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This is the year of the Major Lunar Standstill -- a lunar standstill being very similar to a solar solstice. A northern solstice happens when the zenith (straight overhead) sun reaches its northernmost point for the year, shown on the globe as the tropic of Cancer. A southern solstice takes place some six months later, when the zenith Sun stands at its southernmost point for the year at the tropic of Capricorn. At the solstice, the Sun resides about 23.5 degrees north or south of the equator, with the Sun standing overhead at the tropic of Cancer on or near June 21 and overhead at the tropic of Capricorn on or near December 21.
The Sun, Moon and Zodiac
A lunar standstill is just like a solar solstice, except that it refers to the northernmost and southernmost points of the zenith Moon over the globe of Earth. A northern solar solstice or a northern lunar standstill happens when the Sun or Moon shines in front of the constellation Taurus (almost exactly at Taurus-Gemini border), and a southern solstice or southern lunar standstill happens when the Sun or Moon passes in front of the constellation Sagittarius, a bit northeast of the constellation Scorpius' stinger stars.
The Moon marches full circle through the constellations of the zodiac in a period of about four weeks. That's a much faster clip than the Sun's full circle promenade through the zodiac in one year. A southern lunar standstill comes two weeks after a northern lunar standstill, and then a southern standstill comes two weeks after a northern standstill. Lunar standstills happen twice (and sometimes three times) in a calendar month, whereas solar solstices happen only twice a year.
This being a major lunar standstill year, the Moon's (geocentric) declination will bounce from about 28.5 degrees north to about 28.5 degrees south of the equator every month this year. This contrasts to a minor lunar standstill year, upcoming next in the year 2015, when the Moon's monthly variation in declination swings from about 18.5 degrees north to 18.5 degrees south.
Northern and Southern Major Lunar Standstills
Monthly northern and southern standstills won't vary greatly throughout 2006. Even so, a major lunar standstill comes to pass when the Moon reaches its absolute greatest (geocentric) declination north or south in this 18.5 to 19-year cycle. This year's northern major lunar standstill happens on September 15, at a declination of 28.7228 degrees; and this year's southern major lunar standstill takes place on March 22, at a slightly larger declination of 28.7231 degrees.
It is somewhat unusual for the northern and southern major lunar standstills to be separated by six months. More typically, a southern and northern major lunar standstill fall two weeks apart of one another. For instance, in the following major lunar standstill year of 2025, the northern major lunar standstill falls on March 7, whereas the southern major lunar standstill occurs two weeks later, on March 22.
Lunar Standstills, Equinoxes and Eclipses
A major (or minor) lunar standstill always happens near an equinox; and what's more, it happens when the Moon is at or near quarter phase. A major lunar standstill faithfully occurs within one week of a lunar or solar eclipse, and oftentimes takes place right between a lunar and solar eclipse. For instance, this year's March 22 southern major lunar standstill comes one week after the March 14 lunar eclipse and one week before the March 29 solar eclipse; similarly, the September 15 northern major lunar standstill is flanked by the September 7 lunar eclipse and the September 22 solar eclipse.
Why a Quarter Moon?
If the Moon orbited the Earth on the same plane that the Earth orbits the Sun (Earth's orbital plane is called the "ecliptic"), then we would have no major or minor lunar standstills. Furthermore, we'd always have a solar eclipse at new Moon and a lunar eclipse at full Moon. But as it is, the geometry is such that the plane of the Moon's orbit is inclined about 5 degrees to the plane of the ecliptic. For half the month, the Moon orbits north of the ecliptic, and for the other half of the month, the Moon orbits south of the ecliptic. Twice a month, the Moon intersects the ecliptic at points called nodes. If the Moon is traveling south to north, it's called an ascending node, and if the Moon is traveling north to south, it's called a descending node.
Major and minor lunar standstills are determined by the location of the Moon's nodes on the ecliptic. In a year when the Moon's ascending node is at or near the March equinox point in front of the constellation Pisces, then a major lunar standstill year is in store. If, on the other hand, the Moon's descending node coincides with the March equinox point, then it's a minor lunar standstill year. This map shows you the March equinox point amidst the stars, where the 0 degree hour line intersects the ecliptic. (In addition, the 6 hour line intersects the ecliptic at the northern solstice point, the 12 hour line intersects the ecliptic at the September equinox point, and the 18 hour line intersects the ecliptic at the southern solstice point.)
But still, the reader might ask, why at or near quarter Moon? It's because the tilt of the Moon's orbit relative to the ecliptic is not static but somewhat variable -- depending on the position of the lunar nodes. It's my understanding that the inclination of the lunar orbit is at a maximum whenever the line of the nodes is pointed at the Sun (coincides with the Earth-Sun line). If, on the other hand, the maximum lunar orbital inclination were to take place whenever the line of the Moon's nodes aligned perpendicularly (at a right angle) to the Sun-Earth line, then I think it's likely that the major lunar standstills would be at solstice time & at new and full Moons.
Where in the Sky?
If you have some familiarity with the constellations, you can star-hop to the solar solstice points in the heavens, knowing that major lunar standstills stand some 5 degrees north of the northern solstice and 5 degrees south of the southern solstice.
The northern solstice point stands about halfway between the bright ruddy star Betelgeuse of the constellation Orion, and the star Theta Aurigae of the constellation Auriga. In fact, two stars in Auriga the Charioteer -- Menkalinan & Theta Aurigae -- are aligned north and south one one another. Going north through these two pointer stars escorts you right to Polaris, the North Star. Going southward, using this celestial arrow, you land right on Betelgeuse, the armpit of Orion the Hunter. If you're a deep-sky enthusiast, you may know that M 35 almost coincides with the northern solstice point.
As for the southern solstice point, it's found in front of the constellation Sagittarius the Archer. It resides about 7 degrees north of the star Alnasl. Modern stargazers tend to see Sagittarius as "the teapot." Alnasl marks the tip of the "spout," as is shown in this photo. Again, if you're a deep-sky aficionado, M 8 (Lagoon nebula) nearly aligns with this southern solstice point. At a southern major lunar standstill, the Moon brushes up close to the star Alnasl.
For 2006 monthly standstill dates, please click on this page and scroll down.
copyright 2005 by Bruce McClure
December 2005 Feature * February 2006 Feature
2006 Major Lunar Standstill
A Modern-Day Megalithic Observatory