Image 1: Earth's Moon. Captured with the Celestron 11" SCT on September 21, 2019. The camera was the ZWO ASI294MC Pro. The moon was 50% illumination and was entering a "mansion" near 6h of Right Ascension. The time of year was the Autumnal Equinox, a fact which could be determined by the phase and mansion.
Some time ago I realized something interesting about the full moon.
We know that the moon is full when it is in the opposite part of the ecliptic from the sun. At this point the Sun, Earth and Moon form a straight line.
When the sun is in the north, from the vernal equinox through the autumnal equinox, then the full moons will be located in the south. In winter, then, the full moons will be high in our northern skies. In summer at my observatory I cannot observe the full moon, far from it, because it is lost in the southern trees. The further south the sun is (closer to the winter solstice) the higher in the northern sky will be the full moons, and vice versa.
I stumbled upon the concept of “Lunar Mansions” recently and had quite a difficult time understanding what these were, and then why they were important to the ancients. I suspect astrologers are still interested in this subject but others have mostly forgotten. History shows that they were important for determining the season. How is that?
The Earth’s moon is the brightest celestial object (after the sun) by a huge margin. It would have been extremely obvious to the ancient Arabs in the desert. The varying location of the moon and its changing phases would all have been well known.
The moon (apparently) follows roughly the same path through the fixed stars as the sun. This path is known as the ecliptic. The ecliptic runs through 12 constellations, the zodiac. The sun moves through the zodiac in one year, 365 days, or 360 degrees. The moon moves through the entire zodiac and returns to the same point in a period of approximately 27.3 days. The length of the lunar cycle (which includes Earth's motion about the sun) is 29.5 days.
In a period of 29.5 days the moon will have gone completely around the ecliptic and will have arrived approximately 30 degrees advanced from where it started. This matches the length of a zodiacal sign: The moon's motion no doubt contributed to the fact that we have 12 signs of the zodiac each spanning 30 degrees.
Like the zodiac, the lunar path can be broken up into segments representing the moon’s apparent travels through the stars in a week or in a day. The number 27.3 is rather inconvenient, isn’t it? Some cultures used 27 segments, some 28. Each segment was like a day’s journey, ending with a night out under the skies in a camp. These one-day journeys and the rest camps are the “mansions’. Since the monthly orbit is not an even number of days the moon doesn’t always camp out in the proper campsite, but it is following the trail and the general ‘mansion’ does tell us where it is in the journey.
Note that the word “mansion” comes from Latin “mansio” meaning way-station or stopover. A camp-site to me.
We can now imagine the moon traveling through the entire zodiacal path in 27.3 days, and that path is broken up into segments such that the moon enters a new segment roughly each day. The fact that there are 27 or 28 segments and 27.3 days just means that it is not starting at the exact zero point of the specific mansions.
Let’s assume that segment #1 starts at the modern zero point (equivalent to the first point of Aries), zero Right Ascension. Let’s have 27 segments, so each segment will have 13.33 degrees in length. Thus the first mansion extends from 0 to 13.33 degrees of Right Ascension. In 2022 there should be 12 or so dates when the moon will be in this mansion. What are they?
Aside: Right Ascension is usually measured in hh:mm:ss. 24 hours is 360 degrees, and one hour of Right Ascension is 15 degrees, so 13.33 degrees is 00:53:19. Roughly 53 minutes, 20 seconds.
TABLE 1: Lunar characteristics and time when entering first "mansion" at the first point of Aries.
This table tells us that the moon returns to the starting point among the fixed stars roughly 13 times in a year. In fact it goes around the stars one extra time relative to the stars than we expect because of the sun's (apparent) annual motion. At this point in the orbit the moon is moving slowly in Right Ascension and takes actually a bit more than a day to come to the end of the first “mansion”: 27 ½ hours instead of 24. I checked and at 6hours of right ascension the moon takes roughly 24 hours to complete a “mansion” and is quicker when at right ascension 18hr, racing through that mansion in only 20 hours and 20 minutes. These anomalies are related to the ellipticity of the moon’s orbit. See Table 2.
TABLE 2: Lunar characteristics and time when entering lunar "mansions" at other points in the orbit (6h, 12h, 18h).
In final column of Table 1 we see the moon’s percentage illumination. There is a simple cycle of lunar illumination over the course of the year if we look at those days when the moon crosses the zero RA line (first point of Aries). This pattern is roughly the same other years: On March 22, 2012, and March 26, 1990 the moon is at 0% illumination when at RA zero.
When the new moon is near the first point of Aries the time of year is near the Spring equinox. When the full moon is near the first point of Aries, we are close to the Autumnal equinox.
Consider the image shown at the top: It can be shown that if the moon is at 50% illumination and waning, and is in a mansion located at 6 hours of Right Ascension, then you can conclude that the season is near the Autumnal equinox.
If the full moon is in a mansion in the south, it can be shown that the season is summer. If the full moon is in a mansion in the north, it must be winter. Or, as I have been saying for a few months now, the full moon in summer is always in the south.
The ancients could look at the phase of the moon and its location among the stars (the mansion) and quickly deduce from this the season. They could see the moon cycling around and changing its phase as it entered and re-entered the mansions. Each change of the pattern reflected the slow passage of the seasons.
The lunar mansions provided a simple yearly time-keeping device. We know that they were used by the ancients to determine when to plant crops and when to sow.
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