Friday, February 29, 2008


Time of rising and setting of the moon is important, if there is a need to work at night it would be nice to know if there will be moonlight, when it will be light and how long. Times of moonrise and moonset are listed in the Nautical Almanac. These times can be computed from the Nautical Almanac for any point on the earth. The listed times in the Almanac, however, are LMT of moonrise and moonset at the Greenwich meridian.

Finding Time of Moonrise and Moonset
Finding the time of moonrise and moonset is similar to finding the time of sunrise and sunset with one important difference. Since these moon phenomena occur later from one day to the next and at variable rates of change, which are rather large (on the average about 51 minutes a day), there could be a considerable error from using time corrected only for latitude and zone time. The arguments for determining the time of moonrise and moonset are the observer's longitude and the differences in times on the two Greenwich dates (tabulated latitudes) that straddle the local date. For ordinary purposes of navigation, however, you would be sufficiently accurate to interpolate between consecutive moonrise or moonset at the Greenwich meridian. Since apparent motion of the moon is westward, relative to an observer on the earth.

Interpolation in west longitude is between the phenomenon on the given date and the following one:
In east longitude it is between the phenomenon on the given date and the
preceding one.

For the givin date, enter the daily-page table for latitude, and extract the LMT for the tabulated latitude next smaller than the observer's latitude, (unless this is an exact tabulated value). Apply a correction from Table I of the Nautical Almanac "Tables for Interpolating Sunrise, Moonrise, etc." to interpolate for latitude, determining the sign of correction by inspection. Repeat this procedure for the date following the given date, if in west longitude, or for the day preceding, if east longitude. Using the daily difference between the times for the nearest tabular latitude, and the longitude, enter Table II ofthe Almanac "Tables for Interpolating Sunrise, Sunset, etc." and take out the correction. Apply this correction to the LMT of moonrise or moonset at the Greenwich meridian on the given date to find the LMT at the position of the observer. The sign to be given the correction is such as to make the corrected time fall between the times for the two dates between which interpolation is being made. This is nearly always positive (+) in west longitude and negative (-) in east longitude. Convert the corrected LMT to ZT.