Interactive Spreadsheets for Celestial Navigation

Sextant altitude corrections

The spreadsheet *alt_corr.xls* performs the corrections to the sextant altitude Hs (cell B1) that are needed to produce the apparent altitude Ha (cells B6, B7, B8) and the observed altitude Ho (cells B12, B13, B14). Cell B4 contains a yes/no (Y/N) answer to the question whether a reflecting artificial horizon was used. The index correction goes to cell B2. The semidiameter correction is entered in cell B9; this is positive for lower limb and negative for upper limb observations. The height of eye in cell E1 (enter “ft” for feet or “m” for meters in cell F1) determines the dip correction. Cells E2, E3, and F3 control the refraction correction (in cell F3 state “C” for Celsius, or “F” for temperature in Fahrenheit); the standard values are Temperature = 10 ºC and Pressure = 1010 mb. Cell E6 contains the value of the horizontal parallax (HP) in arc minutes. The Moon parallax can also be corrected for the oblateness of the Earth by entering the latitude (E8) and azimuth (E9). The semidiameter value from either cell E11 (Sun - typical preset value, or from the almanac) or E12 (Moon - computed from the HP) is to be copied (with the appropriate sign characterizing the limb) into cell B11.

The following image shows the spreadsheet ** alt_corr.xls**.

**Summary for spreadsheet alt_corr.xls:**

The preset example contained in the spreadsheet is the upper limb Moon sight from p. 281 of the Nautical Almanac, 2009 Commercial Edition.

**2. Precomputed sextant altitude**

The spreadsheet *alt_prec.xls* is a reversed version of *alt_corr.xls*. It provides the altitude Hs to which the sextant may be preset before an observation. The observed altitude Ho (computed with intercept.xls) is now input in cell B12 and the sextant altitude is displayed in cells B1 and C1. The remaining cells have the same meaning as in *alt_corr.xls*.

The following image shows the spreadsheet ** alt_prec.xls**.

**Summary for spreadsheet alt_prec.xls:**

**3. Averaging sights: 1) precomputed slope**

Random errors can affect every individual sight. This problem can be mitigated by taking a set of measurements and averaging them. The spreadsheet

The following image shows the spreadsheet

**4. Averaging sights: 2) fitted slope**

The spreadsheet *average2.xls* performs the same function as *average1.xls*, but for observed altitude data (Ho) in column B, while allowing the procedure to also choose the slope of the fit. Additional input data include the speed (cell F1) and course (cell F2) of the vessel, the hourly declination change rate (cell F5, in arcminutes), and azimuth (cell F7, in degrees) of the observed body. The remaining cells serve the same function as in *average1.xls*. The weights in column D should come out neither all 1.000, nor all (but two) very small.

The following image shows the spreadsheet ** average2.xls**.

**Summary for spreadsheet average2.xls:**

The spreadsheet *dip_short.xls* implements the formula behind Table 14 in Bowditch. The height of eye can be entered in meters or feet (enter "m" or "ft" in cell C1). The distance to the waterline in cell B2 is in nautical miles. The resulting dip is output in cell B3 in nautical miles.

The following image shows the spreadsheet ** dip_short.xls**.

**Summary for spreadsheet dip_short.xls:**

The spreadsheet *distance.xls* implements the formula from Bowditch to calculate the distance by vertical angle between the waterline and the top of an object. Select "ft" or "m" in cells C1 and C2, and enter the corrected vertical angle in cell B3. The distance in nautical miles is displayed in cell B4.

The following image shows the spreadsheet ** distance.xls**.

**Summary for spreadsheet distance.xls:
Input cells:**

The spreadsheet *alt_move.xls* calculates the effective observed altitude associated with a line of position that is advanced or retarded by dead reckoning. In this technique of compensating for the vessel motion the assumed position is unchanged and only the final adjusted LOP needs to be plotted. Enter the original observed altitude and azimuth from spreadsheet *intercept.xls* in cells A2 and B2. Enter ground speed and course made good in cells C2 and D2. The time interval in cell E2 is positive to advance the LOP and negative to retard the LOP. The (signed) distance traveled in nautical miles is displayed in cell F2. Reenter the adjusted observed altitude from row 6 in cell E2 of *intercept.xls* to obtain the new intercept.

The following image shows the spreadsheet

Summary for spreadsheet

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