Heliacal Rise


As the celestial sphere rotates, every star moves constantly around the line of a particular “latitude” or declination. This means that from any place on earth, every star that is not circumpolar will rise and set in the same positions, day in and day out, year in and year out (although this is not the case on longer time scales). However, since the (diurnal) rotation period is slightly less than a day—approximately four minutes shorter—this means than the time of rising or setting will not remain the same but will be approximately four minutes earlier each day.

For any given place and any given star (apart from circumpolar ones), there will be a time of year, typically lasting a few weeks, when that star is not visible in the night sky at all, because it is rising at approximately the same time as the sun and setting at about the same time as the sun. In other words, it is up in the sky during the day, when it cannot be seen, and below the horizon at night. Gradually, however, the rising time will progress earlier and earlier each day until there comes a day when the star can be seen briefly in the morning sky before the sky brightens too much prior to sunrise. This is known as the heliacal rise. After this, the star will rise progressively earlier, getting ever higher in the sky before it is lost in the pre-dawn twilight. Eventually, the star will reach a point, typically five months or so after the date of heliacal rise, where it crosses the sky from east to west during the night and can just be seen to set before the sky brightens. This first visible setting is often referred to as the acronical set, although the term strictly applies when the star sets at sunrise, an event that will already have passed unnoticed some days earlier. At around the same date, the star’s rising time will have moved back through the night and be approaching the previous sunset. The last time the star can be seen to rise before this occurrence becomes lost in the evening twilight is often referred to (again loosely) as the acronical rise. During the ensuing months the star will be rising in daylight but setting ever earlier during the night. Finally, the day will come when it can only be seen briefly in the evening sky after sunset before itself setting, after which it will once again become invisible. This is known as the heliacal set.

This general series of events—disappearance, heliacal rise, acronical rise/set, heliacal set, and then disappearance again—holds for all stars apart from circumpolar ones, which never rise or set. On the other hand, details such as the length of disappearance, and whether acronical rise occurs before or after acronical set, will depend upon the brightness of the star, the latitude of the place, the declination of the star, and even atmospheric conditions (since these can affect the visibility of a star in the evening or morning twilight). Nonetheless, for any particular star and place of observation, the approximate dates of the various events can be reliably specified.

The terms heliacal rise and heliacal set are also applied to planets and have the same meaning of “first appearance in the pre-dawn sky” and “last appearance in the evening sky,” respectively. However, the apparent motions of the planets are more complex than those of the stars, and those of inferior and superior planets are fundamentally different from each other.

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