A "Celestial Summit Meeting" Set For December 1st main content.

A "Celestial Summit Meeting" Set For December 1st

by Joe Rao on


Without a question of doubt, the most spectacular celestial sight during these final days of November is reserved for the early evening sky. One of my astronomy mentors, Dr. Ken Franklin (1923-2007), a former Chairman and Chief Astronomer here at the Hayden Planetarium often made reference to our dynamic and ever-changing night sky.  Such an eloquent description certainly fits our current evening sky, as we now have a celestial summit meeting in the making in the western evening twilight.

Moon-Jupiter-Venus Conjunction Nov 2008
Venus and Jupiter is joined by the waxing crescent Moon after sunset. This diagram is drawn for North America and the Moon's size is exaggerated for clarity.
Image courtesy Sky & Telescope.

The highlight will be the gradual approach relative to each other of the two brightest planets—Venus and Jupiter—with the waxing crescent Moon joining them on the first evening of December.  For the remainder of November, skywatchers will observe Jupiter and Venus slowly converge on each other in the southwestern sky, right after sunset.  Venus and Jupiter will appear a similar distance apart on both the evenings of November 30 and December 1.  The moment of closest approach will actually come during the early morning hours of December 1, unfortunately when this dynamic duo is below the horizon for North America.  They'll be separated by just over 2°, which corresponds to roughly one-half the distance that separates Pollux from Castor, the Twin Stars of Gemini (The width of your fist, held at arm’s length roughly corresponds to 10°).

The pinnacle, however, will come on early on Monday evening, December 1.

Every once in a while, something will appear in the night sky that will attract the attention of even those who normally don't bother looking up.  It's likely to be that way on that Monday evening when a slender crescent Moon, just 15% illuminated, will appear in very close proximity to Venus and Jupiter.

Venus, Moon, Jupiter Photo 1998
Jupiter, Venus, and the Moon photographed on April 23, 1998.
Image courtesy Steve Irvine.

People who are unaware, or have no advance notice, will almost certainly wonder, as they cast a casual glance toward the Moon on that night, what those two large, silvery stars happen to be?  Sometimes, such an occasion brings with it a sudden spike of phone calls to local planetariums, weather offices, and even police precincts.  Not a few of these calls excitedly inquire about the UFOs that are hovering closely in the vicinity of our natural satellite.

A very close conjunction of the crescent Moon and a bright star or planet can be an awe-inspiring naked-eye spectacle. Those involving Venus, Jupiter and the Moon—the three brightest objects in the night sky—always attract the greatest attention of all. Ken would have called it, A beautiful celestial tableau!

The English poet, critic, and philosopher, Samuel Taylor Coleridge (1772-1834) used just such a celestial sight as an ominous portent in his epic, The Rime of the Ancient Mariner.  In addition, there are juxtaposed crescent Moon and star symbols that have appeared on the flags of many nations, including Turkey, Pakistan, Malaysia, Algeria, Mauritania, and Tunisia.


Also on Monday evening, you may be able to see the full globe of the Moon, its darkened portion glowing with a bluish-gray hue interposed between the sunlit crescent and not much darker sky.  This vision is sometimes called the old Moon in the young Moon's arms.Leonardo da Vinci (1452-1519) was the first to recognize it as what we now call Earthshine.

Earthshine on Young Moon
Earthshine on the young, crescent Moon always appear after sunset or before sunrise, when the Moon is a thin crescent.
Image courtesy Dan Bush www.missouriskies.org

As seen from the Moon, the Earth would loom in the sky nearly 3.7 times larger than the Moon does for us.  In addition, the land masses, the oceans, and clouds make the Earth a much better reflector of sunlight as compared to the Moon.  In fact, the Earth's reflectivity varies as clouds, which appear far more brilliant than the land and seas, cover greater or lesser parts of the visible hemisphere.  The result is that the Earth shines between 45 and 100 times more brightly than the Moon.  The Earth also goes through phases, just as the Moon does for us, although they are opposite from what we see from Earth.  The term for this is calledcomplementary phases.

On Thanksgiving Day, for example, when the Moon is New for us, as seen from the surface of the Moon, the Earth appears Full. A few nights later, as the sliver of a crescent Moon begins to appear in our western twilight sky, its entire globe may be glimpsed.  Sunlight is responsible for the slender crescent, yet the remainder of the Moon appears to shine with a dim blush-gray tone. That part is not receiving sunlight, but shines by virtue of Earthlight: the nearly full Earth illuminating the otherwise dark lunar landscape. So Earthshine is really sunlight which is reflected off Earth to the Moon and then reflected back to Earth.

Venus and Jupiter

Those using binoculars or a small telescope will certainly enjoy the almost three-dimensional aspect of the Moon, but Venus will be rather disappointing appearing only as a brilliant blob of light, for right now, it's a small, featureless gibbous disk.  That will change in the coming weeks, however, as Venus approaches Earth and the angle it makes between us and the Sun allows it to evolve into a half-Moon phase in mid January, and a lovely crescent phase of its own during the latter part of February and March.  Jupiter on the other hand is a far more pleasing sight with its relatively large disk, cloud bands and its retinue of bright Galilean satellites.

No question about it.  It will hardly be an evening spending time watching TV!  If the weather is clear between about 5 and 7 p.m. next Monday, be sure to get out and take a look.