NOON SIGHT

Observing the sun at LAN (local apparent noon) allows you to determine the position from a single sight. The need for a second line of position is eliminated by the additional piece of information implicitly contained in a noon sight: i.e. that the sun and the observer are on the same meridian. Thus the geometric arrangement is reduced to a 1-dimensional problem along this meridian, with the sun bearing either directly north or south from the observer. In the spreadsheet noon_sight.xls the Ho at LAN is entered in cell D1 and the UT in cell D13. From an almanac you find the sun’s declination (D3) and the correction for equation of time; the latter is entered in D14 if positive, or in D15 if negative (without its sign). This correction is always less than one hour, therefore it is to be entered as 00:MM:SS. It can be (optionally) copied from cell B16 that interpolates between cells B14, B15; therein you may enter the equation of time values (without their signs) for the 00h and 12h instants that straddle your noon UT in cell D13. (This interpolator works in the vast majority of cases when the two equation of time values are of the same sign.) The final two entries identify the observer’s hemisphere (D4) and the sun’s bearing (D5); enter N for north and S for south. In the special case of the sun in zenith (Ho = F1 = 90°) your position is the sun’s GP and the sun’s bearing (D5) is as unimportant as it is undefined. The latitude of the position is displayed in cells D9, D10, D11 and cells D16, D17, D18 contain the longitude.