Mohawk Valley Astronomical Society

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Wayward Moon

by Perry Pezzolanella, MVAS

Sometime during the early summer months this year, you may be surprised to find that the Moon crosses the sky at an unusually low angle above the southern horizon even though it rose many hours ago. The Moon always travels low in the night sky during the summer, especially the full moon that falls closest to the summer solstice. However, this year's full moons near the summer and winter solstices are extraordinary.

Normally during the summer, the full moon is low in the night sky, while the Sun blazes high during the day. The reverse is true during the winter. This is due to the orbital path of the Earth around the Sun and the fact that the Earth is tilted on its axis 23.5 degrees to the plane of its orbit. It would not be a big deal except for the fact that the Moon does not orbit the Earth neatly around its equator, and therefore does not always follow the same orbital plane as the Sun and planets do in our sky, which known as the ecliptic. It orbits the Earth at an angle of 5.1 degrees above or below the equator and therefore can appear the same amount above or below the ecliptic. The point where the Moon's orbit crosses the ecliptic is called the node and there are always two nodes with one almost exactly opposite the other. When the Moon crosses the node from above the ecliptic to below, this is called the descending node. If the Moon is crossing the ecliptic from below to above, then the Moon is in the ascending node. The nodes slowly shift along the ecliptic taking 18.6 years to complete a full cycle, known as the lunar Metonic cycle. Twice a year the Moon will cross the nodes resulting in lunar and solar eclipses.

When the nodes align with the summer and winter solstice points, the Moon will not venture to any extremes regardless if it is ascending or descending at those nodes. When the nodes align with the vernal and autumnal equinox points and the Moon is descending at the autumnal equinox node and ascending at the vernal equinox node, the Moon will then end up 5.1 degrees above the ecliptic near the summer solstice point in Gemini and 5.1 degrees below the ecliptic near the winter solstice point in Sagittarius. In this case the Moon can appear unusually high up among the heads of the Gemini Twins or way down low near the bottom of the Sagittarius Teapot. At such extremes, the Moon then has a major declination range from 28.6 to -28.6 degrees, which is the case during 2006. At the other extreme, when the Moon is ascending at the autumnal equinox node and descending at the vernal equinox node, the Moon then has a minor declination range from 18.4 to -18.4 degrees and not as readily noticed except to those who really observe the sky. This will occur in 2015.

This may be an impressive exercise for the mind with regards to celestial motion, but the harmonics, rhythms, and cycles among the objects of the Solar System can be calculated and predicted and can produce some interesting events and curiosities. With the Moon now in a major declination swing, it can be seen crossing the Pleiades or occulting Antares at times when it would otherwise remain far away from them. The Full Moon of June 11 will rise farther south than the Sun can ever rise and it will climb no higher than 18 degrees above the southern horizon at best during the middle of the short night, easily hiding among the trees and buildings, before setting farther south of where the Sun can ever set. On December 4 the reverse will occur; the Full Moon will rise farther north than the Sun could rise and climb to a neck-breaking 76 degrees above the southern horizon, seemingly overhead, during the middle of the long night, and eventually setting farther north than the Sun can ever set. Such an extreme in the Moon's position was last seen during 1987 and will still be noticeable during 2007, but it is best to observe now as the next opportunity will not occur until 2024.