Tuesday April 16, 2019

5 steps to a line of position.

STEP 1: Determine Ho

a). Take your sextant sight. That is Hs (Hight of sextant)
b). Correct for Index Error and Dip ( from the Nautical Almanac). This yields Ha (Apparent Altitude).
c). Finally apply your Main Correction (from the Nautical Almanac) to get your Ho (Observed Altitude).

Note: The ONLY difference between the sun, stars, planets, moon will be the step from Ha to Ho, as the Main Corrections will differ.

STEP 2: Determine the GP of the celestial body.

Using the current nautical almanac, determine the Declination (Dec) and Greenwich Hour Angle (GHA).

STEP 3: In order to enter publication 229 (Sight Reduction Tables for Marine Navigation), you will need

a). LHA ( Local Hour Angle)
– Remember in the western hemisphere, LHA is GHA minus Longitude, in the eastern hemisphere it is GHA plus longitude.
b). Latitude
c). Declination

These values Must be to a whole degree. The “extra” minutes from the declination will be corrected with the d value in Step 4.

STEP 4: From publication 229, determine

a). Hc. (Calculated Height)
b). The d value (a correction for the extra minutes from the declination used in step 3. This value will be added/subtracted fro the Hc value).
c). Z

Convert Z to Zn using the rule found on each page of pub 229.


a). Compare Hc with Ha (use HoMoTo or CGA)
b). Plot

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