The heliacal rising
of a star
(or other body such as the Moon
or a planet
) occurs when it first becomes visible above the eastern horizon
at dawn, after a period where it was hidden below the horizon or when it was just above the horizon but hidden by the brightness of the Sun. Each day after the heliacal rising, the star will appear to rise slighly earlier and remain in the sky longer before it is hidden by the Sun (the Sun appears to drift east relative to the stars along a path called the ecliptic
). Eventually the star will be no longer visible in the sky at dawn because it has already set below the western horizon: this is called the heliacal setting
. Eventually the star will appear again in the eastern sky at dawn, and this will be approximately one year after the previous heliacal rising.
Not all stars have heliacal risings: some may (depending on the latitude of observation on the Earth) remain permanently above the horizon so will be always visible in the sky at dawn, before they are hidden
by the brightness of the Sun.
The heliacal rising of particular stars has been the basis of various calendars. The ancient Egyptians based their calendar on the heliacal rising of Sirius and devised a method of telling the time at night based on the heliacal rising of stars called decans. The ancient Greeks used heliacal rising of various stars for the timing of agricultural activities.