Heliacal rising

Sirius is the fixed star with the greatest apparent magnitude and one which is almost non-variable. The Pleiades, a key feature of Taurus shown across Orion in the same photograph also experience an annual period of visibility ("rising and setting").

Relative to the other stars, the Sun appears to drift eastward (about ​¹⁄_365.24219 of Earth's orbit—hence almost one degree—per solar day) along a path called the ecliptic (specifically appearing in front of 12 constellations considered the zodiac constellations from a total of 88 modern constellations) which is, by definition, the plane of the earth's orbit. While the Sun appears in front of (or south or north of) a relatively small group of stars they can no longer be seen either before dawn, during daytime or after sunset[lower-alpha 1]—their appearance coincides with that of the Sun above the horizon.

Depending on the observer's latitude many stars are subject to annual heliacal risings and settings. Rising means the local latitude of the earth has moved along its orbit such that the star, star cluster or galaxy emerges for part of the year to within months be visible for the whole night and then for the early portion of the night. Thus the star's first emergence, an annual rise, is immediately before dawn. The risen status of each star is easiest considered day-on-day in the tropics where the time of dawn varies less.

The rising of a star which has had its annual rising ("heliacal rising"), typically over months, rises earlier at night and so at dawn figures more toward its annual highest point (meridian) and then in later dawns more toward the west all by about ​¹⁄₁₈₂ of its arc (about ​¹⁄₃₆₅ of the circle) per day, until, observing the western sky after sunset, it has already disappeared. This is called the cosmical setting.[4] The same star will reappear in the eastern sky at dawn approximately one year after its previous rising. For zodiac and near-zodiac constellations (near the ecliptic, the apparent daily path of the Sun), Earth's precession means the date of their rising increases gradually, completing one cycle in about 26,000 years (for example at the March Equinox, the position of the Sun relative to the stars—at which Right Ascension is calibrated as zero, the First Point of Aries, is in the preceding constellation of Pisces.)[5]

Some stars, when viewed from latitudes not at the equator, do not rise or set. These are circumpolar stars, which are either always in the sky or never. For example, the North Star (Polaris) is not visible in Australia and the Southern Cross is not seen in Europe, because they always stay below the respective horizons.

The term circumpolar is somewhat localised as between the Tropic of Cancer and the Equator, the Southern polar constellations have a brief spell of annual visibility (thus "heliacal" rising and "cosmic" setting) and the same applies as to the other polar constellations in respect of the reverse tropic.

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.[lower-alpha 2] Since the development of civilization, this has occurred at Cairo on July 19 on the Julian calendar.[7][lower-alpha 3] 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.[6] 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 rising of a planet above the eastern horizon at sunset is called its acronychal rising, which for a superior planet signifies an opposition, another type of syzygy.

When the Moon has an acronychal rising, it will occur near full moon and thus, in a small number of cases, a noticeable lunar eclipse.

• Dog days of summer