We get to experience (sort of) two of these in the night sky this month -- we've already had one. What are they?

This December is a good month for stargazers and planet gazers too. Venus is brilliant in the western sky just after sunset, and ruddy Mars commands the firmament once Venus sets. Binoculars show Venus to be a crescent now. From our perspective, it's always brightest when it's in this phase. Of course, we're unable to see it when it's closest to us (when it's between us and the Sun -- new phase) since it's lost in the Sun's glare. Saturn rises about 3 hours after sunset now and can be found in the constellation Gemini the Twins, while Jupiter is a dominating morning "star," rising well after midnight. In addition, Mercury is a fine specimen nearly all month. Look for it in the morning twilight, low in the east-southeast, to the lower left of Jupiter.

Furthermore, the Geminid Meteor Shower peaked earlier this week. This is one of the most reliable showers in terms of number of meteors, but this year, the full Moon (full on December 15) or nearly full Moon coincided with the Geminids, and therefore, the show wasn't up to par -- many of the dimmer meteors were undetectable in the Moon's incessant glow.

When a full Moon occurs in the middle of the month, there's no chance for a "blue moon," the second full Moon in a calendar month. Because the lunar cycle is about 29 1/2 days in length, you can see that it's not easy to fit two full Moons into the same month. However, with a mid-month full Moon, 2 new Moon's are possible, and in fact, there happen to be two this month! We had one on December 1 and the 2nd one occurs on December 30. Thus, this month, we get to experience a complete cycle of lunar phases; from new, to young crescent, to waxing gibbous, to full, to waning gibbous, finally to old crescent, and finally to new again.

By the way, while the more modern definition of the "blue Moon" is the second full Moon in a given calendar month, according to the Maine Farmer's Almanac, a "blue Moon" is referred to as the 3rd full moon in a season having 4 full Moons. The chances of 2 new Moons occurring within one calendar month are just the same as the occurrence of 2 full Moons in a single month. It turns out that on average, they're 82 months in each century having either 2 full Moons or 2 new Moons. So approximately once every 1 1/4 years, we'll experience a month with 2 full Moons or a month with 2 new Moons.

Of course, since we can't see the New Moon, we don't think nearly as much about it as we do the full Moon. Nonetheless, they're important as religious markers. For example, for Muslims the Islamic calendar is administered by the phases of the Moon -- the beginning of each month is based on the earliest sighting of the young crescent Moon, frequently within 12 hours of the new Moon.

Since the second full Moon in a month has a special name, shouldn't the second new Moon? The Wiccans' consider the second New Moon of a given month to be the "Black Moon," and any magic performed on this night is expected to be particularly potent. Black Moon is certainly appropriate, but maybe we can come up with something a little catchier -- how about calling it the Boo Moon?

At least the new Moon doesn't disappoint us. Fair weather or foul, we're never able to see its majestic rise or it's enchanting light. However, only the new Moon delivers arguably the most impressive spectacle of Earth-based astronomy -- the total solar eclipse. This phenomenon can only be observed when the Moon is new. Sure, the resplendent full Moon has an eclipse too (total lunar eclipse), but it's wan indeed compared to the awe inspiring total solar eclipse. Only when the Moon is new, can astronomers enjoy a full night of observing. Moreover, total lunar eclipses are basically "yawners" to astronomers, but during a total solar eclipse, solar astronomers anxiously point their scopes toward the new Moon. In fact, though song writers seem to have a love affair with the full Moon, most astronomers can do without it.

How many songs can you think of that have to do with the new Moon? Pink Floyd devoted an entire album to the Dark Side of the Moon, but said nothing about the new Moon. Shouldn't there be a new Moon song like Moonlight Serenade, By the Light of the Silvery Moon, Shine on Harvest Moon, or Blue Moon? Actually, there was a popular song in the 50s that could be construed as having to do with the new Moon; Dark Moon by singer/actress Gale Storm (I'm sure this is her real name).

Dark moon, a way up high up in the sky
Oh, tell me why, oh, tell me why you've lost your splendor
Dark moon, what is the cause your light withdraws
Is it because, is it because I've lost my love

Good grief! In spite of these lachrymose lyrics, Dark Moon was one of the top hits in 1956, somehow eclipsing (sorry) Teenage Crush, Short Fat Fanny and That'll be the Day in popularity.

If you just have to look at our lustrous Moon this month, some good times to do so include the 18th and 19th when Saturn is in close proximity. Jupiter sidles up to the waning crescent Moon in the pre-dawn sky on December 26 and 27, and on the 29th, the ever elusive Mercury is nearby the old crescent Moon. On the evening of January 1, 2006, Venus can be seen cozying up to the young, crescent Moon. By then, that hangover should be waning, so take a look -- not a bad way to start off the New Year.

It's worth noting that this is the first time in years that the first day of Hanukkah falls on Christmas Day. Actually, a blue or boo Moon is much more commonplace than is the coexistence of the first day of Hanukkah with Christmas -- hasn't happened since 1959. The squaring of Christmas and Hanukkah this year is due to both happenstance and to the occurrence of the new Moon in late December -- the Hebrew calendar is partially based on the lunar cycle.

For more about the Moon, see the following sites:

Once in a Blue Moon: http://www.obliquity.com/astro/bluemoon.html

Abrams Planetarium: http://www.pa.msu.edu/abrams/SkyWatchersDiary/Diary.html

Moon Phases: http://tycho.usno.navy.mil/vphase.html

More About the Moon: http://www.netaxs.com/~mhmyers/moon.tn.html

Earth and Moon Viewer: http://www.fourmilab.ch/earthview/vplanet.html

Moonscape: http://www.moonxscape.com/More_Moon_Lore.shtml

What's a Blue Moon?: http://SkyandTelescope.com/observing/objects/moon/article_127_1.asp

Lunar Picture of the Day: http://www.lpod.org/

This week's question comes from by Dr. James Foster. Dr. Foster originated this series and did it as a solo project for the GSFC website for SEVEN YEARS! Two years ago, Dr. Foster has decided to share the enthusiasm he has for this project with other Goddard scientists and will be posing questions on a semi-regular basis.