The Geographical Position (GP) of a celestial body is the location on the surface of the Earth from which this body appears directly overhead (at a given point in time). The measurement of the body’s altitude above the horizon (Ho) with a sextant tells us how far we are from the GP (Zenith distance = ZD = 90° - Ho). Therefore, in order to derive our position we need to know the observed body’s GP at the moment its altitude was measured. The GP’s and other data are published in almanacs as a function of Universal Time (UT). The spreadsheets sun, moon, mercury, venus, mars, jupiter, saturn, uranus, neptune, and aries_stars.xls calculate the GP’s of these bodies from the year, month, day, hour, minute, and second of UT. Thus there is no need for interpolation (increments and corrections). Data calculated by the solar system spreadsheets also include the semidiameter (SD) and horizontal parallax (HP) data. The equation of time is provided by the sun.xls spreadsheet.

Summary for the Sun, Moon, and planetary spreadsheets:

Input cells: A2, B2, C2, D2, E2, F2 (Universal Time)

Output cells: B5, C5, E5, F5 (GP); A8 (SD), C8 (HP, both in arcminutes); E8, F8

The spreadsheet aries_stars.xls provides the GHA of Aries from which the GHA of any star can be determined by adding it to its SHA (sidereal hour angle). The UT is entered in row 2. The GHA of Aries is displayed in cells B5 and C5. You may enter the SHA of the observed star in cells E10, F10 (compound fractions are not used here) and retrieve its GHA from cells B11, C11. If the observed star is one of the 57 main navigation stars you may also scroll down the spreadsheet and find its GP there. The SHA’s and declinations of these main stars are in columns E, F. The spreadsheet calculates these quantities from the UT taking into account the effects of light aberration, Earth’s precession and nutation, and the star’s proper motion. The SHA for each star is then added to the GHA of Aries resulting in the star’s GHA in columns B, C. The numbers next to the star’s name are its almanac ID (column B) and magnitude (column C).

Summary for spreadsheet aries_stars.xls:

Input cells (in green): A2, B2, C2, D2, E2, F2 (Universal Time), E10, F10 (SHA)

Output cells (in cyan): columns B and C (GHA), columns E and F (declination) Intermediate cells (in yellow): columns E and F (SHA) - from row 15 down

The Sun and planetary data are computed using the VSOP87 theory by Bretagnon and Francou. The Moon data are calculated from the improved Chapront ELP-2000/82 lunar theory.