Heliacal rising
Encyclopedia
The heliacal rising of a star
(or other body such as the moon
, a planet
or a constellation
) occurs when it first becomes visible above the eastern horizon
for a brief moment just before sunrise, after a period of time when it had not been visible.
Each day after the heliacal rising, the star will rise slightly earlier and remain visible for longer before the light from the rising sun makes it disappear (the sun appears to drift eastward relative to the stars by about one degree a day along a path called the ecliptic
). Over the following days the star will move further and further westward (one degree per day) over the dome of the pre-dawn sky, until eventually it is no longer visible in the sky at dawn because it has already set below the western horizon. This is called the cosmical setting. The same star will reappear in the eastern sky at dawn approximately one year after its previous heliacal rising. Because the heliacal rising depends on the observation of the object, its exact timing can be dependent on weather conditions.
Some stars, when viewed from a particular latitude
on Earth
, will not have a heliacal rising or setting. Circumpolar star
s remain above the horizon throughout the whole year, making them always visible in the sky at dawn. Conversely, some stars are never seen in some locations. For example, the North Star is not visible in Australia and the Southern Cross is not seen in Europe, because they always stay below the horizon.
Constellations containing stars that rise and set were incorporated into early calendar
s or zodiac
s. The ancient Egypt
ians based their calendar on the heliacal rising of Sirius
and devised a method of telling the time at night based on the heliacal risings of 36 stars called decan stars
(one for each 10° segment of the 360° circle of the zodiac/calendar). The Sumerians, the Babylonians, and the ancient Greek
s also used the heliacal risings of various stars for the timing of agricultural activities. 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 corresponding rising of a celestial body above the eastern horizon at nightfall is called its acronychal rising.
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
(or other body such as the moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
or a constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
) occurs when it first becomes visible above the eastern horizon
Horizon
The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting...
for a brief moment just before sunrise, after a period of time when it had not been visible.
Each day after the heliacal rising, the star will rise slightly earlier and remain visible for longer before the light from the rising sun makes it disappear (the sun appears to drift eastward relative to the stars by about one degree a day along a path called the ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...
). Over the following days the star will move further and further westward (one degree per day) over the dome of the pre-dawn sky, until eventually it is no longer visible in the sky at dawn because it has already set below the western horizon. This is called the cosmical setting. The same star will reappear in the eastern sky at dawn approximately one year after its previous heliacal rising. Because the heliacal rising depends on the observation of the object, its exact timing can be dependent on weather conditions.
Some stars, when viewed from a particular latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, will not have a heliacal rising or setting. Circumpolar star
Circumpolar star
A circumpolar star is a star that, as viewed from a given latitude on Earth, never sets , due to its proximity to one of the celestial poles...
s remain above the horizon throughout the whole year, making them always visible in the sky at dawn. Conversely, some stars are never seen in some locations. For example, the North Star is not visible in Australia and the Southern Cross is not seen in Europe, because they always stay below the horizon.
Constellations containing stars that rise and set were incorporated into early calendar
Calendar
A calendar is a system of organizing days for social, religious, commercial, or administrative purposes. This is done by giving names to periods of time, typically days, weeks, months, and years. The name given to each day is known as a date. Periods in a calendar are usually, though not...
s or zodiac
Zodiac
In astronomy, the zodiac is a circle of twelve 30° divisions of celestial longitude which are centred upon the ecliptic: the apparent path of the Sun across the celestial sphere over the course of the year...
s. The ancient Egypt
Ancient Egypt
Ancient Egypt was an ancient civilization of Northeastern Africa, concentrated along the lower reaches of the Nile River in what is now the modern country of Egypt. Egyptian civilization coalesced around 3150 BC with the political unification of Upper and Lower Egypt under the first pharaoh...
ians based their calendar on the heliacal rising of Sirius
Sirius
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...
and devised a method of telling the time at night based on the heliacal risings of 36 stars called decan stars
Decans
The Decans are 36 groups of stars which rise consecutively on the horizon throughout each earth rotation...
(one for each 10° segment of the 360° circle of the zodiac/calendar). The Sumerians, the Babylonians, and the ancient Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
s also used the heliacal risings of various stars for the timing of agricultural activities. To the Māori of New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
, the Pleiades
Pleiades (star cluster)
In astronomy, the Pleiades, or Seven Sisters , is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky...
are called Matariki
Matariki
In the Māori language Matariki is both the name of the Pleiades star cluster and also of the season of its first rising in late May or early June - taken as the beginning of the new year...
and their heliacal rising signifies the beginning of the new year (around June).
The corresponding rising of a celestial body above the eastern horizon at nightfall is called its acronychal rising.