On Christmas Eve 2015, Millstone’s, Brent Eades, observed and photographed (above) a beautiful Full Moon over the Mississippi River . He mentioned that this is the first Christmas Full Moon since 1977 (see Moonrise.) That there was an interval of 38 years nagged at my consciousness. until I realized that the interval is roughly twice the Saros Lunar periodic cycle that the moon completes every 18+ years.
No two months are exactly the same, the lunar orbit is not circular, and it’s path is ever-changing. The plane of the Moon’s orbit is tilted by 5.1° relative to the Earth’s orbit (which is called the ecliptic.) The nodes in the orbit, and the distance to the earth slowly vary over several years. However, The Saros Cycle puts the moon ‘back’ in exactly the same place with respect to earth as follows:
“The Earth-Sun-Moon geometry will be nearly identical: the Moon will have the same phase and be at the same node and the same distance from the Earth. In addition, because the Saros is close to 18 years in length (about 11 days longer), the earth will be nearly the same distance from the sun, and tilted to it in nearly the same orientation (same season).” – courtesy “Saros” article in Wikipedia
Since the moon’s orbit is tilted to the earths’ orbit (the ecliptic) with the Ascending and Descending nodes (intersection points ) of its orbit regressing rapidly, it completes a full cycle with the regression of the nodes in 18.61 years. – courtesy RASC Observer’s Handbook 2016 , Lunar Observing (by Bruce McCurdy)
It is largely due to the effects of the Sun’s gravity, the Moon’s orbital plane does not stay fixed in space, but “precesses” with an 18.6 year cycle, while still maintaining the 5.1° tilt relative to the ecliptic. – See Precession of the Moon’s Orbit
So this precession has brought us back to a Full Moon on Christmas Eve 2015. It was a rare event, and a multiple of the Saros Cycle. The next Full Moon on Christmas Eve is just a Saros cycle away, 2034. See How rare is a full moon on Christmas
What is the likelihood we will see a similar image for this next Christmas Full Moon in the next Saros Cycle ?
“Many of the cycles that fascinated the ancients can be observed directly. Understanding these cycles is interesting in its own right. It is also of value to telescopic lunar observer who can benefit when the Moon is favourably placed in the sky, as well as to dark Night Sky observers , who may be more interested in knowing when it is below the horizon”. – (after Bruce McCurdy (RASC) Lunar Observing, )
In Mississippi Mills, thanks to our Night Sky Conservation ,we are able to enjoy both the light of the Full Moon as well as the much more distant light of the stars when the Moon is ‘out of the picture’.
