Gordon Gillet, ESO
The dazzling full moon sets behind the Very Large Telescope in Chile's Atacama Desert in this photo released June 7, 2010, by the European Southern Observatory. The moon appears larger than normal because of an optical illusion of perspective.
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updated 3/18/2011 1:52:17 PM ET 2011-03-18T17:52:17

Thanks to a fluke of orbital mechanics that brings the moon closer to Earth than it has been in more than 18 years, the biggest full moon of 2011 will occur on Saturday, leading some observers to dub it a "supermoon."

On Saturday afternoon at 3 p.m. Eastern Daylight Time, the moon will arrive at its closest point to the Earth in 2011:  a distance of 221,565 miles away. And only 50 minutes earlier, the moon will officially be full.

At its peak, the supermoon of March may appear 14 percent larger and 30 percent brighter than lesser full moons (when the moon is at its farthest from Earth), weather permitting. Yet to the casual observer, it may be hard to tell the difference.

The supermoon will not cause natural disasters, such as the Japan earthquake, a NASA scientist has stressed.

Spotting the supermoon
The moon has not been in a position to appear this large since March 1993.

In December 2008, there was a near-supermoon when the moon turned full four hours away from its perigee — the point in its orbit that is closest to Earth. But this month, the full moon and perigee are just under one hour apart, promising spectacular views, depending on local conditions.

Although a full moon theoretically lasts just a moment, that moment is imperceptible to ordinary observation.

During the day or so before and after, most will speak of seeing the nearly full moon as "full," with the actual shaded area of the lunar surface being so narrow — and changing in apparent width so slowly — that it is hard for the naked eye to tell whether it's present, or which side it is. 

Supermoon making waves
In addition, the near coincidence of Saturday’s full moon with perigee will result in a dramatically large range of high and low ocean tides. 

The highest tides will not, however, coincide with the perigee moon but will actually lag by up to a few days depending on the specific coastal location. For example, in Wilmington, N.C., the highest tide (5.3 feet) will be attained at 11:21 p.m. ET on Sunday.

In New York City, high water (5.9 feet) at The Battery comes at 10:49 p.m. ET on Monday, while at Boston Harbor, a peak tide height of 12.2 feet comes at 1:31 a.m. ET on Tuesday, almost two and a half days after perigee.

According to the Observer’s Handbook of the Royal Astronomical Society of Canada, residents of regions along the shores of the Bay of Fundy in eastern Canada, the 10- to 20-foot (3- to 6-meter) swell in the vertical tidal range makes it obvious when the moon lies near perigee, regardless of clear skies or cloudy.

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Any coastal storm at sea around this time will almost certainly aggravate coastal flooding problems. 

Such an extreme tide is known as a perigean spring tide, the word spring being derived from the German springen – to "spring up," and is not, as is often mistaken, a reference to the spring season. 

In contrast, later this year, on Oct. 11, the full moon will closely coincide with apogee, its farthest point from the Earth.  In fact, on that night the moon will appear 12.3 percent smaller than it will appear this weekend.

Big full moon's appearance is deceiving
And while this weekend’s moon will be — as the Observer’s Handbook suggests — the "largest full moon of 2011," the variation of the moon's distance is not readily apparent to observers viewing the moon directly.

Or is it?

When the perigee moon lies close to the horizon, it can appear absolutely enormous. That is when the famous “moon illusion” combines with reality to produce a truly stunning view.

For reasons not fully understood by astronomers or psychologists, a low-hanging moon looks incredibly large when hovering near trees, buildings and other foreground objects. The fact that the moon will be much closer than usual this weekend will only serve to amplify this strange effect.

So … a perigee moon, either rising in the East at sunset or dropping down in the West at sunrise, might seem to make the moon look so close it almost appears that you can touch it. You can check out this out for yourself by first noting the times for moonrise and moonset for your area by going to this website.

Happy mooning!

If you'd like to share your supermoon photos of the March 19 full moon, contact Space.com managing editor Tariq Malik: tmalik@space.com.

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, N.Y.

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Video: Moonstruck: 'Supermoon' to rise Saturday

Photos: 50 years of views from the moon

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  1. Up-close exploration of the moon, Earth's only natural satellite, began in 1959 when the Soviet Union launched its Luna 1 spacecraft on a flyby mission. NASA quickly followed up with missions of its own. Since then, the Europeans, Japan, China and India have launched their own lunar exploration programs. This view shows the moon as seen from the international space station. Click the "Next" arrow above to check out 11 images from the moon made over the last 50 years. (NASA) Back to slideshow navigation
  2. 1959: Far side in full view

    In October 1959, the Soviet Union's Luna 3 spacecraft - the third successfully launched to the moon - made history as the first probe to image the far side of the moon. The photos were fixed and dried on the spacecraft and beamed back to Earth. Though fuzzy by today's standards, the images showed stark differences from the near side, including relatively few dark areas, called lunar maria. (RSA via NASA) Back to slideshow navigation
  3. 1966: A restored ‘Earthrise'

    In 1966 and 1967, NASA sent a series of Lunar Orbiter spacecraft to collect detailed images of moon's surface in preparation for the Apollo program. The tapes were then put in storage. Decades later, researchers with the Lunar Orbiter Image Recovery Project collected the vintage hardware required to play back the imagery. That imagery was digitized , reproducing the images at a much higher resolution than previously possible. On Nov. 11, 2008, the project researchers released this enhanced photograph of Earth rising above the lunar surface, originally made by Lunar Orbiter 1 in 1966. (LOIRP / NASA) Back to slideshow navigation
  4. 1968: The most famous 'Earthrise'

    On Christmas Eve 1968, Apollo 8 astronauts Frank Borman, Jim Lovell and William Anders made history as the first humans to orbit the moon. They were scouting its surface for a suitable landing spot for future missions. But the sight of Earth rising above the moon's horizon caught their - and the world's - attention. The photograph, called "Earthrise," is among the most famous ever made from the moon. (NASA) Back to slideshow navigation
  5. 1969: One small step

    On July 20, 1969, an estimated 1 billion people around the world were glued to television screens to watch astronaut Neil Armstrong, commander of Apollo 11, climb down from the lunar module spacecraft for a stroll on the moon. As his foot touched the lunar surface, he famously said "That's one small step for (a) man, one giant leap for mankind." This image is a black-and-white reproduction from the telecast, showing Armstrong stepping down from the lunar module's ladder. (NASA Johnson Space Center) Back to slideshow navigation
  6. 1969: Man on the moon

    Apollo 11 astronaut Buzz Aldrin, shown here, accompanied Armstrong for the famous walk on the moon. This iconic image is one of the few that shows Armstrong on the lunar surface - as seen in the reflection on the spacesuit's visor. The astronauts walked around on the lunar surface for about two and a half hours. (NASA) Back to slideshow navigation
  7. 1994: Looking for ice

    This mosaic image of the moon's southern polar region, made by the Clementine spacecraft in 1994, suggested that the region could harbor water ice within regions of its craters that are never lit by the sun. The water ice would be left over from impacting comets. Scientists have debated the evidence for and against water ice at the poles ever since the Clementine discovery. The current era of lunar exploration could resolve the debate. If water ice exists, it could help quench the thirst of future human colonists and be used to make fuel for rockets. (NASA / JPL / USGS) Back to slideshow navigation
  8. 2006: European moon probe crashes

    On Sept. 3, 2006, the European Space Agency's SMART-1 spacecraft went out with a bang - a planned crash landing into a volcanic plain called the Lake of Excellence. The impact, shown here, was captured by the Canada-France-Hawaii Telescope. (The black lines are a processing error due to the brightness of the event.) The spacecraft was launched in 2003 primarily to test an ion propulsion system, which uses energy captured by the sun to produce a stream of charged particles. The slow-and-steady propulsion system may be used on future interplanetary missions. (Christian Veillet / CFHT via AP) Back to slideshow navigation
  9. 2007: China targets the moon

    China made its first major strides in the lunar exploration game with the launch of the Chang'e 1 spacecraft in October 2007. The orbiter was sent to make a detailed, 3-D map of the moon's surface. Premier Wen Jiabao unveiled the first image at a ceremony in Beijing, shown here. Chang'e 1's 16-month mission ended with a controlled crash. The country reportedly plans to launch lunar rovers in 2010 and 2017, and a manned mission to the moon by 2020. (Huang Jingwen / XINHUA NEWS AGENCY) Back to slideshow navigation
  10. 2008: India joins the lunar club

    The Indian Space Research Organization successfully launched its Chandrayaan 1 spacecraft on Oct. 22, 2008, for a mapping mission to the moon. A probe released from the mothership took this picture of the lunar surface during its descent to a planned crash landing at the south pole. The Indian space agency plans to use this and other data for a lunar rover mission in 2011 and, eventually, a manned mission. (ISRO via EPA) Back to slideshow navigation
  11. 2009: Japan orbiter watches eclipse

    Some eclipse enthusiasts travel the globe to glimpse alignments of the sun, Earth and moon. Japan's Kaguya probe did them one better: It shot this sequence of a Feb. 10, 2009, eclipse from its lunar orbit. The image shows the view of the sun from the moon mostly covered by Earth. The "ring" appears dark at the bottom because it is obscured by the night-darkened limb of the moon. The Kaguya orbiter was launched in September 2007 to study the moon's origin and evolution. It made a controlled crash landing on the moon in June 2009. (JAXA / NHK) Back to slideshow navigation
  12. 2009: NASA goes back to the moon

    On June 18, 2009, NASA launched two spacecraft to the moon to map its surface in unprecedented detail, scout for future landing sites, and smash probes into a permanently shaded crater in hopes of resolving a longstanding debate over whether such regions contain water ice. NASA's Lunar Reconnaissance Orbiter will orbit both poles for a year, and its mission could be extended to serve as a communications relay for future lunar missions. This is one of the first pictures sent back by the orbiter. LRO's sibling, the crater-smacking LCROSS probe, is due to impact the moon's south pole in October. (NASA / GSFC / ASU) Back to slideshow navigation
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  1. NASA
    Above: Slideshow (12) 50 years of moon shots
  2. Image:
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    Slideshow (12) Month in Space: January 2014

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