Lunar standstill
Encyclopedia
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
.
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.
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 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 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°.
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.
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.
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.
, 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.
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
of the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
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
Bronze Age
The Bronze Age is a period characterized by the use of copper and its alloy bronze as the chief hard materials in the manufacture of some implements and weapons. Chronologically, it stands between the Stone Age and Iron Age...
societies who built the megalithic monuments in Britain
Prehistoric Britain
For the purposes of this article, Prehistoric Britain is that period of time between the first arrival of humans on the land mass now known as Great Britain and the start of recorded British history...
and Ireland
Prehistoric Ireland
The prehistory of Ireland has been pieced together from archaeological and genetic evidence; it begins with the first evidence of Mesolithic hunter-gatherers settling in Ireland around 7000 BC and finishes with the start of the historical record, around AD 400. The prehistoric period covers the...
, 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
Chimney Rock Archeological Site
Chimney Rock Archaeological Area is an archeological site within the San Juan National Forest in Colorado. This area is located in Archuleta County, Colorado between Durango and Pagosa Springs and is managed for archaeological protection, public interpretation, and education.-Geography:Chimney...
in Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
and Hopewell Sites in Ohio
Ohio
Ohio is a Midwestern state in the United States. The 34th largest state by area in the U.S.,it is the 7th‑most populous with over 11.5 million residents, containing several major American cities and seven metropolitan areas with populations of 500,000 or more.The state's capital is Columbus...
.
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 latitudeLatitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
and longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
, we measure positions of stars on this sphere in right ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...
(equivalent to longitude) and declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
(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
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
and Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
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
Summer solstice
The summer solstice occurs exactly when the axial tilt of a planet's semi-axis in a given hemisphere is most inclined towards the star that it orbits. Earth's maximum axial tilt to our star, the Sun, during a solstice is 23° 26'. Though the summer solstice is an instant in time, the term is also...
to −23.5° at the Winter Solstice
Winter solstice
Winter solstice may refer to:* Winter solstice, astronomical event* Winter Solstice , former band* Winter Solstice: North , seasonal songs* Winter Solstice , 2005 American film...
. 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
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
Precession
Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...
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 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.
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, parallaxParallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
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
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
- 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 ThomAlexander Thom
Alexander "Sandy" Thom was a Scottish engineer most famous for his theory of the Megalithic yard, categorization of stone circles and his studies of Stonehenge and other archaeological sites.- Life and work :...
, in his 1971 book Megalithic Lunar Observatories (Oxford University Press). The term "solstice
Solstice
A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...
", 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
- http://www.chimneyrockco.org/standstill1new.htm Lunar standstill alignment at Chimney Rock
External links
- Major Lunar Standstill 2006 A photographic digital mosaic of the 2006 event from Greece