The data for the following example are from John Karl’s “Celestial navigation in the GPS age” (First Edition, 2007), pp. 63-65. With three spreadsheets we reproduce the computations for the sight reduction of the Moon observation presented in Figure 6.4 on p. 64. Given the dead-reckoning (DR) position and the recorded time of observation (UT), the Moon sextant altitude (lower-limb) is reduced to intercept distance and azimuth needed to plot the associated line of position (LOP) according to the intercept method of Marcq St. Hilaire.

**Input data:**

Body: Moon, lower limb

Date: 11 May 2005

UT: 02^{h} 19^{m} 14^{s}

Hs: 25º 21.6′

Sextant index correction: -3.3′

Height of eye: 9 feet

assuming standard atmospheric conditions

DR position: S 34º 13′, E 161º 43′

*moon.xls:*

**Input:**

Date: 11 May 2005

UT: 02^{h} 19^{m} 14^{s}

In cells A2-F2 entered: 2005 5 11 2 19 14

**Output:**

GP (row 5)

GHA: 182º 25.4′ (same as 181º 85.4′ in the book)

Dec: N 27º 42.9′ (difference of only 0.1′)

Semidiameter (SD, cell A8): 15.0′

Horizontal parallax (HP, cell C8): 55.1′

Note that there is no need for increments and corrections (*v* or *d* Corr^{n}).

**alt_corr.xls:**

**Input:**

Height of eye (cells E1, F1): 9 ft

Using standard conditions for temperature and pressure:

Pressure (cell E2): 1010 mb

Temperature (cells E3, F3): 10 degrees Celsius

HP (in arcminutes from *moon.xls* cell C8): 55.1 entered in cell E6

Sextant altitude (Hs): 25º 21.6′, entered as 25 216/600 in cell B1

Index correction: -3.3′, entered as – 33/600 in cell B2

Artificial horizon was not used: entered N in cell B4

SD (in degrees from *moon.xls* cell A8): entered (+)15/60 (positive for lower limb observations) in cell B11

**Output:**

Observed altitude (Ho) in cells B12-B14: 26º 18.2′ (difference of only 0.1′)

Note the intermediate result for apparent altitude (Ha) in cells B6-B8: 25º 15.4′ (same as in the book)

**intercept.xls**

**Input:**

Assumed position (AP) is taken to be the dead-reckoning (DR) position:

Latitude: S 34º 13′, entered as -34 13/60 in cell A2

Longitude: E 161º 43′, entered as (+) 161 43/60 in cell B2

GP of Moon from row 5 of *moon.xls*:

GHA: 182 254/600 entered in cell C2

Dec: (+) 27 429/600 entered in cell D2

Ho from *alt_corr.xls* cells B13, B14: 26 182/600 entered in cell E2

**Intermediate results:**

LHA (cells F2, F3): 344º 08.4′ (same as in the book)

Computed altitude (Hc in cells A6-C6): 26º 16.4′ (difference of only 0.1′)

**Main results:**

Intercept (cells D6, E6): 1.8 Toward (difference of only 0.2′)

Azimuth (cell F6): 15.7 agrees with 16 from the book after rounding

*worksheet.pdf:*

The data from this example are summarized in the worksheet below; input data are marked as **green**, and spreadsheet results (some of which are relayed as input for subsequent calculations) are **blue**.

(first published on August 17, 2010)