Enjoy the beauty of a thin crescent moon

By Joe Rao

updated 6:31 p.m. ET April 24, 2009

Do you have a favorite phase of the moon?  Romantics will almost certainly be drawn to the full moon, but during its 29.5-day cycle, going from one new moon to the next, our satellite offers up plenty of other choices. 

There's the precise semicircle of a first or last quarter moon.  And maybe you've been surprised in the morning to see the moon, about two-thirds illuminated and still clearly visible against the blue daytime sky. But for sheer beauty, nothing beats a razor-thin crescent moon hanging over the western horizon in the fading evening twilight. 

This weekend is a great time to spot one. Below, tips on what to look for and when you can see it. First, the mechanics of it all:

Young spring crescents
The key to understanding moon phases: We see the moon because it reflects sunlight. And the moon's 29.5-day orbit around Earth constantly changes the angles involved.

When astronomers refer to a "new moon" they mean that moment when the moon is between the Earth and the sun and appears in close proximity to the sun in our sky: Basically, we can't find it in the glare of the sun.  It rises roughly when the sun rises, and sets when the sun does.

The new moon is invisible because sunlight is shining on the part of the moon that is turned away from us. (On those special occasions during a solar eclipse, we can see the new moon, at least in silhouette against the sun.)

In the days after new phase, the moon begins to emerge as a slender crescent into the western evening sky.

To obtain an approximate age for a young crescent moon, simply add the number of hours or days that have elapsed from the time of new moon through the date and time of sunset at your location on a particular evening.  Most folks – just casual observers – are unlikely to notice a young crescent moon until three or four days after it is new, when the fattening crescent has moved out of the evening twilight and into the dark night sky. 

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A two-day old moon is not difficult to see if you know where to look.  But to see a moon one day old requires planning and patience. 

If you would like to try your hand at sighting a very young crescent moon, good eyesight, an unobstructed western horizon, and clear skies that are mostly free of haze are important. 

Also, your best chance will always come during early spring.  While the age of the moon is one factor that will determine your chance at seeing it, the altitude of the moon above the horizon is also critical; the higher above the horizon the moon is positioned right after sunset, the better your odds of making a sighting.

March and especially April are the best months of the year in this regard because the ecliptic – the line in the sky along which the sun, moon and planets appear to travel – is most steeply inclined to the western horizon.  The moon always travels within several degrees of this line, so, in the springtime, an imaginary line from the sun to the moon will make a steep angle with the horizon around sunset.  The farther south you travel the steeper the angle.

If your local weather cooperates in April and May it may be worthwhile waiting until just after sunset and then head outside to look toward the western twilight sky.

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