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.
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. Here is a short synopsis:
April 25: Here is your chance to catch a view of an exceedingly thin crescent moon just 21 to 24 hours old and only about six-tenths of one percent illuminated. Carefully take note of exactly where on the west-northwest horizon the sun sets. Then, about a half-hour later, check that area of the sky about 6 degrees above and slightly to the right of where the sun had set (your clenched fist measures about 10-degrees across when held at arm's length). Initially, you might want to use binoculars, but assuming you have a good clear sky with no tall obstructions, you should be able to catch a view of the wire-like arc of the waxing crescent moon.
April 26: The still delicately-thin crescent is a bit wider (3.7 percent illuminated) and considerably higher up (18-degrees) above the west-northwest horizon about a half hour after sunset. Thanks to the steep angle that the ecliptic makes with the horizon, the crescent looks like a thin "smile" on the sky, tilted a bit to the left. In addition, a beautiful sight awaits you, especially if you have binoculars or a small telescope with low power. About an hour after sunset, as darkness deepens, you'll be able to see not only the lunar crescent well above the western horizon, but immediately below and to the right of the crescent will be the beautiful Pleiades star cluster. The view through binoculars will be especially pleasing. And if you drop an imaginary line straight down from the moon and the Pleiades you'll see a bright "star" shining with a slight orange hue. But that's not a star, but the planet Mercury, in the midst of its best apparition of 2009 for evening skywatchers.
May 24: Here is your opportunity to try for an observation that conceivably could get you into the record books. The moon is New at 8:11 a.m. EDT on this date. At sunset that evening, the moon will be positioned 6-degrees directly above where the sun has just set. It will be an exceedingly narrow sliver of light; just two-tenths of one percent of its disk will be illuminated and as such it will appear as a mere short arc of faint light extending less than one-quarter of a way along the lunar limb. This will be just over 11-hours after New phase for the U.S. East Coast; just over 14-hours after New phase for the U.S. West Coast. Try scanning the sky along the west-northwest horizon with binoculars (wait until after the sun has set; you don't want to accidentally get a quick glance at the sun's potentially blinding light!). Look quick, for the moon will set only about 45 minutes after the sun. The record for the youngest moon ever seen with optical aid, 11 hours 40 minutes past new, goes to Mohsen G. Mirsaeed of Tehran who saw it on September 7, 2002. The youngest crescent ever seen by the naked eye, 15 hours 32 minutes, is still that observed in May 1990 by American observer, Stephen James O'Meara. Odds are good that you will not make a sighting this evening, but nothing ventured, nothing gained!
May 25: One day later, the moon – now 35 to 38-hours old and 2.4-percent illuminated – is a much easier target, standing about a dozen degrees above the west-northwest horizon about a half-hour after sunset. As it gets darker, you might notice a second magnitude star about five degrees to the moon's right. That's El Nath, the brighter of the two stars that mark the horn tips of Taurus, the Bull.
Pretend, for a moment, that you are an astronaut on the lunar surface. From the moon, our 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."
For instance: when a sliver of a crescent moon begins to appear in our western twilight sky, you've also noticed the entire outline of the moon, even the dark portion as a faint bluish-gray glow within the thin crescent. 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 earthshine, which is sunlight reflected off Earth and back to the moon. There's a nearly full Earth in the lunar sky illuminating the otherwise dark lunar landscape.
With binoculars, or better yet, a small telescope, peer into that Earth-lit region of the moon and try to pick out some of the prominent craters (like Copernicus or the brightly rayed Tycho). See if you can also identify some of the darker lunar maria. And be sure to examine the crescent itself, which might appear somewhat jagged along the terminator as sunlight plays off of the craters, peaks and rills.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.