Equalized Lunar Standstill
About halfway between a Minor Lunar Standstill and a Major Lunar Standstill, the maximum rising point of the moon during certain monthly standstills will match those the sun reaches each year at the solstices. These further divisions have not received much attention* as they overlap with the same positions as the sun at solstices, but given that those positions were of importance to some ancient observers, when the moon reached the same positions they would likely have at least taken some notice. It is possible that the matching up of the yearly solar rising/setting positions with the slow oscillation between maximum and minimum lunar rising/setting points would have been of great interest in connecting these two cycles.
It is proposed by the Archaeoastronomy Database (and for use therein) that these points in the cycle of standstills should be termed Equalized Lunar Standstills, one with maximum positions increasing from Minor Lunar Standstill toward Major Lunar Standstill, and another with positions decreasing from Major Lunar Standstill back to Minor Lunar Standstill. Northern Equalized Lunar Standstill and Southern Equalized Lunar Standstill can be used to distinguish the range of maximum declination values at each extreme of the monthly swing.
The position of the sun and moon are often measured using geocentric declination which is an angle referenced from the center of the Earth, but because actual observers on the surface of the Earth are positioned at higher or lower latitudes, matching a position of the moon on the sky and horizon to that of the sun requires adjustment for lunar parallax. The sun is relatively far away and so the effect of parallax is smaller for solar positions, but the moon is much closer and so the observed angle to its position from different latitudes can vary widely. This also causes the matching positions with the northern solstice and southern solstice to take place at different points in the Equalized Lunar Standstill 'season' separated by several months.
At approximately 1:30 UTC on 5 March 2020 the geocentric declination of the moon will reach 23.4 degrees as it aligns with the direction of the tilt of Earth's axis. Because of the canceling effect of crossing through the ascending node of its tilted orbit during that orientation, it will match with the geocentric declination of the sun at the Northern Solstice. As the lunar cycle progresses to a point occurring at 14:00 UTC on 17 March 2020, the moon will pass through the descending node and align with the direction of Southern Solstice to attain the closest matching standstill geocentric declination of -23.5 degrees. Because lunar parallax has such an effect depending on the latitude of the observer, these geocentric declination matches mark something of a center point in the Equalized Lunar Standstill 'season' and place it in 2020 for convenience of reference.
In September 2019 the moon will begin to match the position of the northern solstice (23.4°) for observers in the southern hemisphere continuing on for observers nearer and nearer the equator through January 2020 around the same time that positions matching the southern solstice (-23.4°) will be reached and then continue on through August 2020 for lower and lower southern latitudes. For the northern hemisphere, high latitudes will match the southern solstice positions in September 2019 with lower latitudes seeing the matching positions from their angle closer to January 2020. In early 2020 the northern latitudes will begin to experience matching positions for the northern solstice with the apparent position for higher and higher latitudes reaching the correct values on through August.
Depending on how closely local moonrise and moonset correspond to the moment the declination of the moon reaches any given value depends on longitudinal position as well and may cause the preceding or following rise/set event to be closer to the target value, so it is best to check values on either side of a monthly standstill to ensure the best observations. Timing of certain brightly illuminated moon phases that rise or set during when the sun is below the horizon can also be an important factor in some observations.
Note: During an Equalized Lunar Standstill eclipses occur near Solstices.
During Major Standstills the lunar nodes are oriented in line with the equinoxes and the high point in the lunar orbit is aligned with the direction of Northern Solstice and the low point with the direction of Southern Solstice to produce an additive effect to achieve maximum declinations to the north and south of the ecliptic. During Minor Standstills the orientation is exactly reversed to match the low point in the orbit with the position of Northern Solstice and the high point with the position of Southern Solstice for a subtractive effect that limits the range of monthly declination to a minimum.
The Major and Minor Standstills are much discussed and had some significance for certain ancient cultures. Since the cycle between them is 18.6 years there are not many opportunities to make direct observations and to document them at the myriad sites that have possible connections that still exist to any extent. For many ancient sites with putative precision alignments, actual observation, while intriguing, is of limited use because the extreme positions of the sun or moon at the time the alignments were created are no longer attained due to the decreasing obliquity of Earth's orbit over the last few thousand years. One exciting aspect of the concept of Equalized Lunar Standstills for archaeoastronomy beyond its possible interest (if any) to ancient cultures, is the opportunity to use the moon as a proxy for the ancient positions of the sun at solstices.
The maximum lunar declination values to the north and south for the moon during the Increasing Equalized Lunar Standstill of 2019-2020 will continue to increase beyond those matching the current solar maximum values as the cycle progresses towards Major Lunar Standstill. During each subsequent monthly lunar standstill, values will be reached that are slightly higher than are currently possible for the sun but are analogous to certain times in the past. As the year goes on the monthly standstill positions of the moon will pass through successively more extreme values and after adjusting for parallax based on latitude, these can be matched to specific ancient positions of the sun. Although the moon will continue to swing back through all other values previously attained during the increase, it is only at the monthly Northern and Southern Standstill events that the apparent slowing and stopping of declination change (and corresponding positions in the sky and on the horizon) that observations close to target declinations are likely to be achieved. Each target declination will likely only be observed within precision parameters once during the increasing phase in the cycle, perhaps with an alternate observation in the preceding or following lunar month depending on desired precision or one day before or after a standstill depending on positioning at local rise/set events. Sometimes it is possible to have very close values to the target repeated one moonrise or moonset apart as the declination does not increase steadily, but trends over time in incremental steps at varying rates.
In early 2020 the monthly standstill positions correspond with the declination of the winter solstice sun during the Neolithic period in Britain during which many astronomically aligned sites in that area were constructed, but many of these are during unfavorable phases of the moon for observation. A few acceptable observations might still be possible depending on local timing through March and April with less precise observations continuing into early June. By September 2020 parallax adjusted position of the moon will match the position of the Neolithic summer solstice sun. After the window of modern and historic values following the current Increasing Equalized Lunar Standstill period, the moon will continue toward maximum monthly values outside those reached by the sun at any time. The ancient values will not be attained during the slowing of a monthly standstill again for almost a decade until after the Major Lunar Standstill (2025) is over and the lunar orbit makes sufficient progress towards the following Decreasing Equalized Lunar Standstill (2029). For this reason the current opportunity for scientific observation is very important. Even volunteers with limited experience and equipment can gather crowd-sourced data in the form of timestamped photographic evidence of moonrise and moonset positions at various sites to document possible or established alignments with solstice positions, and more in-depth investigations can also be undertaken as opportunities arise.
Years of Occurrence for Events in the Standstill Cycle of the Moon:
2015 Minor Lunar Standstill
2020 Equalized Lunar Standstill (increasing)
2025 Major Lunar Standstill
2029 Equalized Lunar Standstill (decreasing)
2034 Minor Lunar Standstill
Possible Major Sites for Initial Documentation:
Stonehenge
Newgrange
Maeshowe
Slieve Gullion
Townley Hall Passage Tomb
Ballynoe
Beaghmore
Chûn Quoit
Bryn Celli Ddu
*Research shows some discussion of the concept by Brennan, Murphy, Moore, and Byrne.
(proposal is to formalize terminology of matching solar/lunar positions in lunar standstill cycle to aid research/discussion)
It is proposed by the Archaeoastronomy Database (and for use therein) that these points in the cycle of standstills should be termed Equalized Lunar Standstills, one with maximum positions increasing from Minor Lunar Standstill toward Major Lunar Standstill, and another with positions decreasing from Major Lunar Standstill back to Minor Lunar Standstill. Northern Equalized Lunar Standstill and Southern Equalized Lunar Standstill can be used to distinguish the range of maximum declination values at each extreme of the monthly swing.
The position of the sun and moon are often measured using geocentric declination which is an angle referenced from the center of the Earth, but because actual observers on the surface of the Earth are positioned at higher or lower latitudes, matching a position of the moon on the sky and horizon to that of the sun requires adjustment for lunar parallax. The sun is relatively far away and so the effect of parallax is smaller for solar positions, but the moon is much closer and so the observed angle to its position from different latitudes can vary widely. This also causes the matching positions with the northern solstice and southern solstice to take place at different points in the Equalized Lunar Standstill 'season' separated by several months.
At approximately 1:30 UTC on 5 March 2020 the geocentric declination of the moon will reach 23.4 degrees as it aligns with the direction of the tilt of Earth's axis. Because of the canceling effect of crossing through the ascending node of its tilted orbit during that orientation, it will match with the geocentric declination of the sun at the Northern Solstice. As the lunar cycle progresses to a point occurring at 14:00 UTC on 17 March 2020, the moon will pass through the descending node and align with the direction of Southern Solstice to attain the closest matching standstill geocentric declination of -23.5 degrees. Because lunar parallax has such an effect depending on the latitude of the observer, these geocentric declination matches mark something of a center point in the Equalized Lunar Standstill 'season' and place it in 2020 for convenience of reference.
In September 2019 the moon will begin to match the position of the northern solstice (23.4°) for observers in the southern hemisphere continuing on for observers nearer and nearer the equator through January 2020 around the same time that positions matching the southern solstice (-23.4°) will be reached and then continue on through August 2020 for lower and lower southern latitudes. For the northern hemisphere, high latitudes will match the southern solstice positions in September 2019 with lower latitudes seeing the matching positions from their angle closer to January 2020. In early 2020 the northern latitudes will begin to experience matching positions for the northern solstice with the apparent position for higher and higher latitudes reaching the correct values on through August.
Depending on how closely local moonrise and moonset correspond to the moment the declination of the moon reaches any given value depends on longitudinal position as well and may cause the preceding or following rise/set event to be closer to the target value, so it is best to check values on either side of a monthly standstill to ensure the best observations. Timing of certain brightly illuminated moon phases that rise or set during when the sun is below the horizon can also be an important factor in some observations.
Note: During an Equalized Lunar Standstill eclipses occur near Solstices.
During Major Standstills the lunar nodes are oriented in line with the equinoxes and the high point in the lunar orbit is aligned with the direction of Northern Solstice and the low point with the direction of Southern Solstice to produce an additive effect to achieve maximum declinations to the north and south of the ecliptic. During Minor Standstills the orientation is exactly reversed to match the low point in the orbit with the position of Northern Solstice and the high point with the position of Southern Solstice for a subtractive effect that limits the range of monthly declination to a minimum.
The Major and Minor Standstills are much discussed and had some significance for certain ancient cultures. Since the cycle between them is 18.6 years there are not many opportunities to make direct observations and to document them at the myriad sites that have possible connections that still exist to any extent. For many ancient sites with putative precision alignments, actual observation, while intriguing, is of limited use because the extreme positions of the sun or moon at the time the alignments were created are no longer attained due to the decreasing obliquity of Earth's orbit over the last few thousand years. One exciting aspect of the concept of Equalized Lunar Standstills for archaeoastronomy beyond its possible interest (if any) to ancient cultures, is the opportunity to use the moon as a proxy for the ancient positions of the sun at solstices.
The maximum lunar declination values to the north and south for the moon during the Increasing Equalized Lunar Standstill of 2019-2020 will continue to increase beyond those matching the current solar maximum values as the cycle progresses towards Major Lunar Standstill. During each subsequent monthly lunar standstill, values will be reached that are slightly higher than are currently possible for the sun but are analogous to certain times in the past. As the year goes on the monthly standstill positions of the moon will pass through successively more extreme values and after adjusting for parallax based on latitude, these can be matched to specific ancient positions of the sun. Although the moon will continue to swing back through all other values previously attained during the increase, it is only at the monthly Northern and Southern Standstill events that the apparent slowing and stopping of declination change (and corresponding positions in the sky and on the horizon) that observations close to target declinations are likely to be achieved. Each target declination will likely only be observed within precision parameters once during the increasing phase in the cycle, perhaps with an alternate observation in the preceding or following lunar month depending on desired precision or one day before or after a standstill depending on positioning at local rise/set events. Sometimes it is possible to have very close values to the target repeated one moonrise or moonset apart as the declination does not increase steadily, but trends over time in incremental steps at varying rates.
In early 2020 the monthly standstill positions correspond with the declination of the winter solstice sun during the Neolithic period in Britain during which many astronomically aligned sites in that area were constructed, but many of these are during unfavorable phases of the moon for observation. A few acceptable observations might still be possible depending on local timing through March and April with less precise observations continuing into early June. By September 2020 parallax adjusted position of the moon will match the position of the Neolithic summer solstice sun. After the window of modern and historic values following the current Increasing Equalized Lunar Standstill period, the moon will continue toward maximum monthly values outside those reached by the sun at any time. The ancient values will not be attained during the slowing of a monthly standstill again for almost a decade until after the Major Lunar Standstill (2025) is over and the lunar orbit makes sufficient progress towards the following Decreasing Equalized Lunar Standstill (2029). For this reason the current opportunity for scientific observation is very important. Even volunteers with limited experience and equipment can gather crowd-sourced data in the form of timestamped photographic evidence of moonrise and moonset positions at various sites to document possible or established alignments with solstice positions, and more in-depth investigations can also be undertaken as opportunities arise.
Years of Occurrence for Events in the Standstill Cycle of the Moon:
2015 Minor Lunar Standstill
2020 Equalized Lunar Standstill (increasing)
2025 Major Lunar Standstill
2029 Equalized Lunar Standstill (decreasing)
2034 Minor Lunar Standstill
Possible Major Sites for Initial Documentation:
Stonehenge
Newgrange
Maeshowe
Slieve Gullion
Townley Hall Passage Tomb
Ballynoe
Beaghmore
Chûn Quoit
Bryn Celli Ddu
*Research shows some discussion of the concept by Brennan, Murphy, Moore, and Byrne.
(proposal is to formalize terminology of matching solar/lunar positions in lunar standstill cycle to aid research/discussion)
