The Nautical Almanac

A celestial navigator measures a celestial body's height at a precisely noted time of observation. Making use of this information requires knowing the GP of the observed object at the time of the measurement. The chief purpose of the Nautical Almanac is therefore to provide the GPs of celestial objects for any date and time of the year.

The main table has the GPs listed, at every hour on the hour for every day of the year, of the Sun, the Moon, Venus, Mars, Jupiter, and Saturn (plus GHA Aries). If the time of an observation is some number of minutes and seconds past an exact hour, one uses other "interpolation" tables in the Nautical Almanac to calculate the amounts the GPs have changed for any number of minutes and seconds past the hour.

The point on the Celestial Equator where the Sun crosses into the Northern Hemisphere (at the first instant of Spring) has passing through it a unique celestial meridian, called "the First Point of Aries". The GHA (Greenwich Hour Angle) measures the angle from the Earth's Prime Meridian Westwards to the meridian of the First Point of Aries.