- The Chicago Astronomer -
Copyright 2004-2014 All rights reserved by Joseph Guzman Administrator/Founder/Chief Astronomer.
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Below is a copy of the e-mail that I sent yesterday to my friend Tom Skilling in response to that article on his station's website. _____________________________________________
On your website is an article that purports to list the “exact moment of fullness” for Full Moons as viewed from Chicago. The original source seems to be the USNO with possible additions by your staff. Actually those times are based on the moment that the geocentric longitude of the Moon differs from that of the Sun by 180°. First, Chicago is not located at the Earth’s center. Second, even for the Earth’s center that is not the “exact moment of fullness”. And to be really precise, the Moon is never completely full, since that would theoretically be during a lunar eclipse when the Moon is obscured by the Earth’s shadow.
It has become traditional to state the times of Full Moons for that moment the geocentric longitude of the Moon differs from that of the Sun by 180°. That is because that moment is relatively easy to calculate, which was especially true before computers became pervasive. In reality, the Moon always has some geocentric celestial latitude which causes a shift in the moment for the geocentric greatest illumination of the Moon’s apparent disc. An observer’s location on Earth also significantly shifts the time.
For my monthly astronomical calendars, which many people use for their computers’ wallpaper, for each Full Moon I calculate the times for the moment of greatest illumination of the Moon’s apparent disc as viewed from Chicagoland. That’s what I list as the time for the Full Moon. Admittedly, half the time that occurs with the Moon beneath the horizon, so a fictitious X-ray vision is assumed. The next occurrence will be February 18 at 00:21 (12:21 a.m.) CST with 99.81% illumination. That’s nicely near the middle of the night of February 17/18.