Theheliacal rising (/hɪˈlaɪ.əkəl/hih-LY-ə-kəl)[1][2][3] of astar or aplanet occurs annually when it becomes visible above the easternhorizon atdawn just beforesunrise (thus becoming "themorning star").[4] A heliacal rising marks the time when a star or planet becomes visible for the first time again in thenight sky after having set with the Sun at the western horizon in a previoussunset (itsheliacal setting), having since been in thesky only duringdaytime, obscured by sunlight.
Historically, the most important such rising is that ofSirius, which was an important feature of theEgyptian calendar andastronomical development. The rising of thePleiades heralded the start of theAncient Greek sailing season, usingcelestial navigation,[5] as well as the farming season (attested byHesiod in hisWorks and Days). Heliacal rising is one of several types of risings and settings, mostly they are grouped into morning and evening risings and settings of objects in the sky.Culmination in the evening and then morning is set apart by half a year, while on the other hand risings and settings in the evenings and the mornings are only at the equator set apart by half a year.
Relative to the stars, the Sun appears to drift eastward about one degree per day along a path called theecliptic because there are 360 degrees in any complete revolution (circle), which takes about 365 days in the case of one revolution of the Earth around the Sun. Any given "distant" star in the belt of the ecliptic will be visible at night for only half of the year, when it will always remain below the horizon. During the other half of the year it will appear to be above the horizon but not visible because the sunlight is too bright during the day. The star's heliacal rising will occur when the Earth has moved to a point in its orbit where the star appears on the eastern horizon at dawn. 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 theacronycal setting.[6]
The same star will reappear in the eastern sky at dawn approximately one year after its previous heliacal rising. For stars near theecliptic, the small difference between thesolar andsidereal years due toaxial precession will cause their heliacal rising to recur about onesidereal year (about 365.2564 days) later, though this depends on itsproper 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 thezodiac in about 26,000 years due toprecession of the equinoxes.
Because the heliacal rising depends on the observation of the object, its exact timing can be dependent on weather conditions.[7]
Heliacal phenomena and their use throughout history have made them useful points of reference inarcheoastronomy.[8]
Some stars, when viewed fromlatitudes not at theequator, do not rise or set. These arecircumpolar stars, which are either always in the sky or never. For example, theNorth Star (Polaris) is not visible in Australia and theSouthern Cross is not seen in Europe, because they always stay below the respective horizons.
The termcircumpolar 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 earlycalendars orzodiacs. TheSumerians,Babylonians,Egyptians, andGreeks 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 starSirius inAncient Egypt occurred not over a period of exactly onesidereal 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 aJulian year of 365.25 days.[9] Since the development ofcivilization, this has occurred atCairo approximately on July 19 on theJulian calendar.[10][a] Its returns also roughly corresponded to the onset of theannual flooding of theNile, 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 theproleptic Gregorian Calendar. At that time, the sun would be somewhere nearRegulus inLeo, where it is around August 21 in the 2020s.) The ancient Egyptians appear to have constructed their 365-daycivil calendar at a time whenWep Renpet, itsNew Year, corresponded with Sirius's return to the night sky.[9] Although this calendar's lack ofleap years caused the event to shift one day every four years or so,astronomical records of this displacement led to the discovery of theSothic cycle and, later, the establishment of the more accurateJulian andAlexandrian calendars.
The Egyptians also devised a method of telling the time at night based on the heliacal risings of 36decan 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 theMāori ofNew Zealand, thePleiades are calledMatariki, and their heliacal rising signifies the beginning of the new year (around June). TheMapuche ofSouth America called the PleiadesNgauponi which in the vicinity of thewe tripantu (Mapuche new year) will disappear by the west,lafkenmapu orngulumapu, 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, announcedwe tripantu.
When a planet has a heliacal rising, there is aconjunction with the sun beforehand. Depending on the type of conjunction, there may be asyzygy,eclipse,transit, oroccultation of the sun.
The rising of a planet above the eastern horizon atsunset is called itsacronycal rising, which for asuperior planet signifies anopposition, another type ofsyzygy. When the Moon has an acronycal rising, it will occur nearfull moon and thus, two or three times a year, a noticeablelunar eclipse.
Cosmic(al) can refer to rising with sunrise or setting at sunset, or the first setting at morning twilight.[12]
Risings and settings are furthermore differentiated betweenapparent (the above discussed) and actual ortrue risings or settings.
The use of the terms cosmical and acronycal is not consistent.[13][14] The following table gives an overview of the different application of the terms to the rising and setting instances.
| Daytime | Visibility | Rising (east) | Setting (west) |
|---|---|---|---|
| Morning (matutinal) | True (in daylight) | Cosmical | Acronycal[14]/Cosmical[13] |
| Apparent (in twilight) | Heliacal (firstnight sky appearance) | Heliacal[14]/Cosmical[13] (last morning appearance) | |
| Evening (vesper) | True (in daylight) | Acronycal | Cosmical[14]/Acronycal[13] |
| Apparent (in twilight) | Heliacal[14]/Acronycal[13] (first evening appearance) | Heliacal (last night sky appearance) | |
| [13][14] | |||
Heliacal risings occur after a star has been behind the Sun for a season and it is just returning to visibility. There is one morning, just before dawn, when the star suddenly reappears after its absence. On that day it "blinks" on for a moment just before the sunrise and just before it is then obliterated by the Sun's presence. That one special morning is called the star's heliacal rising.
