Heliacal rising

The heliacal rising (/hɪˈlaɪəkəl/ hil-EYE-ə-kəl)^[1][2][3] or star rise of a star occurs annually, or the similar phenomenon of a planet, when it first becomes visible above the eastern horizon at dawn just before sunrise (thus becoming "the morning star"), after it has spent a season behind the sun rendering it invisible.^[4] Historically, the most important such rising is that of Sirius, which was an important feature of the Egyptian calendar and astronomical development. The rising of the Pleiades heralded the start of the Ancient Greek sailing season, using celestial navigation.^[5]

Cause and significance[]

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 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.^[6]

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 due to axial precession will cause their heliacal rising to recur about one sidereal year (about 365.2564 days) later, though this depends on its proper motion. For stars far from the ecliptic, the period is somewhat different and varies slowly, but in any case the heliacal rising will move all the way through the tropical year (i.e. the seasons) in about 26,000 years.

Because the heliacal rising depends on the observation of the object, its exact timing can be dependent on weather conditions.^[7]

Non-application to circumpolar stars[]

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.

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 in Ancient Egypt occurred not over a period of exactly one sidereal year but over a period called the "Sothic year" (from "Sothis", the name for the star Sirius), The Sothic year was about a minute longer than a Julian year of 365.25 days.^[8] Since the development of civilization, this has occurred at Cairo approximately on July 19 on the Julian calendar.^[9][a] Its returns also roughly corresponded to the onset of the annual flooding of the Nile, although the flooding is based on the tropical year and so would occur about three quarters of a day earlier per century in the Julian or Sothic year. (July 19, 1000 BC in the Julian Calendar is July 10 in the proleptic Gregorian Calendar. At that time, the sun would be somewhere near Regulus in Leo, where it is around August 21 in the 2020s.) 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.^[8] 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 (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.

Acronycal and cosmic(al)[]

The rising of a planet above the eastern horizon at sunset is called its acronycal rising, which for a superior planet signifies an opposition, another type of syzygy. When the moon has an acronycal rising, it will occur near full moon and thus, two or three times a year, a noticeable lunar eclipse. The last observation of rising at evening twilight after season of earlier risings is also distinguished as "acronycal rising". Cosmic(al) can refer to rising with sunrise or setting at sunset, or the first setting at morning twilight.^[11]

The apparent acronycal and cosmic rising or setting is the observal position before or after the actual acronycal and cosmic rising or setting.

See also[]

• Dog days of summer

Notes[]

References[]

Wikisource has the text of the 1911 Encyclopædia Britannica article "Heliacal".