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updated 9/10/2010 1:47:58 PM ET 2010-09-10T17:47:58

To the ancient Greeks the moon was an object of reverent admiration, but even today Earth's nearest neighbors pulls on the heartstrings of skywatchers.

On Sept. 18, moon lovers around the world will unite in the first International Observe the Moon Night. The skywatching event is part lunar lovefest and part outreach to spur public interest in the moon and amateur astronomy.

But modern skywatchers are no pioneers when it comes to honoring and gazing at the moon.

In 1609, when Galileo first focused it in his crude telescope, he expressed his admiration and wonder by writing:

"A most beautiful and rapturous sight to behold It does not possess a smoothe and polished surface but is rough and uneven and, similar to the earth itself, is everywhere full of vast protuberances, deep chasms, and sinuosities."

And much earlier in 550 B.C., the moon factored into the construction of one of the seven wonders of the ancient world in the city of Epheseus in what is now Turkey. It was the Temple of Artemis, built to honor a deity associated with the Greek moon goddess Selene.

In fact, modern study of lunar topography is known to this day as selenography.

Our ever-changing moon

One of the most complex of all astronomical problems is calculating the motion of the moon. But this need not concern any of us who enjoy watching the ever-recurring lunar phases, or the varied aspects of the moon with relation to the horizon, the planets and the stars during each month. [More: How Moon Phases Work ]

About 29.5 days elapse from one new moon to the next, and on all but a few nights in this time the moon can be seen in some part of the sky.

First, it adds delicate beauty to the western twilight as a thin arc of light enclosing a ghostly ball. This was the signal by which the ancients set their calendars.

For nearly a week after the new moon, sunlight reflected from the Earth illuminates the night side of the moon, making its entire disk glow faintly with a blue-gray hue and fitting the old saying, the old moon in the new moon's arms." 

Here is one of nature's beautiful sights, easily seen by everyone but better appreciated by amateur astronomers, who know the stage setting that makes it possible.

Organizers for International Observe the Moon Night are hoping to use the moon's draw to introduce skywatchers to amateur astronomy. Details on how to host or attend a moon-watching event are available at the group's website: http://observethemoonnight.org/

Sighting half a moon

Every month the moon performs a cycle in which it grows in phase each night, increasing in brightness until it is fully illuminated in a neat half-lit disk. At the moment of first quarter phase, the lunar terminator appears straight to our view, or nearly so depending on the observer-Earth-moon geometry. 

The terminator divides day and night on the moon, and the long shadows of lunar mountains and crater rims make fascinating watching in a small telescope as the sun rises over the quarter moon. Sometimes within a few minutes one can see sunlight suddenly illuminate the summit of a lunar mountain. The shadow of a peak near the terminator can shorten noticeably from hour to hour.

It is not always possible, however, to see the terminator precisely straight when the astronomical almanacs and handbooks predict first quarter, for the moon might be below your horizon at that moment.  This week, that time is 5:50 at the Greenwich meridian, or 1:50 a.m. (after moonset) for New York City. 

The evening moon of Tuesday, Sept. 14 will still be a slight crescent. The next night the phase will be noticeably bigger.

Not until Oct. 14 at 5:27 p.m. EDT (2127 GMT) will all observers be able to view the moon at the moment  it is only half-full.

But this will happen during the daytime hours in the afternoon sky, for the first quarter moon does its shining half by day, half by night.

It will rise about six hours later than the sun, around noon, and climb high during the afternoon, about half a sky away from the sun.  The moon will be at its highest about sunset and shine through the first half of the night. 

When a full moon isn't full

A week after the first quarter phase in September comes the full moon, which will also hold the title of Harvest Moon a full moon that occurs nearest in the calendar to the autumnal equinox.

This year we have an almost perfect example of a Harvest Moon, since the moon will turn full only 6 hours and 8 minutes after the equinox on Thursday, Sept. 23 at 5:17 a.m. EDT (0917 GMT).

I'm quite sure that on that Thursday evening, many of my fellow broadcast meteorologists on radio and television will tell their listeners and viewers to go out that night and enjoy the sight of the Harvest full moon. But that announcement should actually come on Wednesday, since by Thursday evening the moon will be more than half a day past the moment it turns full.

But on Wednesday night, the moon will be much closer to appearing full and will actually turn full during the overnight hours. Indeed, on the West Coast, the moment of full phase will come just a couple of hours past midnight during the early hours of Thursday morning.

If it's clear during the evenings of Sept. 21-23, try this experiment: Pick a time when it's convenient for you to step outside to look at the moon.

Let's say, for example, 9 o'clock. Although at first glance it will appear "full" on all three evenings, closer inspection will show that on Sept. 21, it will appear ever-so-slightly less than full, with the terminator appearing as a narrow slice of darkness along its left side.

On Sept. 22 it will appear "full" in all aspects, while on the evening of Sept. 23 the night almanacs and calendars tell us that it's a "full" moon it will once again appear slightly less than full, but this time the narrow slice of darkness will appear along its right side.

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.

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