Declination.  Declination corresponds to terrestrial latitude and is defined as the angular distance of a heavenly body north or south of the celestial equator, measured in units of arc (ie degrees, minutes and seconds).  Declination is the result of the earth’s movement/orbit around the sun and the fact that the earth’s poles are not perpendicular to the plane of that orbit, ie the plane of the earth’s rotation around it’s axis does not coincide with the plane of the earth’s orbit around the sun.

This change in inclination of the earth’s axis with respect to the sun results in the seasons we experience during the year. As can be seen from the above diagram the declination of the sun changes from 23½° North to 23½° South and back again in 12 months.

Because of their great distance from the earth, the declination of the stars changes very slowly and may be considered constant for up to one month.

Since they are relatively close to the earth, the declination of the four navigational planets (Venus, Mars, Jupiter and Saturn) changes between fairly wide limits.

Tabulation of declination.
The declination of the sun and planets are tabulated on the daily pages of the Nautical Almanac for every hour of GMT/UT (Greenwich Mean Time/ Universal Time). To find their values at any other time, interpolation is required. To assist, a quantity “d” is tabulated at the foot of the column for the body concerned. “d” is the mean hourly difference for that page and the amount to be added or subtracted from the hourly value is found in the increment/correction tables at the back of the almanac. Each page covers two minutes in time and covers increments to the tabulated GHA (of the sun, planets, Aries and moon) and the corrections to be applied to the declination. The latter correction is found under the heading “v or d corrn”.The actual correction to be applied is obtained by entering the table under the heading “v or d” with the tabulated mean hourly difference and reading the amount next to it under the heading “corrn”

The moon’s declination varies less uniformly and more quickly than that of the sun and planets. It is tabulated in the same way but the hourly difference has to be given for every hour. The correction is obtained from the interpolation tables in the same way as the others.

For the purpose of this course, only the declination of the sun will be considered.