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October 2003 Feature: Halloween First Quarter Moon



Trick or treaters have a first quarter Moon lighting up the sky this Halloween night. Quarter Moon, technically a fleeting moment, comes to pass when it's exactly 50% full, at 11:25 p.m. Eastern Standard Time.

In keeping up with the festivities, the Moon has a trick or two of its own for Halloween ghosts and hobgoblins. As always, one of the Moon's favorite tricks is to rotate on its axis in the same amount of time that it takes to orbit the Earth. That way, the same side of the Moon always faces the Earth while its back side remains forever hidden.

For a Halloween treat, follow the quarter Moon's terminator (the line dividing night from day) upward to find the Moon's north pole at the top of the lunar disk, then downward to find the Moon's south pole at bottom. The terminator aligns with the Moon's prime meridian of 0 degrees longitude, crossing the center of the lunar disk at the equator, or 0 degrees latitude. Incidentally, the Moon's 0 degrees latitude and longitude point resides quite near the crater Bruce.

And, yes, the first quarter Moon provides another trick for entertainment and edification. The daylight edge of the lunar disk highlights 90 degrees of longitude to the east of the prime meridian, whereas the nighttime edge outlines 90 degrees of longitude to the west.

Though the prime meridian always divides the Moon down the middle, regardless of the phase, there's nothing like a quarter Moon to illuminate the two hemispheres. The eastern hemisphere shines to the right of the prime meridian, while the western hemisphere lingers in shadow to the left.*

At first quarter, sunrise coincides with the prime meridian, with noon presiding at the Moon's right limb and midnight on its left. One week later, at Full Moon, the noonday Sun strikes the prime meridian, as the Sun sets on the right limb and rises upon the left.

If you were at the Moon, you'd find that the Earth stays in the same place in the lunar sky in much the same way that Polaris, the north star, stays put in the Earth's sky. But in contrast to Polaris, which stands at zenith - or straight overhead - at the Earth's North Pole (90 degrees North latitude), the Earth stands at zenith over the Moon's 0 degrees latitude and 0 degrees longitude.

My gosh, think how easy it'd be to navigate on the Moon, using the planet Earth as your guidestar! With no clouds or foliage to block the view, the Earth would light your way to 0 degrees latitude, 0 degrees longitude, enabling you to find your way back to the crater Bruce everytime!

Alas, I think I discern some rumblings before I even have a chance to type my final period. Yes, I admit it, I totally ignored lunar libration, saving it as a separate article for my more persnickety readers.

copyright 2003 by Bruce McClure

* Yes, the Moon's east limb points in the Earth's westward direction, and vice versa, so hopefully the terms right and left lessen the confusion. East on the Moon as well as Earth refers to the direction of the Sun at sunrise.
Cool Moon Map

Background courtesy of The Background Sampler