Mark Atwood (fallenpegasus) wrote,
Mark Atwood
fallenpegasus

This is probably important only if you are a werewolf

In a geeky discussion in a chat, the conversation turned computing the phases of the Moon, I wondered if the POM calculator in emacs needed the observer's lat/lon numbers.

someone: Phases of the moon don't depend noticeably on lat/long - the moon is far enough away compared to the diameter of the earth that to all intents and purposes you see the same hemisphere of the moon no matter where you are on earth. I think they're ususually just calculated wrt the centre of the earth. http://aa.usno.navy.mil/faq/docs/moon_phases.php says "For practical purposes, phases of the Moon and the percent of the Moon illuminated are independent of the location on the Earth from where the Moon is observed."
mra: (looks up numbers, does trig)
mra: the sin of the max angle of view is (12750km / 2) / (384403km / 2)
mra: so the max angle of divergence from plumb is 0.0331743996 radians
mra: which is .005282547 of the full circle, 3.46 hours of the lunar orbit
mra: so the apparent phase can be 3.5 hours early or late, depending on where you are standing on the earth
mra: when something is computing the time of the phase to the closest minute, it looks like your lat/long is significant


Did I get my math right?
Tags: geek, math, moon
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