coordinate - Why is right ascension measured in hours?

In order to know when a star will be above horizon, you'll need an equation with times, not "celestial latitudes".

So you end up adding RA and sidereal time.

EDIT

In order to find out the position of a star you need to find out how its Hour Angle varies with time.

The Hour Angle is how far the star is from the observer's meridian:

In this image, it is the angle between the site's celestial meridian (North Celestial P ole-Z enith-S outh horizon) and the star's celestial meridian (P-X-Y). It is the same angle as between where X culminates and X.

So it comes than $HA=ST-RA$ and since you may not have a "local sidereal time" clock at hand, you can use Greenwich's Sidereal Time: $HA=GST-Lat-RA$

• $HA$ (local) Hour Angle of a star


• $ST$ Local Sidereal Time


• $GST$ Greenwich Sidereal Time


• $RA$ Right Ascension of a star


• $Lat$ (local) Geographical Latitude