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Summary

Description
English: Absolute value of declination for lunistices in 2006, a year of a major lunistice. At the bottom are shown the times of phases and ecliipses.

The declination of the lunistices follows a 18.6-year cycle, but with a small 173-day perturbation. The graph shows that in the short term, this perturbation causes much greater changes in declination than the slow change due to moving around the 18.6-year cycle. These perturbations peak when the sun is lined up with the moon's nodes, which is also near the times when eclipses occur. So there are always eclipses, lunar and solar, near the time of the major lunistices, when the moon reaches its furthest north or south point of the 18.6-year cycle, as well as near minor lunistices, when the declinations are closest to zero. These major and minor lunistices occur when the moon's node is near an equinox direction (right ascension 0° or 180°). This means that when the sun lines up with the nodes it is also near an equinox direction, and therefore the major and minor lunistices occur near the moments of the vernal or autumnal equinoxes. If the moment of the sun lining up with the nodes is more than two or three weeks away from an equinox, then the major or minor lunistice will occur around a later or earlier equinox.

Within two weeks before or after the moment of the sun lining up with the nodes there will be both a lunar and a solar eclipse (two weeks apart). About one week after the solar eclipse, in the spring, or three weeks before it, the moon will arrive at right ascension 90° and one of these will be the northern lunistice. In the autumn, this will be about one week before or three weeks after the solar eclipse. In any case, this will happen within about half a month of the sun lining up with the nodes, which as said will be within a couple of weeks of the equinox. The same goes for the southern lunistice.

In 2006, the moment when the node crossed zero right ascension was in June, but the furthest north lunistices were in early April and in mid-September, near the equinoxes, as seen in the graph. Likewise, the furthest south lunistices were in late March and late September.

The dates, inclination, and node were calculated using the mean longitude, mean anomaly, mean elongation, mean node, mean argument of latitude, and mean solar anomaly and the periodic terms for the node and inclination given in "Numerical expressions for precession formulae and mean elements for the Moon and the planets" by the group at the Bureau des Longitudes, Paris, 1994. From the node and inclination, and the obliquity of the earth (assumed constant), one can calculate the location of the lunistice.
Date
Source Own work
Author Eric Kvaalen

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Captions

Absolute value of declination for lunistices in 2006. At the bottom are shown the times of phases and ecliipses.

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3 March 2024

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Date/TimeThumbnailDimensionsUserComment
current19:37, 9 March 2024Thumbnail for version as of 19:37, 9 March 2024605 × 340 (28 KB)Eric KvaalenAdded more periodic terms.
09:47, 3 March 2024Thumbnail for version as of 09:47, 3 March 2024605 × 340 (27 KB)Eric KvaalenUploaded own work with UploadWizard

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