Editing Heliacal rising (section)

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