Global Rumblings: Astronomy - Pictures - Major Moon Standstill Explained re: Stonehenge

http://www.umass.edu/sunwheel/pages/moonteaching.html

MOON TEACHINGS FOR THE MASSES AT THE UMASS SUNWHEEL & AROUND THE WORLD -- THE MAJOR LUNAR STANDSTILL OF 2006

Dr. Judith S. Young
Dept. of Astronomy
Univ. of Mass., Amherst
Jan. 10, 2005

The text below is based on an Education Press Conference which I was invited to present at the January 2005 meeting of the American Astronomical Society in San Diego.

ABSTRACT

With the culmination of the 18.6 year cycle of the Moon in 2006, also called the Major Lunar Standstill, we are afforded the unique opportunity to teach the public about the monthly, annual, and 18.6-year wanderings of the Moon. The 18.6 year cycle is caused by the precession of the plane of the lunar orbit, while this orbit maintains a 5° tilt relative to the ecliptic. At the peak of this cycle, the Moon's declination swings from -28.8° to +28.8° each month. And even though we are more than one year away from the peak of the 18.6-year cycle, already the Moon's declination ranges each month between -28° and +28°. What this means is that each month for the years 2005-2007, the Moon can be seen rising and setting more northerly and also more southerly than the solar extremes, and will transit monthly with altitudes which are higher in the sky than the summer Sun and lower in the sky than the winter Sun.

The U.Mass. Sunwheel is a stone circle calendar which I created in 1997 on the campus of U.Mass. Amherst, with 8'-10' tall stones marking the cardinal directions, the solstice sunrise and sunset directions, and the northernmost and southernmost moonrise and moonset directions. Over 13,000 people have visited the Sunwheel since its construction, and over 5,000 have attended the seasonal sunrise and sunset gatherings which I host. Late in 2004, I began showing the public the Moon at its extremes, and there will be monthly opportunities over the next several years for all of us to notice the very high or very low transiting Moon. The following material explains the monthly behavior of the Moon along with the 18.6 year cycle of the Moon, including why there is an 18.6 year cycle, and what the observable consequences of this cycle are. To understand the patterns in the movements of the Moon requires some familiarity with the somewhat simpler pattern in the movements of the Sun.

PATTERNS IN THE MOVEMENTS OF THE SUN

From day to day, the Earth revolves, or orbits, around the Sun in a plane which we call the ECLIPTIC, completing one orbit around the Sun in a time we call the YEAR. At the same time that the Earth revolves around the Sun, our planet also rotates, or spins, around an imaginary axis running from the North pole to the South pole, and completes one rotation in a time we call a DAY.

The Earth's equatorial plane does not coincide with the plane of the ecliptic, but rather there is a tilt of 23.5° between the two planes; this is equivalent to a tilt of 23.5° between the Earth's rotation axis and the direction which is perpendicular to the ecliptic.

As Earth revolves around the Sun with a tilted axis of rotation, Earth's axis always points toward the same direction in space. We know this to be true because Polaris, the North Star, never appears to rise or set, and is always seen in the same place in the sky from hour to hour and night to night as viewed from a particular location.

From day to day during the year, as Earth orbits the Sun in the plane of the ecliptic and maintains a tilted axis of rotation which points in a fixed direction in space, we see 4 things change:

(1) the sunrise direction changes
(2) the sunset direction changes
(3) the length of daylight changes
(4) the altitude of the Sun at noon above the southern horizon changes

This is illustrated in the following figure, which shows the daily tracks of the Sun on the solstices and equinoxes, as seen by on observer at mid-northern latitudes.

[All of the 4 changes listed above arise because of Earth's tilted axis as our planet orbits the Sun. If there were no tilt of the Earth's axis of rotation in relation to our orbit, none of these things would change. If there were no tilt, (or equivalently, if Earth's equatorial plane coincided with the ecliptic plane), then every day the path of the Sun would be like an equinox for everyone on Earth -- the Sun would rise due East, set due West, be up for 12 hours and down for 12 hours. The only difference for observers at different latitudes would be the noontime altitude of the Sun.]

Like a pendulum, the sunrise direction changes from NE @ summer solstice to SE @ winter solstice, and back to NE, completing one full swing back and forth, or one full cycle, each year. Also like a pendulum, there is a stopping or 'standstill' at the extremes, or solstices. In fact, the word solstice means 'standstill of the Sun'.

Based on keeping track of the sunrise and sunset directions, human beings discovered and created calendars by marking the extremes of the sunrise and sunset directions, or azimuths, along the horizon. The best known stone circle calendar in the world is Stonehenge.

PATTERNS & MOVEMENTS OF THE MOON

Where to find the Moon in the sky on any given day depends on 3 things:

(1) the phase of the Moon on that day
(2) where the Sun is in the sky
(3) where the Moon is in its 18.6 year cycle

Moon Phases:

The Moon orbits, or revolves, around Earth and completes one revolution in 29.5 days. This is the same length of time that it takes the Moon to complete one cycle of phases. Because of the shorter period of revolution for the Moon around the Earth, compared to the revolution of the Earth around the Sun, the Moon completes between 12 and 13 cycles of lunar phases in 1 year.

The phases themselves arise because the light which lights the Moon is coming from the Sun, while our vantage point is the Earth. To understand the relative locations of the Sun and Moon in relation to the lunar phases, the simplest guides to remember are:

(1) the full Moon is opposite the Sun in the sky
(2) the new Moon is in the same direction as the Sun in the sky (and the side of the Moon which is illuminated by the Sun is the side we cannot see)
(3) the crescent Moon is always near the Sun in the sky

[Contrary to popular misconception, the phases of the Moon do NOT arise from the Earth's shadow. The only time the Earth's shadow falls on the Moon is during a total lunar eclipse.]

Changing Direction of Moonrise and Moonset:
The pattern in the Moon's movements is more complex than the Sun's for several reasons:

(2) The plane of the Moon's orbit is close to the plane of Earth's orbit around the Sun, but not identical -- the Moon orbits us in a plane which is tilted by 5.1° relative to the ecliptic.

Because of the rapid orbit of the Moon around us in a plane which is close to the plane in which we always see the Sun, THE MOON DOES IN A MONTH WHAT THE SUN DOES IN A YEAR, in terms of the changing rising and setting direction along the horizon. That is, the moonrise and moonset directions change like a pendulum, swinging back and forth along the horizon and completing one full swing each month. The Sun's pendulum-like swing continues much slower, with the Sun completing one full swing each year.

Because of the tilt of the Moon's orbital plane in relation to our orbit around the Sun, the outer extremes of the Moon's monthly range of rising and setting are NOT THE SAME as the outer extremes of the Sun's yearly range of rising and setting.

Furthermore, the outer extremes of the Moon's monthly range of rising and setting are not fixed, but change slightly from year to year with an 18.6-year cycle.

THE MOON'S 18.6-YEAR CYCLE -- What and Why?

The plane of the Moon's orbit is tilted at an angle of 5.1° to the plane of our orbit around the Sun.

This means that the Moon can at times be seen to rise and set more northerly and more southerly even than the solar extremes. [Recall that the changing sunrise and sunset direction every day is a result of the tilt of the plane in which we see the Sun in the sky in relation to the plane through Earth's equator.]

Due to the effects of the Sun's gravity, the Moon's orbital plane does not stay fixed in space, but "precesses" with an 18.6 year cycle, while still maintaining the 5.1° tilt relative to the ecliptic.

and

The Moon's 18.6-year cycle peaks in 2006 (and every 18.6 years thereafter), with observable consequences extending from 2005-2007.

MAJOR LUNAR STANDSTILL = maximum monthly range of Moon rising and setting
MINOR LUNAR STANDSTILL = minimum monthly range of Moon rising and setting (9.3 years later)

For the years 2005-2007, EACH MONTH the Moon will rise and set more northerly and ~2 weeks later more southerly than the solar extremes. Also, EACH MONTH the Moon will transit higher in the sky than the summer Sun and ~2 weeks later lower than the winter Sun. This is the MAJOR LUNAR STANDSTILL. [The Moon's Declination (effectively the latitude on the sky) ranges each month between -28 and +28° .]

Click here for the table of dates when the Moon will be at the northern and southern extremes during 2005-2007.

9.3 years after a major standstill, the monthly range of moonrise and moonset shrinks. What we see is that each month the Moon will rise and set LESS northerly and ~2 weeks later LESS southerly than the solar extremes. Also, EACH MONTH the Moon will transit NOT AS HIGH in the sky as the summer Sun and ~2 weeks later NOT AS LOW in the sky as the winter Sun. This is the MINOR LUNAR STANDSTILL. [The Moon's Declination (effectively the latitude on the sky) ranges each month between -18 and +18°.]

The schematic diagram below shows the placement of the stones in the U.Mass. Sunwheel as seen from above, with large dots for the 8'-10' tall stones and the small dots for the 2'-4' tall stones. The Sunwheel includes stones for cardinal directions, solstices, and the northern and southern extremes of the lunar rising and setting seen at major standstill. The figure indicates the maximum monthly range of moonset and moonrise seen at major standstill, and the minimum monthly range of moonset and moonrise seen at minor standstill.

WHY THE TERM 'STANDSTILL' ?

The monthly range of lunar Declination changes very little from 2005-2007. Also during this time, the monthly range of moonrise and moonset directions changes little. The fact of little change in the outer extremes of monthly range of rising and setting directions for ~3 years is what leads to the use of the term STANDSTILL.

Solstice means 'STANDSTILL OF THE SUN' and refers to the fact that the Sun's Declination changes little for the ~2 weeks around solstice. The sunrise and sunset directions change little over this time as well.

THE SUN'S ANNUAL CYCLE IN CHANGING SUNRISE DIRECTION BEHAVES LIKE A PENDULUM. The sunrise direction changes rapidly from day to day near the Equinox, like the rapid motion of a pendulum at mid-swing, and there is a stopping or 'standstill' at solstice, like the outer limit of the swing of a pendulum.

Over the 18.6-year cycle of the Moon, THE CHANGING MOONRISE AND MOONSET DIRECTION BEHAVES LIKE A COMPOUND PENDULUM SYSTEM.

More time is spent monthly at the N and S extremes of moonrise and moonset than in the middle -- this behavior is described by the first pendulum.
And, from month to month within the 18.6-year cycle, the Moon spends more time with the monthly range of rising and setting at the outer extremes (major standstill) and at the inner extremes (minor standstill) than at the middle extremes.
It is as if the pendulum representing the monthly range of lunar rising and setting is modulated by 2 additional small coupled pendulums, one at the Sun's northern rising limit and one and the southern. When these 2 pendulums swing out we get MAJOR lunar standstill, and when they swing in we get MINOR lunar standstill.

The fact that the Moon spends more time with the monthly range of rising and setting at the outer extremes (major standstill) and at the inner extremes (minor standstill) than at the middle extremes is the reason for the use of the term STANDSTILL.

WHICH CALENDAR SITES AROUND THE WORLD MARK THE LUNAR STANDSTILL ?

(1) STONEHENGE, located at a latitude of 51° in England, is a 5,000 year old solar and lunar calendar and observatory. The stone locations and sightlines indicate summer solstice sunrise, winter solstice sunset, southernmost moonrise, and northernmost moonset. At the latitude of Stonehenge, the angle between the winter solstice sunset and southernmost moonrise directions is exactly 90°.

(2) CALLANISH, SCOTLAND is located at a latitude of 58°, west of the northern part of of the UK on the Isle of Lewis. Also a 5,000 year old solar and lunar calendar and observatory, stone alignments indicate the northern moonrise and the summer solstice sunrise. The full Moon near summer solstice, which must always be opposite the Sun, is very low in the sky (like the winter Sun). At major standstill, the full Moon near summer solstice reaches only 3.5° above the horizon from Callanish and is viewed through the stones.

(3) CHACO CANYON, NEW MEXICO is a 1,000 year old solar and lunar calendar, located high (400') atop Fajada Butte, inaccessible except by ropes. This site was discovered in 1977 by Anna Sofaer. At noon on the solstices and equinoxes, a dagger of light (called the 'Sun Dagger') pierces a spiral petroglyph carved into the rock face of the cliff. The same spiral petroglyph has been shown to also mark the major and minor standstills of the Moon. The Sun Dagger is accepted by archaeoastronomers worldwide as the best example of a culture keeping track of the Sun and also the 18.6 year cycle of the Moon.

(4) THE U.MASS. SUNWHEEL is an 8 year old stone circle -- a solar and lunar calendar and observatory located on the campus of the University of Massachusetts, Amherst. The stone circle contains 14 stones 8'-10' tall, marking the cardinal directions, the directions along the horizon to the rising and setting Sun at the solstices and equinoxes, and the directions to the rising and setting Moon at major lunar standstill.

WHAT, SPECIFICALLY, WILL WE SEE IN THE NEXT 3 YEARS, WITH THE MOON AT MAJOR STANDSTILL?

At MAJOR STANDSTILL, the Moon will exhibit the maximum monthly range of rising and setting directions.

The full Moon near winter solstice in 2005-07 rises and sets the farthest North that the Moon ever gets, and farther north than the Sun ever gets. At transit, the winter solstice full Moon will be higher in the sky than the summer Sun ever gets. DID YOU NOTICE THIS around Dec. 27, 2004?
The full Moon near summer solstice in 2005-07 rises and sets the farthest South that the Moon ever gets, and farther south than the Sun ever gets. At transit, the summer solstice full Moon will be lower than the winter Sun ever gets. [To notice this requires knowing which way South is, since a low Moon can also be one which is just rising or setting.] DID YOU SEE THIS at summer solstice 2005?
The 1st and 3rd quarter Moons around equinox during 2005-2007 will be at the northern and southern extremes of the Moon rising and setting. In particular, around the spring equinox in 2005, we had high Moon on March 18, and a low Moon 2 weeks later on April 1-2.
The crescent Moon near winter solstice will be the southern rising and setting Moon (see diagram below). The crescent Moon near summer solstice will be the northern rising and setting Moon.

MY VISION OF THE FUTURE
- Teach people to pay attention to the sky and help them understand what they see
- Help enrich people's lives through deepening their connection to the Universe
- Build Sunwheels in National Parks
- Build Sunwheels on University Campuses
- BUILD A SUNWHEEL IN EVERY COMMUNITY!