NavList:

David,

You asked:

...steps one and three require GHA which in turn requires time. But the purpose of the Lunar is to find time. A circular argument!

Where have I gone wrong?

It has been said that one is rarely, if ever, truly "lost". In celestial navigation, one usually starts with an estimate of position (DR or AP) and refines that into a fix with observations and calculations. The same is true for lunars. Unless you've just woken up from a coma in the middle of the ocean with no timepiece whatsoever, you will probably have some idea of the date and time. A lunar is used to refine that estimate. Or, in other words, to check the error of your timepiece. Even if you were very unsure of the time, it is not difficult to calculate the distances for each whole hour and compare them to your observation to see which pair your observation lies between. Once you find the hours at which the distance is greater and lesser than your observation, all that is left is to interpolate to find the minutes and seconds.

To be clear: the only time you need to measure the altitudes is very close to the time you measure the lunar distance. One method is to measure the altitudes just before and after the lunar, and interpolate for the altitude at the exact time of the lunar measurement. The purpose is to get the proper refraction, parallax and semi-diameter values in order to correct your lunar measurement. There is no need to measure the altitudes at, or even near the time of the calculated distances, because these are found using geocentric position data from the almanac. The entire "clearing" process is designed to reduce your topocentric observation to a geocentric value for comparison. Both of the calculated distances may also be computed well before or after the actual lunar measurement is taken, at the leisure of the lunarian.

I hope that helps!

Regards,

Sean C.