Lunar standstill

At a major lunar standstill, which takes place every 18.6 years, the range of the declination of the Moon reaches a maximum. As a result, at high latitudes, the Moon appears to move in just two weeks from high in the sky to low on the horizon. This time appears to have had special significance for the Bronze Age societies who built the megalithic monuments in Britain and Ireland, and it also has significance for some neo-pagan religions. Evidence also exists that alignments to the moonrise or moonset on the days of lunar standstills can be found in ancient sites of other ancient cultures, such as at Chimney Rock in Colorado and Hopewell Sites in Ohio.

Informal explanation

As the Earth spins on its axis, the stars above us appear to move in circles. It appears to us as if all the stars are fixed in a great sphere surrounding us. In the same way that we measure positions on the earth using latitude and longitude, we measure positions of stars on this sphere in right ascension (equivalent to longitude) and declination (equivalent to latitude). If you stand at a place on the earth which has latitude 50°, then the stars directly above you have a declination of 50°.

Unlike the stars, the Sun and Moon do not have a fixed declination. As the Earth travels its annual orbit around the Sun, with its rotational axis tilted at about 23.5° from the "vertical" (a line perpendicular to the orbit), the Sun's declination changes from +23.5° at the Summer Solstice to −23.5° at the Winter Solstice. Thus, in the Northern hemisphere, the Sun is higher in the sky and visible for a longer period of time in June than it is in December. This is why we have seasons on Earth.

The Moon also changes in declination, but it does so in only a month, instead of a year for the Sun. So it might go from a declination of +25° to −25° in just two weeks, returning to +25° two weeks later. Thus, in just one month the moon can move from being high in the sky, to low on the horizon, and back again.

But, unlike the Sun, the maximum and minimum declination reached by the Moon also varies. This is because the plane of the Moon's orbit around the Earth is inclined by about 5° to the plane of the Earth's orbit around the Sun, and the direction of this inclination gradually changes over an 18.6-year cycle, alternately working "with" and "against" the 23.5° tilt of the Earth's axis. As a consequence, the maximum declination of the Moon varies from (23.5° − 5°) = 18.5° to (23.5° + 5°) = 28.5°. The effect of this is that at one particular time (the minor lunar standstill), the Moon will change its declination during the month from +18.5° to −18.5°, which is a total movement of 37°. This is not a particularly big change, and may not be very noticeable in the sky. However, 9.3 years later, during the major lunar standstill, the Moon will change its declination during the month from +28.5° to −28.5°, which is a total movement of 57°, and which is enough to take its zenith from high in the sky to low on the horizon in just two weeks (half an orbit).

Strictly speaking, the lunar standstill is an instant in time: it does not persist over the two weeks that the Moon takes to move from its maximum (positive) declination to minimum (negative) declination, and it most likely will not exactly coincide with either extreme. However, because the 18.6-year cycle of standstills is so much longer than the Moon's orbital period, the change in the declination range over periods as short as half an orbit is very small.

Detailed explanation

Apparent paths of the Sun and Moon on the celestial sphere (angles exaggerated for clarity)

A more detailed explanation is best considered in terms of the path of the Sun and Moon on the celestial sphere, as shown in the first diagram. This shows the imaginary sphere of the sky, surrounding the Earth at the centre. The Earth is aligned so that the North pole is pointing straight upwards.

The Sun follows the ecliptic, which is tilted at an angle of e = 23.5° to the equator, and completes one revolution around the Earth in one year.

The moon follows its path (shown dotted) which is inclined at an angle of about i = 5° to the ecliptic, and completes one revolution around the Earth in one month. The two points at which its path crosses the ecliptic are known as the nodes, shown as N1 and N2, and the line connecting them is known as the line of nodes. Due to precession of the lunar orbit, these crossing points, the nodes, slowly move round the ecliptic, taking 18.6 years to complete one cycle.

From the diagram, it can be seen^{[clarification needed]} that when the line of nodes aligns with the equator, the Moon's orbit reaches either the steepest or the least steep angle with the equator: the 5° tilt of the Moon's orbit either adds to or subtracts from the declination of the ecliptic. This is when a standstill occurs.

The declination of the Moon in 2005–6.

The effect of this on the declination of the Moon is shown in the second diagram. During the course of each month, as the Moon follows its path around the earth, its declination swings from –m° to +m°, where m is a number in the range (e – i) ≤ m ≤ (e + i). At a minor standstill (which will happen in 2015), its declination during the month varies from –(e – i) = –18.5° to +(e – i) = 18.5°. During 2006, which was a major standstill, the declination of the Moon varied during each month from about –(e + i) = –28.5° to +(e + i) = 28.5°.

The maximum declinations of the Moon in 2005–6.

However, an additional subtlety further complicates the picture. The Sun's gravitational attraction on the Moon pulls it towards the plane of the ecliptic, causing a slight wobble of about 9 arcmin with a 6-month period. In 2006, the effect of this is that, although the 18.6 year maximum occurs in June, the maximum declination of the Moon is not in June but in September, as shown in the third diagram.

Other complications

All discussion above refers to the Geocentric Declination, which is the declination of the Moon as viewed from the position of the centre of the Earth. However, because the Moon is relatively close to the Earth, parallax causes a change of declination (up to 0.95°) when the Moon is observed from a position on the Earth's surface. Thus the geocentric declination discussed above may be up to about 0.95° different from the observed declination. Because the amount of this parallax is not the same for all maxima shown above, it may be sufficient, for example, to make the April 2006 maximum a higher declination, when viewed from a particular site, than the September 2006 maximum. Thus the date of the observed maximum will change from place to place in the world.

Another effect is refraction - the bending of the light from the Moon as it passes through the Earth's atmosphere - which will also change the observed declination of the Moon. This is especially significant at low elevation.

In addition, not all the maxima are observable from all places in the world - the Moon may be below the horizon at a particular observing site during the maximum, and by the time it rises it may have a lower declination than an observable maximum at some other date. Or the maximum may occur in the middle of the day, and thus be effectively invisible.

2006 standstill

Events in Sydney, Australia	Date / Time	RA	Dec	Az.	Elev	Lunar phase
Closest viewing of "true maximum" on 15 September during civil twilight	September 14 19:53	04:42:57.32	+29:29:22.9	3	27	46% waning
Highest visible maximum during civil twilight	April 4 07:49	06:03:11.66	+29:30:34.5	350	26	38% waxing
Highest visible maximum during darkness	April 4 09:10	06:05:22.02	+29:27:29.4	332	21	39% waxing
Lowest visible minimum during civil twilight	March 22 19:45	18:10:57.40	−28:37:33.2	41	83	50% waning
Lowest visible minimum during darkness	March 22 18:36	18:09:01.55	−28:36:29.7	80	71	50% waning
Events in London, UK	Date/Time	RA	Dec	Az.	Elev	Lunar phase
Highest visible maximum during civil twilight	September 15 05:30	06:07:12.72	+28:19:52.6	150	64	42% waning
Highest visible maximum during darkness	March 7 19:43	05:52:33.05	+28:18:26.9	207	64	60% waxing
Lowest visible minimum during civil twilight	September 29 17:44	17:49:08.71	−29:31:34.4	186	9	43% waxing
Lowest visible minimum during darkness	September 2 20:50	18:15:08.74	−29:25:44.0	198	7	70% waxing

Note that all dates and times in this section, and in the table, are in UTC, all celestial positions are in topocentric apparent coordinates, including the effects of parallax and refraction, and the lunar phase is shown as the fraction of the Moon's disc which is illuminated.

In 2006, the minimum lunar declination, as seen from the centre of the Earth, was at 16:54 UTC on 22 March, when the Moon reached an apparent declination of -28:43:23.3. The next two best contenders were 20:33 on 29 September, at a declination of -28:42:38.3 and 13:12 on 2 September at declination −28:42:16.0.

The maximum lunar declination, as seen from the centre of the Earth, was at 01:26 on 15 September, when the declination reached +28:43:21.6. The next highest was at 07:36 on 4 April, when it reaches +28:42:53.9

However, these dates and times do not represent the maxima and minima for observers on the Earth's surface.

For example, after taking refraction and parallax into account, the observed maximum on 15 September in Sydney, Australia took place some hours earlier, and then occurred in daylight. The table on the right shows the major standstills that were actually visible (i.e. not in full daylight, and with the moon above the horizon) from both London, UK, or Sydney, Australia.

For other places on the Earth's surface, positions of the Moon can be calculated using the JPL ephemeris calculator.

Origin of name

The term "lunar standstill" was apparently coined by the archaeologist Alexander Thom, in his 1971 book Megalithic Lunar Observatories (Oxford University Press). The term "solstice", which derives from the Latin solstitium: sol- (sun) + -stitium (a stoppage), describes the similar extremes in the sun's varying declination. Neither the sun nor the moon stands still, obviously; what stops, momentarily, is the change in declination.

Acknowledgement

Data shown in this article were calculated using the JPL ephemeris calculator (HORIZONS), who are thanked for providing such a valuable resource to the community.

Further reading

• Lunar Standstill Season
• A webcamera at Calanais I (Lewis, Scotland) recording the lunar standstill events in 2005, 2006 and 2007
• A project to study the major standstill events in 2005, 2006 and 2007 at (pre-)historic buildings
• [1] Lunar standstill alignment at Chimney Rock

External links

• Major Lunar Standstill 2006 A photographic digital mosaic of the 2006 event from Greece