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December 2005: Northernmost Full Moon Preludes Major Lunar Standstill
The full Moon this December 15 reigns as the northernmost full Moon until December 27, 2023. Glory be! The moonlit nights this middle December make up for the waning days of sunlight in our northern hemisphere. On December 15 in particular watch as the Moon rises around sunset and sets around sunrise, lighting up the sky all through the night.
Traditionally, in our northern hemisphere, the full Moon closest to the December (Southern) solstice goes by the moniker of Long Night Moon. Depending on the year, the Long Night Moon can fall on the day of the solstice, or up to two weeks before or after. (This year's December solstice, incidentally, falls on December 21, at 1:35 p.m. EST.) Like the Sun in summer, Long Night Moons rise far north of due east and set far north of due west, and oftentimes stay out for more hours than the Sun does on the longest day of the year.
This year's Long Night Moon noticeably rises farther north on the eastern horizon and sets farther north on the western horizon than the northernmost Sun ever gets on the day of the June (Northern) solstice. At midnight, the Moon soars even higher in our northern hemisphere skies than the year's highest noonday Sun. Moreover, the Moon stays above the horizon considerably longer than the Sun on the longest summer day.
18 to 19-Year Cycle
This December presents the northernmost full Moon till December 27, 2023, standing at a declination of over 28 degrees north of the celestial equator.* This contrasts to the northernmost declination reached by the Sun on the June solstice, when the Sun is at zenith at the tropic of Cancer, at a latitude of nearly 23.5 degrees north. (See your globe!)
Year after year, the nearest full Moon to the December solstice mirrors the approximate position of the June solstice Sun amidst the stars. However, during a peak year, the December full Moon gets as far as 5 degrees (10 Moon diameters) north of this northern solstice point. Thereafter, the December full Moon gradually descends southward year by year -- till some 9 to 10 years later, when the Moon reaches its southern extreme of more than 5 degrees (10 Moon diameters) south of this northern solstice point. Then the December full Moon bounces back northward again, returning to its northermost point in a cycle of 18 to 19 years.
(It may be of interest to note that 5 years from now, on December 21, 2010, the December full Moon happens to fall on the same day as the December solstice -- and what's more, lodges at the northern solstice point. The full Moon of December 21, 2010, will pass right through the Earth's umbral shadow, featuring the northernmost total lunar eclipse in the 21st century.)
Why December of this Year and 2023?
If the Moon orbited the Earth on the same plane that the Earth orbits the Sun, then the declination of the December full Moon would equal -- or nearly equal -- the declination of the June solstice Sun, year after year. But the geometry is such that the plane of the Moon's orbit is inclined at some 5 degrees to the plane of the ecliptic (Earth's orbital plane).
That means that the Moon is north of the ecliptic (Earth's orbital plane) for half the month, and south of the ecliptic for the other half of the month. Twice a month, the Moon crosses the ecliptic at points called nodes. If the Moon is traveling from south to north, it's called an ascending node; and if the Moon's traveling from north to south, it's called a descending node. As the Moon reaches a place in its orbit that's halfway between the ascending node and descending node, the Moon reaches its maximum point north of the ecliptic -- then some two weeks later, the Moon reaches its maximum point south of the ecliptic (at a point in its orbit that's halfway between the descending node & ascending node).
If the December full Moon happens appreciably close to the time of the December solstice, and in addition, finds itself sufficiently close to its maximum distance north of the ecliptic, then the full Moon will be especially far north of the celestial equator -- hence, the reason for the full Moons of December 2005 and December 2023 reigning as the northernmost full Moons.
Moon's Nodal Cycle
The Moon's nodes are not static, but travel in retrograde (or westward) through the constellations of the zodiac, going full circle in period of about 18.6 years. Therefore, northernmost (and southernmost) full Moons tend to repeat themselves in cycles of 18 to 19 years, since they are very much linked to the cycle of the Moon's nodes.
(December full Moons take place north of the constellation Orion, near the June solstice point at the border of the constellations Taurus and Gemini -- shown on this star chart, where the ecliptic intersects the 6-hour meridian extending from the north celestial pole to the south celestial pole. And this map helps you to envision the Moon's place amongst the stars when it's 5 degrees north or south of the June solstice point.)
Looking ahead to next year, the southernmost full Moon will take place on June 11, 2006, followed by another southernmost full Moon on June 22, 2024, some 18 years later. But since the full Moon of June 22, 2005 was actually a little more south than the June 2006 full Moon will be, the southernmost full Moons this time around are separated by 19 years.
Major Lunar Standstill
This December's northernmost full Moon preludes the year of Major Lunar Standstill, the subject of the upcoming January 2006 feature. By way of preview, I'll tell you right now that a Major Lunar Standstill always takes place near an equinox. In the year 2006, the southernmost Major Lunar Standstill comes to pass on March 22, whereas the northernmost Major Lunar Standstill makes its showing on September 15.
copyright 2005 by Bruce McClure
* It must be emphasized that this (geocentric) declination of the Moon is measured from the center of the Earth. Due to parallax, the Moon's declination as seen from the Earth's surface may vary from this figure by up to about one degree. Plus, at the instant of this month's full Moon, the Moon is high in the sky in eastern Asia but not yet above the horizon in most of the Americas.
My thanks to Jean Meeus, who provided me with some of the information presented in this article.
November 2005 Feature * Dawning of the New Year * January 2006 Feature
|Phases of the Moon by the US Naval Observatory|
|Ephemeris of the Moon by Fred Espenak|
|Major Lunar Standstill 2006|