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What is a 'milk moon' anyway?

The full moon of May, also known as the "Milk moon," will occur at 7:07 p.m. EDT Thursday. What better time to discuss this brightest object in the night sky?

By Geoff GahertyStarry Night Education/SPACE.com / May 26, 2010

A jet is silhouetted on the full moon in St. Petersburg, Russia, early Thursday, April 29, 2010. This month's full moon is known as a 'milk moon' in English-speaking countries.

Dmitry Lovetsky/AP

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To ancient peoples without complex calendars, the moon was probably the most important marker of the passage of time — especially at times like this Thursday, when May's full moon arrives.

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The full moon of May, also known as the "Milk moon," will occur at 7:07 p.m. Eastern Daylight Time this Thursday, May 27. This makes it a good time to explore lunar myths and mysteries as well as the mechanics of the moon.

The moon is visible most nights at some point, and is the brightest object in the night sky. It is also visible in daylight much of the month, though most modern humans may be unaware of it. Here's how it works:

The moon is a sphere, lit from varying angles by the sun. As it moves around the Earth in its orbit, it goes through a series of phases from new moon, when the moon lies on or near a line between Earth and sun; first quarter, when lit from one side; full moon, when lit by the sun directly behind the Earth; and third quarter, lit from the other side.

IN PICTURES: The full moon

The full moon is the most striking of these phases: a seemingly huge moon rising in the east, fully illuminated, just as the sun is setting in the west. [More: How Moon Phases Work]

Check it out

The full moon of May is located in the constellation Scorpius, very close to the southernmost point in its monthly journey around the Earth. As a result, the moon spends less time than average above the horizon and never gets very far above the horizon, as seen from the Northern Hemisphere.

The moon will also be very close to the bright red star Antares on full moon night. It won't be close enough to actually pass in front of Antares, as it sometimes does, but still close enough to make a very pretty sight.

While a full moon is not the best time to observe the moon through a telescope, there's really never a bad time to explore the moon. At full moon, the shadows of the moon's many craters are less pronounced. Still, any small telescope, or even binoculars, will reveal rich details of the larger craters — each one evidence of a long-ago impact by a space rock.

Just an illusion

The large size of the full moon is actually an optical illusion, known as "the moon illusion," caused by our mind's attempt to make sense of the moon in relation to earthly objects on the horizon. In fact, the full moon on the horizon is no larger than the moon at any other time or location.

You can verify the moon illusion yourself by holding a small object, such as a pencil eraser at arm's length and compare its size to that of the rising moon. Then go back out a couple hours later, when the moon is higher and seems smaller, and make the same comparison to the eraser. Alternately, you can take two pictures of the moon, with your camera at the same settings, then print and compare them.

Various peoples around the world have given special names to the full moons throughout the year, a simple way of recognizing the passage of time.

Probably the best known of these names are those used in the English language, but other well known naming systems are used by the Algonquian peoples of northeastern North America, the Hindus of India, and the widespread Buddhist religion.

May's full moon is known as the Milk moon in English, the Flower moon in Algonquian, Buddha Poornima in Hindi, and Vesak Poya in the Sinhala Buddhist tradition.

It is also known as Corn Planting moon, Corn moon, and Hare's moon.

Also, one of our main calendar components is the month, named for the moon.

Never really full

Now here is something strange. Full moon occurs at 7:07 p.m. Eastern Daylight Time, yet the moon doesn't rise until around 8:30 p.m. The exact time of full moon is determined strictly by the geometry of the sun, Earth, and moon: all three fall in a straight line with the Earth in the middle. This is an instantaneous event, and happens at the same instant everywhere in the world, in this case happening when the moon is below the horizon in eastern North America.

Full moon happens in England at seven minutes past midnight British Summer Time, with the moon high overhead, because Britain is about a quarter of the way around the earth from eastern North America. The moon itself is quite indifferent to where on Earth people are observing it from.

Even though full moon occurs at a very specific instant each month, it looks full for a day or two on either side of that instant, at least to the naked eye. In a telescope, you can see the terminator, the line of sunset or sunrise on the moon, and see that it is not quite full.

In fact, the moon is never truly full. When everything lines up perfectly, so that the moon's face would be 100 percent sunlit from our point of view, Earth gets in the way, blocks the lights from the sun, and causes a total lunar eclipse.

This article was provided to SPACE.com by Starry Night Education, the leader in space science curriculum solutions.

IN PICTURES: The full moon