Time Angle

In Astronomy, the time angle of a star is defined like the difference between the sidereal Temps and its Right ascension. This expression comes owing to the fact that the angles and the hours are closely dependant. Indeed, the right ascension is the name of an angle, measured in hours compared to an origin; it has a value given for a given star. Sidereal time is also an angle depend on the place of observation, and in particular of its Longitude; it is noted in hours, but it expresses the passage of a celestial meridian line (Right ascension) to the local Méridien: it is the time angle of the vernal Point. The time angle (AH) is defined like:

AH_objet = TSL - $\backslash \; alpha$_objet

where $\backslash \; alpha$ is the right ascension, and TSL indicates the sidereal Temps local. The value of the right ascension of a star does not change appreciably on a short time interval. On the other hand, sidereal time obviously constantly changes because of the diurnal Mouvement. When sidereal time is equal to the right ascension, it is said that the star passes by the meridian line . As it is then in its highest position in the sky for the place considered, it is the best moment to observe it, since the value of the corresponding Masse of air is minimal.

The solar hours marked by a Sundial express the variation of the time angle of the Sun during the day.

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