To raise heliacal

In Astronomy, the to raise heliacal of a star is the moment when it becomes visible with the Is above the horizon to the paddle, after one period when it was hidden under the horizon or was located just above the horizon but was drowned by the luminosity of the Sun.

Description

Each day, the Sun seems to shift towards the East compared to stars on the celestial Sphère, along the ecliptic . Consequently, each day which follows to raise it heliacal, a star appears a little earlier and remains visible longer in the sky before being hidden by the Sun. At the end of a certain number of days, the star will not be visible any more in the sky at dawn because it will have passed under the horizon western, which corresponds to its sleeping heliacal.

All the stars do not have to raise heliacal. According to the Latitude of observation, some remain perpetually above the horizon and are thus always visible in the sky at dawn.

The heliacal rising generally relates to a star, but it is also possible to consider it for the the Moon or a Planet.

If one considers the moment when the star raises when the Sun lies down, one speaks about raising acronych (or acronic).

Heliacal year

A star reappears above the horizon east at dawn approximately a Année after its precedent heliacal rising, one period which corresponds to the heliacal year of star.

For a star located on the ecliptic , one year heliacal is very close to a sidereal Année (on average: 365.256363051 D in 2000, is 365 D 6 H 9 min 9 S), put aside the differences due to the own Mouvement of star and to the Précession of the equinoxes. If it is not the case, of the significant differences can appear, as for example for Sirius to which the heliacal year (named sothiaque Année) is close to the Année Julienne (: 365.25 d).

History

In the ancient Egypt, the calendar was based on the to raise heliacal of Sirius. They moreover had conceived a method making it possible to give the hour during the night on the basis of heliacal rising of 36 stars (for each segment of 10° of the circle zodiacal).

The Babylonian Sumériens, and the Greek ancients used the heliacal rising several stars to synchronize the agricultural activities.

At the Māori, in New Zealand, to raise it heliacal of the Pléiades (named Matariki) the beginning of the New Year's Day indicates (about June).

See too

Internal bonds

To raise heliacal of Sirius

External bonds

To raise and lay down stars, to raise heliacal

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