Hour angle

The hour angle and the declination are the coordinates of a star in the fixed equatorial coordinate system with which its position can be described in terms of an observation site on the earth. One speaks of the hour angle, because it is, among other things for the time measurement and is therefore usually given in the time dimension. It corresponds to the time that has elapsed since the last culmination of the star.

Definition

The hour angle of a celestial body is the angle between two planes:

• Of the great circle through the celestial poles ( hour circle ) and the celestial body and the plane spanned
• The plane spanned by the meridian plane.

The hour angle is measured in the plane of the celestial equator from the meridian in the direction of the apparent diurnal motion of the celestial sphere, ie positive in a westerly direction (for a south-facing observer " to the right" ) and negative in an easterly direction ( " left "). If a heavenly body over the meridian, it has the hour angle zero at this moment.

Hour angle of some celestial bodies

Most celestial bodies - in particular, the star - move in the sky seemingly westward. Your hour angle is always growing in a positive direction. Some earth satellites orbiting the earth from west to east, with the hour angle increases in the negative direction. The hour angle of geostationary satellites remains constant.

The hour angle of a star grows in a sidereal day of 23h 56m ( equal to 24 hours sidereal time ) to 360 ° in an hour so to 15.04 °. As the sun moves relative to the star by about one degree per day in an easterly direction, their hour angle is growing slightly slower than that of a star. It grows in the central 15 ° per hour (periodic small deviation from the mean: see equation of time ). This smooth numerical value follows from the original definition of the hour as 1/24 of a solar day, the period between two meridian crossings of the Sun, in which her ​​hour angle increases by 360 °. During 1/24 of this period, the hour angle increases by 360 ° / 24 = 15 °.

And the angle h of a star can be used to measure time. It is equal to the time elapsed since the last meridian passage of the star sidereal time.

Due to the suitability for timing the hour angle is measured in degrees instead often also in the measure of time (in hours, minutes, and seconds) specified (15 ° = 1 hour). This is true not only for the sun, but also for the stars. When we are familiar with solar time a change in angle of the sun of 15 ° occurs in one hour solar time, when used by astronomers Stardate happens a change in angle of a star of 15 ° in an hour sidereal time. The time minutes and seconds of time used in both cases should not be confused with the arc minutes and arc seconds of a given angle in degrees.

Hour angle and equatorial mount

Equatorial Mounts of astronomical observation instruments allow the separate measurement of the fixed equatorial coordinate hour angle and declination. They each contain a rotation axis in order to follow the change of the coordinates. The hour angle changes smoothly as with stars, so the instrument can automatically by means of a drive to the celestial axis parallel to the right ascension (or hour axis ) tracking. Because the declination of the star does not change in the observation of which the declination axis can be locked.

Hour angle and right ascension

Hour angle and right ascension are the mutually corresponding coordinates in the stationary or rotating in the equatorial coordinate system. Reference plane of the right ascension angle is not the fixed meridian plane, but the rotating plane, which is spanned by the great circle through the vernal point and the celestial poles. The right ascension is positive in an easterly direction - that is opposite to the hour angle - counted. The declination is the same in both systems, because it relates both times to the equatorial plane.

Hour angle and right ascension are the sidereal time linked to each other:

The hour angle of the vernal equinox is identical with the sidereal time at the site. It is null if the fictitious spring equinox the ( local ) Meridian happens. At this moment, hour angle and right ascension are the same for all celestial bodies are different, only the sign.

The conversion between hour angle and right ascension is described inter alia in the main article Astronomical coordinate systems.