Major and Minor Standstills of the Moon
The Metonic Cycle
While the monthly path of the Moon causes it to swing high and low in the sky, successive Fullmoons - the time when we notice the Moon most - are always opposite the Sun. Thus, as the Sun moves higher in the sky to the North in the Summer, successive Fullmoons rise further to the South and their paths are lower in the sky. In the approach to Winter it is just the opposite. The Sun moves lower, and it is the turn of the Fullmoons to ride high in the Winter sky.
If the Moon is New or Full when it Crosses the Ecliptic, as in the illustration above, there will be an Eclipse. But as you can see in the illustration below, most of the time, this is not the case.
Major and Minor Standstills of the Moon
Our ancient forefathers and mothers were interested in eclipse prediction. One piece of evidence we have is their interest in what is called the Major and Minor Standstills of the Moon.
The Moon deviates from the ecliptic by as much as 5 degrees 08 minutes (5.1°), thus making a total deviation from the ecliptic of 11.2°. It takes 18.67 years for the Moon to go from one extreme to the other and back again. This 18.67 year cycle is called the Metonic Cycle after Meton, the Greek who supposedly identified it. The Fullmoons' rises and sets mirror the Sun's throughout the year. The Fullmoon closest to the Winter Solstice rises around the point where the Summer Solstice Sun rises - and vice-versa, the Fullmoon nearest the Summer Solstice rises around the point on the horizon where the Winter Solstice Sun rises. We will see evidence shortly that the Neolithic people of Britain were well aware of the major standstills of the Moon and more than a millennium before the Greeks.
The reason why these major and minor standstills are important is for the prediction of Eclipses. It takes 9.3 years for the Moon to go from the ecliptic out to one of the extremes, and back to the ecliptic. While an inexact Eclipse can occur at other stages of the cycle, the stage where Eclipses are most exact occurs when the Moon is half-way between extremes of declination, when it is on the ecliptic. If, at the time of the Summer Solstice, the Fullmoon rises where the Winter Solstice Sun rises, there's going to be an Eclipse.
As with all previous exercises, at this point connect the minor standstills and the major standstills across the meridian with two parallel lines. Determine half their length, mark this on the latitude line, and connect the two to create the paths of the various major and minor standstills. Bring these lines down to the horizon. Measure the distance where these paths Cross the level horizon from C, transfer them down to C', and draw the four parallel transfer lines.
The major standstill is as far outside the Solstice rise/declination as possible. The minor standstill is as far inside the Solstice Sun's path (ecliptic) as the Moon deviates.
Connect the various major and minor standstill rises and sets by drawing lines that Cross through C'. Note how the major standstill Moonrises and Moonsets are in opposition to each other, as are the minor standstills.
At the 51st latitude, what is the azimuth to the rising point of the Southern major standstill of the Moon?
While all of the azimuths of the major and minor standstills of the Moon can be easily determined with a circular protractor, we are interested here in the rising point of the Southern major standstill of the Moon. By using our circular protractor:
The azimuth of the Southern major standstill of the Moon is 141°.
There is a very interesting thing that happens when you compare the azimuth of the Summer Solstice Sunrise with the azimuth of the Southern major standstill of the Moon at the 51st latitude.
The major axis of Stonehenge is oriented towards the Summer Solstice Sunrise. The four Station Stones create a rectangle whose longer side is a tangent to the major Trilithon Circle. The longer axis of the rectangle is perpendicular to the major axis, and is aligned with the rising point of the Southern major standstill of the Moon.
As you can see from the animated .gif above, the azimuth of the Summer Solstice Sunrise at the 51st latitude is 51° (an interesting coincidence of 51s), and the azimuth of the rising point of the Southern major standstill of the Moon is 141° - exactly at right angles to each other (90°). This is one of the special characteristics of the location of Stonehenge.
You now have the tools to locate the quarter and Cross-Quarter Days and the major and minor standstills of the Moon, all at any angle of elevation to the horizon, at any latitude. It is our goal to have a cgi script that will give the rising and setting points of the Sun for any point on Earth, on any day of the year, at any latitude between the polar circles.
The 51st latitude is only place on Earth above the Equator where the furthest North that the Sun rises is exactly 90o from the furthest South that the Moon rises! This shows clearly that the builders of Stonehenge I, in about 2750 BCE, knew about this 19.67 year cycle of the Moon several thousand years before Meton 'discovered' it.
It is essential to know about the lunar standstills in order to begin to predict eclipses.