Hour Angles

Project the Greenwich meridian onto the celestial sphere. The Greenwich hour angle, GHA, is the angle between Greenwich and the position of the object, to the west; the celestial longitude.

ooking down from the pole star..the local hour angle, LHA, is angle between the local meridian, LM, and the object as measured to the west


Declination is the celestial equivalent of latitude. The declination of any star is constant for example the declination of Polaris is constantly 90 N. The declination of the Sun is 23 North at midsummer and 23 South at midwinter. The path of the Sun across the celestial sphere is called the ecliptic. The point at which the Sun will cross the celestial equator at the Spring equinox defines the First Point of Aries. The SIDERIAL HOUR ANGLE is measured westwards from Aries around the celestial equator towards the object: sun, moon, star or planet.

The values of GHA, LHA, SHA and declination for a given time are the tools which, when applied to the sight reduction tables, yield the azimuth and intercept values to construct a position line.

We do not have to understand the mathematics of the method in order to use the tables. For a  mathematical analysis follow the link to Henning Umland on the contacts page.