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Major and Minor Lunar Standstills
by Simon at 18:33 30/07/15 (Blogs::Simon)
The attached PDF graphs the declination of the Moon over the span of 2012 to 2026 as seen from Stonehenge, to help you spot when the next lunar standstills will occur.

The graphs are generated using a Java applet available on http://www.jgiesen.de/moondata/

Explanation:

Because the Moon's orbit is inclined to the plane of the Earth's orbit at about 5.1, and also because the Moon's orbital ellipse precesses (actually regresses) around the Earth, there are years when the Moon's maximum and minimum declination can be far greater than that of the Sun.

In years of a Major Standstill, the Moon's greatest northern declination can reach (approx) +23.5 + 5.1 = 28.6 and its minimum can reach (approx) -23.5 - 5.1 = -28.6.

In Major Standstill years, therefore, the Moon can both rise AND set far further to the north and south than the Sun can ever reach.

So if you're standing in the centre of Stonehenge, in a Major Standstill Year you will be able (weather and phase permitting) to observe the Moon rise well to the left (north) of the position that the Sun reaches at summer solstice.

Similarly, in the same Major Standstill year, you will ALSO be able to observe the Moon rise well to the right (south) of the position that the Sun reaches at winter solstice.

So in a Major Standstill year, the range of azimuths along the eastern horizon that the Moon can have at rising is larger than the range that the Sun can ever have.

In fact, the Moon's rising position can span a range of azimuths of some 100 (from about 41 E of N to about 141 E of N) in Major Standstill years, whereas the Sun can only ever manage 80 (from about 50 E of N to 130 E of N).

In Minor Standstill years, the Moon's declination ranges from approx +23.5 - 5.1 = +18.4 to -23.5 + 5.1 = -18.4.

At Minor Standstill, the range of azimuths that the Moon can have at rising (and setting) is at its minimum.

So if you watch the Moon rise over the course of a Minor Standstill year, you will not see it rise as far north as the Sun does at summer solstice, nor will you see it rise as far south as the Sun does at winter solstice.

In Minor Standstill years, the Moon's rising azimuth range is about 60, very roughly from about 61 E of N to 121 E of N.

This does not mean that in Minor Standstill years the Moon achieves either rising or setting azimuths that it cannot achieve at other times in the cycle. At any time in the 18.6-year-long cycle the Moon might rise at 61 E of N or at 121 E of N. It might do so tomorrow night.

It's only in Major Standstill years that the Moon's rising and setting azimuths reach values that they cannot achieve at any other time in the cycle, and it's these extreme positions that seem to have been important to skywatchers of the past.

Finally, this 18.6 year cycle from one Major Standstill to the next Major Standstill is called "The Regression of the Lunar Nodes" cycle - it's specifically not the Metonic Cycle, which is something else entirely. 18.6 years is the time it takes for the Moon's orbital ellipse to regress once around the Earth.

Finally, finally, Minor Standstills happen halfway between Major Standstills, that is 9.3 years after a Major Standstill.

All clear now? Good :-)
--
simon

NB: Figures and azimuths massively simplified for slightly easier comprehension.

Attachments...
PDF file (1245 K) Moon declinations from 2012 to 2026
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