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Crescent Moons in 2006: just for enjoyment or to help you set a record, here's a guide to sighting the youngest possible moons.

EVERY MONTH, following the time of new Moon, an enchanting spectacle greets skywatchers around the world. After being invisible for a day or two in the Sun's glare, the Moon emerges into view as a beautiful thin crescent low in the western sky after sunset. As an aid to predicting the crescent's appearance, we give the date and time of astronomical new Moon to the nearest minute in each issue (see page 54). Some 16 to 30 hours later, if the Sun has just set at your location, you might be able to spot a very slender crescent very low in the west. Several time zones to your east, others have a chance to see an even younger Moon with binoculars or a carefully aimed telescope.

For the third year in a row, we present a world map that tells at a glance the earliest evening on which you might be able to see each month's young crescent Moon. The date depends mainly on your longitude and to a lesser extent on your latitude. Other important factors include an open view of the western sky right down to the horizon, pristine clear air, and whatever experience and optical aid you bring to bear.

Skywatchers living west (left) of any arc plotted on our map stand at least a small chance of spotting the month's first crescent on the date indicated. To the east (right) of an arc they have virtually no chance and must wait until the following evening to see the Moon.

If a memorable view or an eye-catching photograph is your goal, you may not want to be too near a plotted arc on the date in question. As the hours pass, Earth's rotation causes the sunset zone to migrate westward. In each successive time zone to the west, the crescent becomes a little thicker, higher, and more easily visible as the Moon draws away from Earth's line of sight to the Sun.

For example, note the arc that cuts down the middle of North America on March 29th. That Wednesday evening, skywatchers in the western US will need perfect atmospheric conditions and careful planning if they have any hope of seeing the ultrathin crescent. And since the Moon's age at the arc's optimum viewing point in Louisiana is given as 14.5 hours, westerners can expect to see a crescent that is 15 or 16 hours from new.

On the other hand, those living in the eastern US must wait until the next evening, March 30th, to see the Moon at all. It will be 37 to 38 hours from new there--much easier to see and quite eye-catching.

A midway example occurs on February 28th; the arc for this date crosses Arabia. By the time twilight reaches the eastern US, the crescent will be about 23 or 24 hours old and not too hard to see with careful planning.

Adding to a crescent Moon's beauty is earthshine, that feeble glow on the orb's dark face that poets have called "the old Moon in the young Moon's arms." Leonardo da Vinci first explained it correctly as the lunar surface being dimly lit by Earth shining brightly in the Moon's night sky.

Setting New Records

When you start asking what's the youngest, thinnest crescent that can be seen, the arcs on the map take on added significance. The widely accepted record for the youngest Moon ever seen, 11h 40m past new, was set by Mohsen G. Mirsaeed of Tehran on September 7, 2002, using giant 150-millimeter (6-inch) binoculars (S&T: February 2004, page 102). It's clear from our chart that no one will duplicate Mirsaeed's feat in 2006, because the minimum ages given on the visibility arcs range from 12.9 hours (in the mid-Pacific on January 29th) to 18.2 hours (east of Madagascar in the Indian Ocean on August 24th).

Two features of the Moon's orbit explain these delays in 2006. Only if the Moon is at perigee (nearest Earth) when it passes through new can it draw away from the Sun most rapidly and become visible the soonest. And only if the Moon's path takes it well north or south of the Sun in a given month (meaning there is no chance of a solar eclipse) does this separation augment the separation produced by the Moon's motion (see last September's issue, page 116). An optimum combination of these effects just doesn't happen this year.

Seventy-five years ago, French astronomer Andre Danjon pointed out that it's really the Moon's elongation (angular separation) from the Sun, rather than the time after new, that governs whether or not the crescent can be seen. Upon analyzing a large collection of sightings, Danjon concluded that whenever the Moon's elongation is less than a certain limiting angle, the crescent is impossible to see in any telescope, perhaps because of shadowing effects on the Moon's rough surface.

Modern studies have put the Danjon limit at 7.5?, and that's the angle we've adopted in the calculations for our map. Indeed, Mirsaeed's record 2002 sighting was made when the Moon was 7.5[degrees] from the Sun. Similarly, Ali-Reza Movahhed-Nezha-d and Hamid-Reza Giahi Yazdi, also in Iran, just managed to spot the crescent when it was 7.6? from the Sun on August 19, 2001.

Breaking the Danjon Limit

But can a crescent even closer to the Sun be seen? Maybe. On October 13, 2004, veteran observers James Stamm and John Polacheck drove up Mount Lemmon in Arizona before dawn. Armed with a Celestron 8-inch telescope and a plot of stars near the waning Moon, first Stamm and then Polacheck spied what they are convinced was a short arc of the lunar crescent. "At 6:09 a.m. [13:09 Universal Time], I looked into the eyepiece, and in about 3 or 4 seconds, I could see the crescent!" Stamm writes. A few minutes later, Polacheck also succeeded. "It was easier to see if we tapped the scope," Stamm continues. "After 6:16 a.m., each of us could only glimpse the image for a second and were never able to confirm each other's description. After 6:21 a.m. neither of us could even venture a claim of a glimpse."

First seen at 13h 39m before new, this crescent was not a record sighting as measured in time. But it was clearly a record for the least elongation from the Sun, 6.5?, that anyone has ever seen the Moon (except, of course, in silhouette at a solar eclipse).

Realizing they were going against conventional wisdom, Stamm and Polacheck tried again to view the waning crescent from the same mountain location on December 11, 2004, when its elongation would have been 7.0??west of the Sun. "We were not able to see the crescent," Stamm concedes, even though he was positive he had the correct field in the 55x eyepiece of his 8-inch telescope.

But word of Stamm and Polacheck's earlier sighting reached Iran, and it turned out that the morning of September 3, 2005, would offer another check. The Moon was near apogee (farthest from Earth) and approaching the Sun slowly, and so it would be a generous 17h before new Moon. But it would also be a mere 7.3??from the Sun for extreme western Iran, and 15 different groups of observers set out for another assault on the Danjon limit.

Mirsaeed himself, along with Ali Koohpaee, Mohammad Almasi, and Amir Alahmani, traveled 100 kilometers east of Tehran with 6- and 10-inch telescopes and 25 ??150 binoculars. "I tried hard to observe the ultrathin crescent," Mirsaeed writes, "but no success." The expeditions to the west also failed to obtain any confirmed sightings.

So is it really possible to see the Moon a mere 6.5??from the Sun, as Stamm and Polacheck confidently claim? Only further observations can tell for sure. While 2006 does not offer a chance for a record young Moon in terms of age (for the reasons explained earlier), every new Moon provides two opportunities to check on the visibility of the thinnest possible crescent, one before and one after new, to see if the conventional Danjon limit really does need amending.

Sky & Telescope welcomes your observations, whether you're reporting success or not. All it takes is to be in the right place at the right time!

Senior editor Roger W. Sinnott has yet to surpass his own naked-eye sighting of a 26-hour Moon.
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Title Annotation:celestial calendar
Author:Sinnott, Roger W.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Feb 1, 2006
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