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Once, when stargazing at Reynolds Observatory, the clouds came drifting in around the midnight hour, wiping out all but the brightest stars. However, we enjoyed a beautiful view of the last quarter Moon rising above the eastern horizon, prompting someone to say, "When the Moon is rising or setting, the terminator - or the Moon's shadow line - must be pointing to celestial north and to Polaris, the North Star. So you can use the rising Moon or setting Moon to figure out cardinal directions here on Earth."
For the fun of it, let's speculate upon this proposition to see what we come up with . . .
Definition of Celestial North
Celestial north is the direction to the north celestial pole - that point in the sky that resides at zenith, or straight overhead, at the Earth's north geographical pole. The north celestial pole is closely marked by Polaris, the present-day polestar.
As on Earth, celestial north on the Moon is the direction to the Moon's north celestial pole - that point in the Moon's sky that is at zenith over the Moon's north geographical pole. But does that mean the polar axes on Earth and the Moon point in the same direction in space?
No! The Earth's north polar axis points about 23.5 degrees away from the north ecliptic pole. In constrast, the Moon's north polar axis points within 1.5 degrees of the north ecliptic pole. For simplicity, we'll say the Moon's north celestial pole points to the north ecliptic pole.
Definition of North Ecliptic Pole
Alas, we have another term to define: north ecliptic pole. The north ecliptic pole is the point in the Earth's (or Moon's) sky that marks the northern perpendicular to the Earth's orbital plane. This point is located about 23.5 degrees away from the Earth's north celestial pole, in the constellation Draco the Dragon. If you can find the "Eyes of Draco" - the stars Eltanin (Etamin) and Rastaban - you can approximate the location of the ecliptic pole. It's about 15 degrees north of the brighter eye star, Eltanin. (Sky chart on right courtesy of Wikipedia)
Star-hop to the North Ecliptic Pole
You can star-hop to the Eyes of Draco via the Summer Triangle, the large configuration made of the three first-magnitude stars Vega, Altair and Deneb. A line drawn from Altair through Vega faithfully escorts you to Draco's Eyes, the stars Eltanin and Rastaban. Look at the sky chart, and you can see that the stars Eltanin and Grumium more or less point in the direction of the north ecliptic pole. You can also star-hop to the north ecliptic pole by drawing a line from the Big Dipper northern pointer star, Dubhe, and through the star Kochab in the Little Dipper, going a bit more 1.5 times the Dubhe-Kochab distance.
Finding the Moon's North and South Poles
Yes, you can use the Moon's terminator - shadow line - to find the approximate position of the Moon's north and south poles. Follow the terminator northward to the edge of the lunar disk to locate the Moon's north pole and southward to locate the Moon's south pole. If you live at mid-northern latitudes and are unsure which way is north, follow the terminator upward to find the Moon's north pole.
However, you can't depend on the Moon's north polar axis to point to the Earth's north celestial pole (closely marked by Polaris, the North Star). The Earth's north polar axis points about 23.5 degrees away from the north ecliptic pole. Depending on time of the month, the Moon's polar axis points up to 23.5 degrees above or below the Earth's celestial poles.
Moon's North Polar Axis Points to Earth's North Celestial Pole Twice A Month
My guess is that the Moon's polar axis points in the direction of the Earth's celestial poles twice a month. When the Moon reaches that point in its monthly journey through the constellations of the zodiac whereby the Eyes of Draco shine in between the Moon and Polaris, I would expect the Moon's north polar axis to point toward the Earth's north celestial pole. Or conversely - when the Moon reaches that point whereby Polaris stands between the the Moon and the Dragon's Eyes - I would again expect the Moon's north polar axis to align with the Earth's north celestial pole.
When the Moon is near the border of the constellations Scorpius and Sagittarius, the Eyes of Draco are between the Moon and Polaris. If you are familiar with the concept of right ascension, the Moon is at or near 18 hours of right ascension, or the December solstice colure, when the Eyes of Draco sit between the Moon and Polaris.
When the Moon is near the border of the constellations Taurus and Gemini, Polaris is between the Eyes of Draco and the Moon. At this juncture, the Moon is at or near 6 hours right ascension, or the June solstice colure.
Moon's Polar Axis Points Maximally Away Twice A Month
When the Moon is at 0 degrees or 12 hours right ascension (the equinox colures), I would expect the Moon's polar axis to point maximally away from Polaris, the North Star. At these two times, the Moon, Polaris and the Eyes of Draco make a right angle in the sky.
If the moon rises or sets when the Eyes of Draco are at zenith over the North Star, I expect to see the Moon's polar axis - as defined by the "horns" of the crescent Moon or the quarter Moon terminator - pointing maximally above Polaris, the North Star. When the Moon rises or sets when the Eyes of Draco are directly below Polaris, I would expect the Moon's polar axis to point maximally below Polaris.
Speculation Versus Observation
I should make it clear that the views stated herein arise from the world of thought and speculation, not thorough observation and investigation. But I'll be watching the Moon in the months and years ahead, to see whether my musings pass the muster!
From North American latitudes on November 12, watch for the waning crescent moon to rise over your eastern horizon as the star Eltanin plunges to lower transit below Polaris at about 2:30 a.m. local time.
copyright 2009 by Bruce McClure
October 2009 Feature * December 2009 Feature