Due to the moon’s elliptical orbit about the earth, there will be a great moon around perigee (closest point to earth)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 (furthest away)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'.
http://science.nasa.gov/media/medialibrary/2011/03/16/movie_strip.jpg
While you're out, here's something else you can do. Look north and find the Big Dipper. You can't miss it. The two bright stars at the end of the bowl, the pointers, always point to the North Star, Polaris, whose distance in degrees above the Northern horizon will be your latitude.
Now follow the curve of the Dipper's handle across the sky and to your east you'll see that arc hit a a bright reddish star, Arcturus, the brightest star in the constellation Bootes, the Charioteer. Continue that arc to the SE and you'll find another bright star, Spica, in the constellation of Virgo the Virgin. This year you'll get an extra treat. Near Spica is a an even brighter star: the planet Saturn.
Thank you for posting this. While I don't have much advanced knowledge about astronomy, I continue to be awed by this part of creation. I can't look up into the night sky without being impressed by the precision, the distances and the sheer beauty of it all. Thanks!
bob23
Yeah, it all gets to me, too. And it certainly fits in with sailing, especially night sailing!
Read This:
http://www.goodoldboat.com/newsletter/febnewslett28.html#night
And don't forget to check up on the minor correction on compass directions I made to the original Great Moon post.
Wow! Wonderful article. Thanks so much for sharing that.
Bob23