Moonlight Madness: The Frustrating Full Phase

This is not going to be a very good week for skygazing, thanks chiefly to the presence of a brilliant moon that will officially turn full on Saturday, Nov. 24. During the early evening hours, a large swath of the southern sky appears almost devoid of any stars, because the moon's light drowns out most of the faint autumn stars and constellations.

Even the visibility of Comet Holmes, which has put on a stunning show in recent weeks, will be adversely affected. The comet, which to the naked eye resembles a dim circular cloud subtending even larger in apparent size than the moon (and in real terms is larger than the sun's diameter), will nonetheless be blotted out by the brilliant moonlight for several days.

It's not even a good time to look at the moon through binoculars or a telescope.

Too bright

Normally, even with just small optical power we can see a wealth of detail on its surface. But around the time of full phase, the moon appears comparatively flat and one-dimensional as well as incredibly bright.

But later in the week, as the moon wanes to a gibbous phase and then to last quarter, those lunar features lying close to the terminator – the variable line between the illuminated portion and the part of the moon in shadow – will appear to stand out in sharp, clear relief.

The moon will arrive at last quarter phase on Saturday, Dec. 1 at 7:44 a.m. EST, when its disk will be exactly 50 percent illuminated. How does it brightness compare at that moment with when it's full?

Most would believe it?s half as bright, but in reality astronomers tell us that the last quarter moon is only 1/11th as bright as full. This is because the moon is not a smooth sphere, but has a myriad of craters, mountains and valleys which cast long, distinct shadows across the lunar landscape. Interestingly, a first quarter moon is actually slightly brighter than a last quarter moon, because at first quarter the illuminated half of the moon displays less of the dark surface features known as the "maria" (pronounced măr-rēa) popularly referred to as lunar "seas."

And believe or not, it isn?t until just 2.4 days before or after full that the moon actually becomes half as bright as full!

Lunar cycles

Here are some interesting calendrical facts that the famed Belgian astronomical calculator Jean Meeus has compiled concerning the phases of the moon:

All are cyclical, the most noteworthy being the so-called Metonic Cycle that was independently discovered by the Greek astronomer Meton (born about 460 B.C.). This is a 19 year cycle, after which time the phases of the moon are repeated on the same days of the year, or approximately so.

Take, for instance, this weekend's full moon of Nov. 24. Nineteen years hence, in 2026 there?ll be another full moon on Nov. 24.

Another interesting cycle: after 2 years, the preceding lunar phase occurs on or very nearly the same calendar date. Thus, in 2009, it will be the First Quarter moon that occurs on Nov. 24. After 8 years, the same lunar phases repeat, but occurring one or two days later in the year. The Greeks called this 8-year cycle the octaeteris. Indeed, in 2015, a full moon occurs on Nov. 25.

Finally, in our Gregorian Calendar, 372 years provides an excellent long period cycle for the recurrence of a particular phase on a given date. Thus, we know with absolute certainty that the same full moon that shines down on us on Nov. 24 of 2007 will also be shining on Nov. 24 in the year 2379.

Mark your calendars!