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View Full Version : Two Wide & Bright Full Moons in January 2018



Centaur
2017-Dec-31, 01:15 AM
As measured geocentrically, the Full Moon during the evening of 2018 JAN 01 will appear the widest since 2016 NOV 14 and until 2034 NOV 24. It will appear the brightest since 2009 DEC 31 and until 2018 JAN 31. The latter will be a total lunar eclipse with great brilliance before and after traversing the Earth’s penumbra.

For my Moon preview graphics including the upcoming total lunar eclipse, please visit my Moon webpage at www.CurtRenz.com/moon.html

Photos and descriptions of the next two Full Moons would be welcome additions to this thread.

Hornblower
2017-Dec-31, 06:33 PM
Let's enjoy the sight of a biggest and brightest full Moon, while the popular media hyperventilate over exaggerated descriptions of it.

Ken G
2017-Dec-31, 08:00 PM
We'll also get to watch them struggle with the meaning of a "blue Moon." It is often reported that a blue Moon is the second full Moon in a month, but actually it happens in years with 13 full Moons instead of 12, and which Moon is officially the "blue" one is not always completely obvious but is intended to be whatever is the extra full Moon in a season, not in a month. There is a good reason for this, some ancient cultures calculated their seasons as three lunar cycles, so if you have 4 full Moons in a season you have to kind of throw one out, as it were, to avoid getting out of synch with the seasons. So it is essentially a "leap Moon', if you will, but it shouldn't have anything to do with the calendar months. The mistake is so prevalent by now that it is starting to show up in definitions, so I guess that horse is out of the barn at this point.

Centaur
2017-Dec-31, 08:17 PM
Let's enjoy the sight of a biggest and brightest full Moon, while the popular media hyperventilate over exaggerated descriptions of it.

We'll also get to watch them struggle with the meaning of a "blue Moon." It is often reported that a blue Moon is the second full Moon in a month, but actually it happens in years with 13 full Moons instead of 12, and which Moon is officially the "blue" one is not always completely obvious but is intended to be whatever is the extra full Moon in a season, not in a month. There is a good reason for this, some ancient cultures calculated their seasons as three lunar cycles, so if you have 4 full Moons in a season you have to kind of throw one out, as it were, to avoid getting out of synch with the seasons. So it is essentially a "leap Moon', if you will, but it shouldn't have anything to do with the calendar months. The mistake is so prevalent by now that it is starting to show up in definitions, so I guess that horse is out of the barn at this point.

I did try to avoid using the hyperbolic term SuperMoon. ;)

The phrase Once in a Blue Moon originated early in the 19th century and was simply an analogy for anything that rarely or never happens, such as the case of the Moon appearing blue in color. That was its commonly understood meaning when I was young. A 1940s suggestion in a minor almanac that a fourth Full Moon in a season should be called blue was largely ignored. Shortly afterward a Sky & Telescope article misinterpreted that almanac article and said it was the second Full Moon in a calendar month. That was similarly ignored; however the error was eventually corrected by the magazine.

Then in 1961 came a popular doo-wop song called Blue Moon, which referred to the Moon being viewed by a momentarily depressed person, i.e. one who has the “blues”. That year during a radio program discussion of popular music, someone asked what is meant by a blue Moon? A participant answered that he thought he heard it referred to the second Full Moon in a calendar month. Finally the word spread rapidly from there, thus giving that silly notion some currency.

The second Full Moon of a month (or fourth in a season) does not appear blue in color. This application of the term blue Moon is ludicrous. Yet hardly anyone questions it, since we’ve heard it repeated so many times. The popular culture grapevine is amazing, especially in this internet age. Unfortunately, nowadays this means that we only hear the term used in its original sense once in a blue Moon.

Centaur
2017-Dec-31, 08:45 PM
It should also be noted that the lists of widest Full Moons and brightest Full Moons are not identical. This despite popular science writers often referring to a particular Full Moon as being the widest and brightest of the year. A Full Moon near perigee is not the only factor. Another is the Earth near perihelion. And the factor most overlooked is nearness of the Moon to the ecliptic. This enhances the oppositional flash (aka surge or effect). Lunar eclipses of course occur with the Moon near the ecliptic, so many of them appear in the list of brightest Full Moons, that is just before and after the Moon traverses the Earth's penumbra. The second Full Moon this coming January will be further from both the Earth and Sun than the first one. But being nearer the ecliptic gives the second one the lead in brightness.

Below are lists I created of both wide Full Moons and bright Full Moons.

22863

Hornblower
2018-Jan-01, 03:18 AM
As always, Centaur's announcements of these interesting phenomena are most welcome and informative.

Let me share some words to the wise with anyone who might look at the Moon tomorrow night and ask what is such a big deal.

The difference in brightness of the Moon between perihelion and aphelion, with all else equal, is less than a tenth of a magnitude. Experienced variable star observers are hard pressed to see such a slight difference when comparing an object with reliable reference objects. I don't think anyone looking at the Moon in isolation would notice it.

The difference between having the full Moon spot-on at perigee and having two successive full Moons straddle the perigee position is going to be an even smaller fractional difference. I am as sure as I can be that nobody would notice it when just gazing at the Moon. It would be detectable in a telescope equipped with a suitable reticle or by comparing and measuring photographs.

The difference between side by side images of the Moon at perigee and apogee are mind-blowing, but I was never particularly aware of it when just gazing at the Moon unless I was looking for it. For me, the position of Mare Crisium with respect to the limb is more eye-catching than the variations in the Moon's angular diameter. At first quarter last week I looked up and saw it hard over against the limb and thought to myself, "Yes, that is consistent with the last new Moon at apogee. The Moon has just passed through the slow part of its orbit, and its rotation is outrunning its orbital motion." Seven months earlier, Mare Crisium had been well separated from the limb at first quarter. It was an easy naked eye observation with my rebuilt eyes. Two years ago my cataracts would have prevented that observation.