Lunar standstills

The Moon in January

The Moon in February

The Moon in March

The Moon in April

The Moon in May

The Moon in June

The Moon in July

The Moon in August

The Moon in September

The Moon in October

The Moon in November

The Moon in December

In 2006 the Moon was at a "major standstill". This page sets out to explain what this means, and how it affects the way the Moon appears in the sky.

The term "lunar standstill" was apparently coined by the archaeologist Alexander Thom, in his 1971 book Megalithic Lunar Observatories (Oxford University Press). It is similar to the term "solstice", which means "sun standstill"; but in neither case does the moon or the sun actually stand still!

Consider the sun first. The earth's rotational axis is tipped to the plane of its orbit:


During the northern-hemisphere summer, the sun appears to be in the northern half of the sky; during winter it appears to be in the southern half. Its varying position is shown in Figure 1. It reaches extremes of 23.4 degrees above and below the celestial equator, at present (these numbers were a little greater in Megalithic times).

Figure 1

Figure 1 The sun's yearly path.

The sun's changing position in the sky controls many aspects of its apparent daily motion: how high it appears at midday; how early it rises and how late it sets; and also exactly where on the horizon it rises and sets. It is the last of these that is probably easiest to observe without instruments.

At the March equinox (marked Aries on Figure 1) the sun rises due east and sets due west; but over the following weeks, for observers in north temperate latitudes, the rising and setting points move northwards along the horizon. (At the same time, it rises earlier and sets later every day, and it climbs higher in the sky at midday.) This movement slows down and halts at midsummer, before reversing; in this sense the sun "stands still" at the midsummer solstice.

So what of the moon? To a first approximation, it's simply a high-speed mimic of the sun, repeating the sun's annual north-south cycle in every monthly orbit. For example, in February the last-quarter moon, being 90 degrees west of the sun, shows us roughly how the sun was behaving three months ago, in November.

But the moon's movement is not quite that simple; its orbit is tilted to the plane of the earth's orbit, by 5.1 degrees. So for most of the time, the moon travels either north or south of the sun's path. It crosses it only twice in each orbit, at the nodes (marked ascending node and descending node on Figures 2 and 3).

Figure 2

Figure 2 The moon's path in 1995.

Figure 2 shows the moon's path in the year 1995. The ascending node of the moon's orbit on the ecliptic lay just to the east of the September equinox Libra ; the descending node lay just to the east of the March equinox Aries. The figure also shows that, in 1995, the extreme northern and ~southern declinations of the moon were less than those of the sun.

But the nodes of the moon's orbit do not remain fixed: they drift steadily westwards along the ecliptic, taking 18.6 years to make one complete circuit. Two years after Figure 2, in February 1997, the moon's nodes had moved westwards from their positions in Figure 2, and lay exactly on the equinoxes. As a result, the moon's motion in declination was reduced to its minimum, between +18.3 degrees and -18.3 degrees; this is what Thom calls a minor standstill.

Figure 3

Figure 3 The moon's path in 2005.

By contrast, Figure 3 shows the moon's orbit at the start of 2005: the ascending node was then a little way east of the March equinox, and the descending node was approaching the September equinox. And the extreme northern and southern declinations of the moon were now greater than those of the sun. When we reached June 2006, the nodes again coincided exactly with the equinoxes, and this time we had a major standstill: the moon can reach declinations of +28.6 degrees and -28.6 degrees.

When the moon is near its extreme northern declination, it climbs higher in the sky than usual, and it rises and sets further north than usual. Alexander Thom theorised that many ancient stone circles were designed to keep track of the changing positions of the sun and moon, by monitoring where on the horizon they rose and set: 

"As I write this (1969), the Moon is coming through a major standstill. One cannot fail to be surprised to see it set and rise almost in the north. A fortnight later one is again surprised to see how far to the south are the rising and setting points, and how very low it is at transit." (Megalithic Lunar Observatories, p.22.)

The Moon's node passed the equinox in the middle of June 2006, but the Moon actually reached its most northerly declination on September 15th 2006, and its extreme behaviour was still noticeable for a couple of years after that. However, as the nodes continue to drift westwards, it is gradually returning to "normal". The next minor standstill is in October 2015, and we won't see another major standstill until April 2025.

A little more detail may be found here.

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