How to Calculate Declination

Written by will charpentier
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How to Calculate Declination
The sextant, altitude and declination.

The declination of the star is the place on the face of the Earth that is directly under the star. If you could draw a line from the centre of the star to the the centre of the earth that passes through the surface of the earth, the declination would be that place on the surface through which the line passes. Determining the physical declination of a star (or the sun, even) uses a "line of position," drawn along a known bearing for a known or calculated distance. The angle from the equator to the declination of the star is published in the nautical almanac.

Skill level:
Challenging

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Things you need

  • Sextant
  • Nautical almanac
  • Charts
  • Pencil
  • Accurate clock or access to WWV Time Service
  • Calculator

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Instructions

  1. 1

    Use the sextant to determine the altitude of the star above the horizon. Note the time of your observation.

    How to Calculate Declination
    The sextant, altitude and declination.
  2. 2

    Consult the tables of the nautical almanac pertaining to that star at that time on that date to ascertain the navigational declination, or angular distance north or south of the celestial equator.

  3. 3

    Do the math. Since you know the distance from the equator to the declination of the star in degrees, convert those degrees to nautical miles by multiplying by 60. Add the number of minutes to the total, since one minute is 1 nautical mile. This will tell you the distance from your boat to the physical declination.

    How to Calculate Declination
    Converting degrees to nautical miles.
  4. 4

    Draw a bearing line on the chart equal to the azimuth from your boat to the star. The length of the line should equal the distance to the physical declination.

    How to Calculate Declination
    Drawing the line.

Tips and warnings

  • Finding the actual declination of a star or the sun is a good exercise in nautical astronomy.

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