Hour Angle
Hour angle is an astronomy coordinate that tells how far a celestial object is from crossing an observer's meridian.
What hour angle is
Hour angle measures how far a celestial object has moved east or west of an observer's meridian. The meridian is the north-south great circle passing through the zenith and the celestial poles. When a star, planet, or spacecraft direction is on that line, its local hour angle is zero.
A local coordinate
Unlike right ascension, hour angle depends on where and when the observer is located. Two observatories at different longitudes can see the same object with different hour angles at the same instant. That local character is exactly why hour angle is useful for pointing an instrument from a particular site.
How it relates to right ascension
Right ascension is a catalog coordinate. Hour angle is a local, time-dependent coordinate. The practical relationship is that local hour angle equals local sidereal time minus the object's right ascension, after wrapping the result into the chosen range. If the result is zero, the object is crossing the meridian.
Before and after transit
Astronomers often talk about an object transiting, culminating, or crossing the meridian. A negative or eastern hour angle means the object has not reached the meridian yet. A positive or western hour angle means it has already crossed. Sign conventions can vary by context, so software and observing guides should state the convention they use.
Why it uses time units
Hour angle is commonly expressed in hours because the sky appears to turn once around in roughly a sidereal day. A full circle is 24 hours, so one hour of hour angle corresponds to 15 degrees. This makes it easy to connect angular distance from the meridian with time before or after transit.
Use in telescope mounts
Equatorial telescope mounts use axes aligned with Earth's rotation and the declination direction. In that geometry, hour angle and declination are natural pointing coordinates. A telescope can track an object by slowly changing hour angle as Earth rotates, while keeping the object's declination nearly fixed for ordinary tracking.
Greenwich and local versions
Hour angle can be measured from a local meridian or from the Greenwich meridian. Local hour angle is the form most useful to an observer at a site. Greenwich hour angle appears in navigation and almanac contexts where Greenwich is the shared reference meridian.
Why it matters
Hour angle translates a star catalog position into the sky a local observer actually sees. It connects sidereal time, right ascension, meridian transit, telescope tracking, celestial navigation, and older observatory practice. It is one of the small pieces of astronomy that turns a coordinate on paper into a direction in the sky.