View source for Heliacal rising

{{Short description|Rising of stars prior to sunrise}}
The '''heliacal rising''' ({{IPAc-en|h|ɪ|ˈ|l|aɪ|.|ə|k|əl}} {{respell|hih|LY|ə|kəl}})<ref name=OED>{{cite OED|heliacal}}</ref><ref>{{cite Merriam-Webster|heliacal}}</ref><ref>{{cite dictionary.com|heliacal}}</ref> of a [[star]] or a [[planet]] occurs annually, when it  becomes visible above the eastern [[horizon]] at [[dawn]] in the brief moment just before [[sunrise]] (thus becoming "the [[Morning Star (disambiguation)#Astronomy|morning star]]").{{efn|
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 {{grey|[overwhelming light of the]}} Sun's presence. That one special morning is called the star's heliacal rising.<ref name=Stanford-show-me/>
}}<ref name=Stanford-show-me>{{cite web |title=Show me a dawn, or "heliacal", rising |publisher=[[Stanford University]] |url=https://solar-center.stanford.edu/AO/dawn-rising.html }}</ref> A heliacal rising marks the time when a star or planet becomes visible for the first time again in the [[night sky]] after having set with the Sun at the western horizon in a previous [[sunset]] (its '''heliacal setting'''), having since been in the [[sky]] only during [[daytime]], obscured by sunlight.

Historically, the most important such rising is that of [[Sirius]], which was an important feature of the [[Egyptian calendar]] and [[Egyptian astronomy|astronomical development]]. The rising of the [[Pleiades]] heralded the start of the [[Ancient Greek]] sailing season, using [[celestial navigation]],<ref>{{cite OED|Pleiad}}</ref> as well as the farming season (attested by [[Hesiod]] in his [[Works and Days]]). Heliacal rising is only one of several types of alignment for stars' risings and settings; mostly the risings and settings of celestial objects are organized into lists of morning and evening risings and settings. [[Culmination]] in the evening and the culmination in the morning are separated by half a year, while on the other hand risings and settings in the evenings and the mornings are only separated by a half-year at the equator, and at other latitudes set apart by different fractions of the year.

== Cause and significance ==
[[File:Kepler_Observatory_Linz_under_the_Stars.jpg|thumb|Sirius is the fixed star with the greatest [[apparent magnitude]] and one which is almost non-variable. The [[Pleiades]], a key feature of [[Taurus (constellation)|Taurus]] shown across [[Orion (constellation)|Orion]] in the same photograph also experience an annual period of visibility ("rising and setting"). Photo taken at sunset.]]

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. 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''.<ref>{{Cite web |title=rising and setting of stars |website=sizes.com |url=https://www.sizes.com/time/risesetstar.htm }}</ref>

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 year|solar]] and [[sidereal year]]s 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 [[zodiac]] in about 26,000 years due to [[precession of the equinoxes]].

Because the heliacal rising depends on the observation of the object, its exact timing can be dependent on weather conditions.<ref>{{Cite web|url=https://archaeoastronomy.wordpress.com/2005/09/10/archaic-astronomy-and-heliacal-rising/|title=Archaic Astronomy and Heliacal Rising|date=September 10, 2005}}</ref>

Heliacal phenomena and their use throughout history have made them useful points of reference in [[archeoastronomy]].<ref name="Schaefer 1987 pp. S19–S33">{{cite journal | last=Schaefer | first=Bradley E. | title=Heliacal Rise Phenomena | journal=Journal for the History of Astronomy | publisher=SAGE Publications | volume=18 | issue=11 | year=1987 | issn=0021-8286 | doi=10.1177/002182868701801103 | pages=S19–S33| s2cid=116923139 }}</ref>

== Non-application to circumpolar stars ==
Some stars, when viewed from [[latitude]]s not at the [[equator]], do not rise or set.  These are [[circumpolar star]]s, 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 [[calendar]]s or [[zodiac]]s. The [[Sumer]]ians, [[Babylonia]]ns, [[ancient Egyptians|Egyptians]], and [[Ancient Greece|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 (calendar)|Julian year]] of 365.25 days.<ref name=teacosy>{{harvp|Tetley|2014|p=[https://web.archive.org/web/20171114202029/http://www.egyptchronology.com/uploads/2/6/9/4/26943741/ch_3_investigating_ancient_egyptian_calendars.pdf 42]}}.</ref> Since the development of [[civilization]], this has occurred at [[Cairo]] approximately on July 19 on the [[Julian calendar]].<ref name=lavia>{{citation |contribution-url =https://web.archive.org/web/20131225071503/https://www.lavia.org/english/archivo/egyptiancalendaren.html|contribution=Ancient Egyptian Civil Calendar |title = La Via |url = https://www.lavia.org/indexIT.html |access-date=8 February 2017 }}.</ref>{{efn|The exact date varies with [[latitude]], so that Sirius's return is observed about 8–10 days later on the [[Mediterranean Sea|Mediterranean]] coast than at [[Elephantine|Aswan]].<ref name=teatime>{{citation |last=Tetley |first = M. Christine |title = The Reconstructed Chronology of the Egyptian Kings, ''Vol. I'' |url = http://www.egyptchronology.com/vols-1--2.html |year = 2014 |page = [http://www.egyptchronology.com/uploads/2/6/9/4/26943741/ch_3_investigating_ancient_egyptian_calendars.pdf 43] |access-date=2017-02-09 |archive-url = https://web.archive.org/web/20170211075421/http://www.egyptchronology.com/vols-1--2.html |archive-date=2017-02-11 |url-status=dead }}.</ref> Official observations were made at [[Heliopolis (ancient Egypt)|Heliopolis]] or [[Memphis, Egypt|Memphis]] near [[Cairo]], [[Thebes, Egypt|Thebes]], and [[Elephantine]] near [[Aswan]].<ref name=teatime/> 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.}} Its returns also roughly corresponded to the onset of the [[flooding of the Nile|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&nbsp;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 (constellation)|Leo]], where it is around August 21 in the 2020s.) The ancient Egyptians appear to have constructed their 365-day [[Egyptian calendar|civil calendar]] at a time when [[Wep Renpet (month)|Wep Renpet]], its [[New Year]], corresponded with Sirius's return to the night sky.<ref name=teacosy/> Although this calendar's lack of [[leap year]]s caused the event to shift one day every four years or so, [[Egyptian astronomy|astronomical records]] of this displacement led to the discovery of the [[Sothic cycle]] and, later, the establishment of the more accurate [[Julian calendar|Julian]] and [[Alexandrian calendar]]s.

The Egyptians also devised a method of telling the time at night based on the heliacal risings of 36 [[Decans|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 people|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 (astronomy)|conjunction]] with the sun beforehand. Depending on the type of conjunction, there may be a [[Syzygy (astronomy)|syzygy]], [[eclipse]], [[Transit (astronomy)|transit]], or [[occultation]] of the sun.

==Acronycal and cosmic(al)==
The rising of a planet above the eastern horizon at [[sunset]] is called its ''[[Wiktionary:acronycal|acronycal]] rising'', which for a [[superior planet]] signifies an [[opposition (planets)|opposition]], another type of [[syzygy (astronomy)|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]].

''[[Wiktionary:cosmical|Cosmic(al)]]'' can refer to rising with sunrise or setting at sunset, or the first setting at morning twilight.<ref>{{Cite journal|url=https://ui.adsabs.harvard.edu/abs/2012AAS...21915001H|title=Acronical Risings and Settings|first=Thomas A.|last=Hockey|journal=American Astronomical Society Meeting Abstracts #219 |date=January 1, 2012|volume=219|pages=150.01|bibcode=2012AAS...21915001H |via=NASA ADS}}</ref>

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.<ref name="Robinson"/><ref name="ESPaCE"/> The following table gives an overview of the different application of the terms to the rising and setting instances.

{| class="wikitable" 
|-
! Daytime
! Visibility
! Rising (east)
! Setting (west)
|-
| rowspan="2" |Morning (matutinal)
| True (in daylight)
| Cosmical
| Acronycal<ref name="ESPaCE"/>/Cosmical<ref name="Robinson"/>
|-
| Apparent (in twilight)
| '''Heliacal <br>(first [[night sky]] appearance)'''
| Heliacal<ref name="ESPaCE"/>/Cosmical<ref name="Robinson"/> <br>(last morning appearance)
|-
| rowspan="2" |Evening (vesper)
| True (in daylight)
| Acronycal
| Cosmical<ref name="ESPaCE"/>/Acronycal<ref name="Robinson"/>
|-
| Apparent (in twilight)
| Heliacal<ref name="ESPaCE"/>/Acronycal<ref name="Robinson"/> <br>(first evening appearance)
| Heliacal <br>(last night sky appearance)
|-
| colspan="4" |<ref name="Robinson">{{cite journal | last=Robinson | first=Matthew | title=Ardua et Astra: On the Calculation of the Dates of the Rising and Setting of Stars | journal=Classical Philology | publisher=University of Chicago Press | volume=104 | issue=3 | year=2009 | issn=0009-837X | doi=10.1086/650145 | pages=354–375| s2cid=161711710 | url=https://discovery.ucl.ac.uk/id/eprint/4840/ | url-access=subscription }}</ref><ref name="ESPaCE">{{cite web | title=Understanding - Rising and setting of stars | website=Encyclopedia FP7 ESPaCE | url=https://promenade.imcce.fr/en/pages6/725.html | access-date=2022-10-29}}</ref>
|}

==See also==
*[[Dog days]]
*[[Steering star]]

== Notes ==
{{Notelist}}

== References ==
{{Wikisource1911Enc|Heliacal}}
{{Reflist}}
{{Portal bar|Astronomy|Stars|Spaceflight|Outer space|Solar System}}
{{DEFAULTSORT:Heliacal Rising}}
[[Category:Observational astronomy]]
[[Category:Stellar astronomy]]
[[Category:Time in astronomy]]
[[Category:Technical factors of astrology]]
[[Category:Egyptian calendar]]