Lunar Navigation Calculations

Calculation of the Hour Angle and Declination of the Moon

Determine the hour angle and declination for January 1 at 01:20 UTC.

Step 1
Locate the nearest value of E☾ in the Nautical Almanac and ensure that it precedes the observation time.
For example, to determine E☾ at 01:20, the value at 01:00 should be used rather than 01:30.

Step 2
Since E☾ is tabulated at 30-minute intervals and varies significantly, obtain the correction for the desired time using the P.P. table in the right-hand column, and apply it to the selected value of E☾.

Step 3
Add the observation time (UTC) to the corrected value of E☾ to obtain the hour angle.

Step 4
The resulting hour angle is expressed in units of time.
To convert it into degrees, multiply by 15.

Step 5
The declination is obtained in the same manner, using interpolation as required.

Calculation of the Meridian Passage Time of the Moon

Determine the meridian passage time of the Moon at 140°E on January 1.

Step 1
Since the Moon moves more rapidly than other celestial bodies, the value to be referenced differs depending on longitude.
For east longitude, use the value from the previous day; for west longitude, use the value from the following day.

Step 2
From the Nautical Almanac, obtain the Moon’s meridian passage time (GMT) for the target day and the adjacent day.
Subtract the correction for east longitude and add it for west longitude.

Step 3
Find the difference between these two meridian passage times.

Step 4
Determine the correction by multiplying the obtained time difference by:
Longitude / 360

Step 5
Apply this correction to the meridian passage time (GMT) of the target day.
Subtract the correction for east longitude and add it for west longitude.

Step 6
To obtain a more accurate meridian passage time, use the meridian passage time (GMT) obtained in Step 5 to determine a new value of E☾, and repeat the same calculation procedure.
By then applying the longitude time corresponding to the standard meridian, the result can be converted into Local Standard Time.

Correction of the Observed Altitude of the Moon

Step 1
The procedure is essentially the same as that used for the Sun.

Step 2
Apply the sextant index error (I.E.) to the observed sextant altitude.
This corrected value is used as the basis for all subsequent corrections.

Step 3
Using the Moon correction tables, obtain the required correction values in the same manner as for the Sun
(e.g., height of eye 10 m, altitude approximately 55°).
Note that, as with the Sun, both upper and lower limbs are used.

Step 4
Obtain the Moon’s semidiameter (S.D.) from the Nautical Almanac and apply the appropriate correction.