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Interactive Nautical Almanac

The Nautical Almanac available here, tabulates the coordinates of the Geographical Position of the selected Celestial Object (in Greenwich Hour Angle and Declination) for each integral hour of the year. Time is recorded in Universal Time (UT).


On activating the almanac tables from the links below (see "Interactive Tables") the almanac data with the Greenwich Hour Angle (GHA) and Declination (Dec) for 6 successive days will be generated in a separate window. This window also contains navigation buttons to display and print appropriate parts of the Nautical Almanac.

Beside the coordinates of the Geographical Position also the following information is listed for each day:

  • the Greenwich Culmination Time (GCT). This is the time in UT at which the selected Celestial Object "passes" the Prime Meridian at Greenwich (at this moment the GHA is 0°).
  • the Semi-Diameter (SD) and the Horizontal Parallax (HP) (both relevant for the Sun and Moon) are given in a four-hour interval (02:00 UT through 22:00 UT) .

The Semi-Diameter and Horizontal Parallax are the geocentric values as observed at the center of the Earth. The topocentric values as observed on the surface of the Earth may differ slightly from these geocentric values as recorded in the Nautical Almanac. Especially for the Moon, due to its close proximity to the Earth, there is a measurable difference in Diameter and Parallax Effect between geocentric and topocentric positions.

Augmentation of the Moon

As explained above, the apparent diameter of the Moon measured on the surface of the Earth (topocentric diameter) is different from the geocentric value recorded in the Nautical Almanac. The difference in Semi-Diameter can be as large as 0.3' when the Moon is in the Zenith of the observer.
This phenomenon of increased apparent diameter when the Moon is high above the Horizon, is called "augmentation of the Moon".


Interactive Tables

With one of the links below, the interactive Nautical Almanac is started in a new window.


The generated pages should fit on a single A4 or Letter paper format if printed out (this may depend on the configuration of your HTML browser and your printer).

To obtain the correct GHA and Declination for the time of observation the "Interpolation and Conversion Tables" may be useful. For the correction of measured Sextant Angle use the "Correction Tables for Sextant Altitudes".
Both the Interpolation and Correction tables are not restricted to a specific year.

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