Heliacal rising

Source: Wikipedia, the free encyclopedia.

The heliacal rising (/hɪˈl.əkəl/ hih-LY-ə-kəl)[1][2][3] of a star or a planet occurs annually when it first becomes visible above the eastern horizon at dawn just before sunrise (thus becoming "the morning star") after a complete orbit of the Earth around the Sun.[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] as well as the farming season (attested by Hesiod in his Works and Days). Helical 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.

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 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 the acronycal 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

precession 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 in

archeoastronomy.[8]

Non-application to circumpolar stars

Some stars, when viewed from

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

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

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

transit, or occultation
of the sun.

Acronycal and cosmic(al)

The rising of a planet above the eastern horizon at

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
.

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 between apparent (the above discussed) and actual or true risings or settings.

Overview

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
(first night 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]

See also

Notes

  1. ^ The exact date varies with latitude, so that Sirius's return is observed about 8–10 days later on the Mediterranean coast than at Aswan.[11] Official observations were made at Heliopolis or Memphis near Cairo, Thebes, and Elephantine near Aswan.[11] The date at any location also slowly varies within the Gregorian calendar by about three days every four centuries. July 19 of the Julian Calendar occurs on August 1 Gregorian in the 20th and 21st centuries.

References

  1. ^ "heliacal". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
  2. ^ "heliacal". Merriam-Webster.com Dictionary.
  3. ^ "heliacal". Dictionary.com Unabridged (Online). n.d.
  4. ^ "Show Me a Dawn, or "Heliacal," Rising". Stanford University. 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.
  5. ^ "Pleiad". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
  6. ^ rising and setting of stars
  7. ^ Archaic Astronomy and Heliacal Rising
  8. S2CID 116923139
    .
  9. ^ a b Tetley (2014), p. 42.
  10. ^ "Ancient Egyptian Civil Calendar", La Via, retrieved 8 February 2017.
  11. ^ a b Tetley, M. Christine (2014), The Reconstructed Chronology of the Egyptian Kings, Vol. I, p. .com/uploads/2/6/9/4/26943741/ch_3_investigating_ancient_egyptian_calendars.pdf 43, archived from the original on 2017-02-11, retrieved 2017-02-09.
  12. ^ Acronical Risings and Settings
  13. ^
    S2CID 161711710
    .
  14. ^ a b c d e f "Understanding - Rising and setting of stars". Encycopedia FP7 ESPaCE. Retrieved 2022-10-29.