appears to drift eastward about one degree per day along a path called the ecliptic. While the sun is moving past a given star, the star cannot be seen because it is only above the horizon during the day. The heliacal rising occurs when the sun has moved far enough past the star that the star rises and becomes visible before the sun rises in the morning. Each day after the heliacal rising, the star will rise slightly earlier and remain visible for longer before the light from the rising sun overwhelms it. Over the following days the star will move further and further westward (about one degree per day) relative to the sun, until eventually it is no longer visible in the sky at sunrise 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. For stars near the ecliptic, the small difference between the solar and sidereal years will cause their heliacal rising to wander through the calendar over long enough periods of time. Because the heliacal rising depends on the observation of the object, its exact timing can be dependent on weather conditions.


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Wikipedia

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