The lunar distance technique allows you to determine Universal Time (UT) in case you cannot rely on a chronometer. This method is not all that accurate by modern standards but it is rigorous and can serve as a viable backup option. The spreadsheet lunar_distance.xls clears the lunar distance and then performs the interpolation that yields an improved UT. Start by entering your best estimate of UT in row 3. Select Time1 and Time2 instants on opposite sides of this estimate and enter the corresponding GPs of the Moon and the other body in rows 7 and 12. The apparent and observed altitudes go to row 17. Enter the observed lunar distance (corrected for index error only) into cell B22. The objects’ semidiameters are placed in cells C22 and E22 (enter near-limb values as positive, and far-limb values as negative). The computed UT is displayed in cell C30, which is Time1 + T_add (i.e. the sum of cells A7 and B30). Verify that the interpolation factor IntF (cell A30) lies between 0 and 1. If that is not the case then the two instants Time1 and Time2 do not “bracket” the “true” UT and need to be changed accordingly.

Summary for spreadsheet lunar_distance.xls:

Input cells: row 3, A7-E7, A12-E12, B17, C17, E17, F17, B22, C22, E22

Output cells: A30, B30, C30

Intermediate lunar distance cells: F7, F12 (computed), F22 (centered), E27, F27 (cleared)

The method and example preset in this spreadsheet can be found in Celestial Navigation in the GPS Age by John Karl, pp. 93-95.