## Unit 3 Part 3 – Altitude Correction tables for the Moon

Note.  Units of this course can be viewed in numerical order under ‘Astro Nav Course’ on the menu bar.

The Moon’s Semi-Diameter.  When the Moon is not full, sometimes only the upper limb will be visible and sometimes only the lower limb.  In such cases, we have no choice other than to measure the altitude of the limb that we can see and to either add or subtract the semi-diameter accordingly.

When the upper limb is used, 30’ must be subtracted.  This is because 30’ is added to the upper limb corrections during compilation of the tables to keep the value small and positive.

Parralax corrections for the Moon. Because the Moon is relatively close to the Earth, parallax will be significant and so a correction has to be made.  These corrections are included in the altitude correction tables and therefore do not have to be applied separately.

Horizontal Parallax. Parallax error is greatest when the celestial body is close to the horizon and decreases to zero as the altitude approaches 90o.  It is negligible except in the case of the Moon which is close to the Earth in comparison with the other celestial bodies.  Because horizontal parallax is significant in the case of the Moon, a separate correction (abbreviated to HP) has to be applied.

The hourly values for HP are given in the daily pages of the Nautical Almanac as shown in the extract below:

Altitude Correction Tables for the Moon There are two correction tables for the Moon in the Nautical Almanac.  One is for apparent altitudes of 0o to 35o and the other is for 35o to 90o.  You will see from the extract below, that the altitude correction table is divided into two parts.  The top part of the table is entered with the apparent altitude and the correction is read from the appropriate column.

The lower part of the table is for horizontal parallax (HP) and has different columns for lower limb and upper limb.  This part of the table is entered with the value of HP (which is obtained from the daily pages of the nautical almanac) and the correction is read from the same column as that for the apparent altitude correction above.

Example:   At 0200 GMT on 24 December, the sextant altitude of the Moon’s upper limb is measured and the following data is obtained:

Sextant Alt. = 33o 15’.0

I.E. = +1’.8

Ht. of eye = 18 ft.

H.P. = 54’.9

Using the extracts given above, the true altitude is calculated as follows:

 Sext. Alt.          33o 15’.0 I.E.                            + 1’.8 Obs. Alt. =         33o 16’.8 Dip.                           – 4’.1 App. Alt.  =        33o 12’.7 Corr. (1)                +  57’.4  (for App. Alt.) Corr. (2)                   + 2’.1 (for HP upper limb) =                           34o 12’.2 .                                – 30’.0 (for Upper Limb.) True Alt.  =       33o 42’.2

Test Question.  On 6 June. Sextant altitude of Moon’s lower limb: 31o 32’.8.  Index error: +2’.4.  Ht. of eye: 15m.  HP: 59’.7.  Weather conditions standard.  What is the true altitude?

 Solution Sext. Alt.          31o 32’.8 I.E.                           + 2’.4 Obs. Alt.  =      31o 35’.2 Dip.                           – 6’.8 App. Alt. =       31o 28’.4 Corr. (1)                + 58’.2  (for App. Alt.) Corr. (2)                  + 7’.2  (for H.P.) True Alt. =       32o 33’.8

(Note. This topic is covered in greater depth in the book ‘Astro Navigation Demystified’).

Unit 4 will explain the importance of altitude and azimuth in astro navigation.