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[Centaur]

For those who get SkyNews magazine there is an article about the first quarter moon which goes on to explain that Half-Moon really isn't Half Moon but First Quarter because your only seeing 1 quarter of the lunar globe, I get that but what I don't get is:

Full Moon, why do we call it Full Moon when your only seeing Half of the lunar globe, it should be called Half-Moon imo.

Jeff

The above quote is actually from a thread on the Cloudy Nights message board. Below is my response.


American Heritage College Dictionary
half-moon, n. 1. The moon when only half its disk is illuminated.

Dictionary.com
half-moon -noun 1. the moon when, at either quadrature, half its disk is illuminated.


Of course, Jeff, you are right that the explanation you were given is nonsensical. I can see why it might have led you to believe that a Full Moon should be called a Half Moon. Whoever wrote that article is simply confounding his readers by passing along a misunderstanding about certain lunar terms. Reference to one-quarter of the lunar globe is based on a silly assumption by the writer. Those who first referred to quarters did not even know the Moon was a globe; most assumed it was a disk. The use of the term Half Moon (commonly understood as one that appears "D" shaped) is quite correct.

The terms first and last quarter do not refer to either the shape of the apparent lunar disk or a fraction of the Moon's globe. Instead, they denote the Moon's 90° quadrature (quarter of a circle in elongation) from the Sun. They are terms that were originated by astrologers who believed that the 90° separation had meaning for human affairs.

In fact, the quarters are each actually about a week in length and do not represent specific moments. The posted time for first quarter marks the end of the Moon's first quarter between conjunction (Dark or New Moon) with the Sun and eastern quadrature. The posted time for last quarter marks the beginning of the Moon's last quarter between western quadrature and conjunction with the Sun. Second quarter is the period between eastern quadrature and opposition (Full Moon) from the Sun. Third quarter is the period between opposition and western quadrature.

At the time of the quarter positions noted in publications, the apparent disk of the Moon is almost exactly 50% illuminated, thus it indeed appears as a Half Moon. Actually, the precise moment of geocentric dichotomy (Half Moon) occurs when the Earth and Sun are separated by 90° relative to the Moon's center. But that happens at an average of only 17 minutes from the posted quarter positions. From the surface of the Earth the actual times of these events can vary by as much as an hour from the geocentric times. In truth, I'm being overly precise since the Moon's terminator is not a sharp boundary.

Since the terms first and last quarter often confuse people, I tend not to use them in my publications. I recommend the terms Waxing Half Moon and Waning Half Moon or Evening Half Moon and Morning Half Moon. See my monthly astronomical calendar which presents the times of Half Moon as the moments when the disk of the Moon appears 50% illuminated from my location in Illinois. Click: www.curtrenz.com/astronomical.html