The time measure, also hour measure, is a common indication of an angle in astronomy in the units hour, minute and second. The full angle of 360° is divided into 24 hours. One hour therefore corresponds to an angle of 15°. The time measure is used exclusively for angles in the equatorial plane. It establishes a connection between the apparent change in position of a celestial body under the Earth’s rotation and the time elapsed in the process. For example, the hour angle of the sun increases by (on average) 15° or 1 (angular) hour in one solar hour. To the right ascension angle 15° of a star corresponds that star hour which passes between the passage of the vernal equinox and that of the star through the celestial meridian.
For example, 1h 23m 45s (in hour measure) and 20° 56′ 15″ (in degree measure) mean the same angle. The abbreviations prime ′ and double prime ″ are not used for minutes and seconds of the hour measure to avoid confusion with the arc minutes and seconds of the degree measure. To distinguish them from the time units of the same name, the abbreviations h, m and s are superscripted for angular units.
|Time||Angle in |
|Angle in |
|Angle in arc dimension|
|1 day||24h||360°||2π ≈ 6,283|
|1 hour||1h||15°||π⁄12 ≈ 0,262|
|4m||1°||π⁄180 ≈ 0,0175 = 1,75 · 10−2|
|1 minute||1m||15′= 1⁄4°||π⁄720 ≈ 0,00436 = 4,36 · 10−3|
|4s||1′ = 1⁄60° (Bogenminute)||π⁄10800 ≈ 0,000295 = 2,95 · 10−4|
|1 second||1s||15″ = 1⁄240°||π⁄43200 ≈ 0,0000727 = 7,27 · 10−5|
|1⁄15s = 0,0667s||1″ = 1⁄3600° (Bogensekunde)||π⁄648000 ≈ 0,00000485 = 4,85 · 10−6|