The heliacal rising (/hɪˈlʌɪəkəl/, hi-LUH-ih-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.
Some stars, when viewed from a particular latitude on Earth, do not rise or set. These are circumpolar stars, which are either always in the sky or never. 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 respective horizons.
History
Constellations containing stars that rise and set were incorporated into early calendars or zodiacs. The Sumerians, Babylonians, Egyptians, and Greeks all used the heliacal risings of various stars for the timing of agricultural activities.
Because of its position about 40° off the ecliptic, the heliacal risings of the bright star Sirius occur over a "Sothic year" almost exactly synchronized with the solar year. Since the development of civilization, this has occurred at Cairo on July 19 on the Julian calendar. Its returns also roughly corresponded to the onset of the annual flooding of the Nile before it was ended by the Aswan Low and High Dams. The ancient Egyptians appear to have constructed their 365-day civil calendar at a time when Wep Renpet, its New Year, corresponded with Sirius's return to the night sky. Although this calendar's lack of leap years caused the event to shift one day every four years or so, astronomical records of this displacement led to the discovery of the Sothic cycle and, later, the establishment of the more accurate Julian and Alexandrian calendars.
The Egyptians also devised a method of telling the time at night based on the heliacal risings of 36 decan stars, one for each 10° segment of the 360° circle of the zodiac and corresponding to the ten-day "weeks" of their civil calendar.
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 of South America called the Pleiades Ngauponi which in the vicinity of the we tripantu (Mapuche new year) will disappear by the west, lafkenmapu or ngulumapu, appearing 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 approximately 12 days before the winter solstice, announced we tripantu.
When a planet has a heliacal rising, there is a conjunction with the sun beforehand. Depending on the type of conjunction, there may be a syzygy, eclipse, transit, or occultation of the sun. The Moon's heliacal rising (a.k.a. the new moon) often determines the start of a month in a lunar calendar, which may have religious or political significance.
The corresponding rising of a celestial body above the eastern horizon at sunset is called its acronychal rising, which for a planet signifies a solar opposition in astrology, another type of syzygy. If the moon has an acronychal rising, it is usually a full moon or potentially a lunar eclipse.