Due to the moon’s elliptical orbit about the earth, there will be a great moon around perigee on May 5, 6, or 7 (Greenwich Time) According to the Nautical Almanac, the moon’s apparent semidiameter will be 16.7 minutes of arc on those dates. The moon will also be full on the 6th (again, that’s GMT). Great Moon is when perigee and full moon come at the same time.

By contrast, around new moon on Apr 21, 22, 23, the moon was at apogee and had a semidiameter of 14.7 minutes of arc.

The Nautical Almanac notes the moon and sun’s semidiameter for every day of the year so this number can be added or subtracted to a sextant observation to get the exact location of the center of the orb. The sun’s semidiameter was greatest this year at 16.3′, and is currently near minimum at 15.8′.