Sun almanac

Reading the Sun's almanac

GHA, the Greenwich hour angle, is how far west of Greenwich the Sun's ground position is: it grows by about 15° every hour as the Earth turns. Declination is the ground position's latitude, north or south of the equator, and changes by at most about a minute of arc an hour.

The almanac lists both for each whole hour of UT. For the minutes and seconds between, add the increment to the GHA (the Sun moves 15° an hour, so 1° every four minutes), and correct the declination by d, its hourly change, in proportion. Enter a time above and the page shows that working.

The figures are computed from the JPL DE440 ephemeris and laid out as the Nautical Almanac's daily pages are.

It is a learning aid, not a navigation tool: at sea, use the official Nautical Almanac and tables, and check every position by other means.

Learn to use it

• Where the Sun is: GHA and declination: the daily pages, increments and the d correction, with practice.
• Angles, time and UT: why the almanac runs on Greenwich time.
• Sight reduction calculator: use the GHA and declination to reduce a sight.