hour (hr)

An hour is one twenty-fourth part of a day, equal to 60 minutes or 3,600 seconds. In one hour, the Earth rotates through 15°. The time of day at any point on Earth is expressed as the number of hours and minutes that have elapsed since midnight for the time zone in which the point is situated, the time zones being fixed intervals behind or ahead of Greenwich Mean Time.


The hour is also the unit of right ascension, equivalent to 15 degrees of arc.