Skywatch: Moon and Venus to pair up

By Joe Rao

Skywatching columnist

updated 2:13 p.m. ET May 11, 2007

Every once in a while, something will appear in the sky that will attract the attention of even those who normally don't bother looking up. It's likely to be that way on Saturday evening, May 19, when a slender crescent moon, just 15-percent illuminated, will appear in very close proximity to the brightest planet in the sky. 

Those who have no advance notice will almost certainly wonder, as they cast a casual glance toward the moon on that night, what that "huge silvery star" happens 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 UFO" that's hovering closely in the vicinity of our natural satellite.

The planet that will keep close company with the moon on that Saturday evening will be dazzling Venus, which has adorned the western twilight sky since late December.  No other star or planet can come close to matching Venus in brilliance.  During World War II, aircraft spotters sometimes mistook Venus for an enemy airplane.  There were even cases where Venus drew antiaircraft fire.  Venus currently is setting about three and a half hours after sunset, which means it's in the sky until past 11 p.m.  For some locations it remains visible until after midnight.

But a very close conjunction of the crescent moon and a bright star or planet can be an awe-inspiring naked-eye spectacle.  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.

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Also on May 19, 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." 

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 called "complementary phases." 

On May 16, for example, there's a New moon for us, but as seen from the surface of the moon that day, there will appear in the sky a brilliant Full Earth. A few nights later, as the sliver of a crescent moon begins to appear in the western twilight sky, its entire globe may be glimpsed.  sunlight is responsible for the 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 reflected back to Earth.

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