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.