A composite picture, based on an image of the moon taken by the Galileo spacecraft, shows the apparent size differences between a full moon at perigee (the closest point in lunar orbit) and one at apogee (the farthest point), as seen from Earth. On December 12, 2008, the full moon will be at an unusually close perigee, offering sky-watchers a view of the biggest and brightest full moon seen since 1993. (Image courtesy NASA, on the National Geographic News website.)
Tonight's Moon is biggest in 15 years
New Scientist 12 Dec 08;
The full Moon will loom larger in the sky on Friday than it has since 1993, as it will be nearly as close as it ever comes to Earth in its orbit.
The Moon does not orbit Earth in a perfect circle. Instead, it follows an elliptical path that brings it 50,000 kilometres closer to our planet on one side of its orbit (called perigee) than the other (apogee).
On 12 December, the Moon will enter its full phase, when its disc appears completely illuminated by the Sun, just four hours after reaching its closest point to Earth. This will make it 14% bigger and 30% brighter than other full Moons in 2008, though the difference will be hard to distinguish by eye (see the difference in the full Moon's size in 2004).
It will be eight years before the Moon appears so big again. "This evening's Moon is not only the largest for 2008 but also during the period 1993-2016," says Anthony Ayiomamitis, who lives in Greece.
For observers in the northern hemisphere, tonight's full Moon will also appear higher in the sky than any other this year. Around midnight, it will shine down from nearly overhead.
Earth set for full moon close-up
BBC News 12 Dec 08
A full moon is set to occur closer to the Earth on Friday evening than it has done for the past 15 years.
The Moon's elliptical orbit means its distance from the Earth is not constant.
It will be a little over 350,000km away as it passes over the northern hemisphere, which is about 30,000km closer than usual.
If the sky is clear it will appear brighter and larger than usual, say astronomers.
Friday's full moon could appear up to 14% bigger and 30% brighter than other full moons this year, Nasa said.
The Moon's orbit is elliptical, meaning it does not follow a circular but rather an oval path.
It is currently approaching the point where this oval orbit is nearest to the Earth.
"It's only every few years that a full moon happens to coincide with the part of the Moon's orbit when its closest to the Earth," said Marek Kukula, an astronomer at the UK's Royal Observatory.
"What people will see is a full moon that's really bright and a bit bigger than what they're used to."
It will appear largest as it rises and sets, but this is a psychological illusion, Dr Kukula said.
"When it's close to the horizon, our brain interprets it as being bigger than it actually is, this is called the moon illusion," he said.
"The size may be striking when it's near the horizon," said Robert Massey of the UK Royal Astronomical Society.
However, he cautioned against expecting too much.
"The Moon may be brighter and may appear somewhat larger, but it's really quite hard for the eye to notice the difference; the eye will compensate for the extra brightness, it's not like going from night to day," said Dr Massey.
The Moon's brightness varies throughout its annual cycle, during the mid-winter in the northern hemisphere it can appear brighter simply because it is higher in the sky.
Sky Show Tonight: Biggest, Brightest Full Moon of 2008
Andrew Fazekas, National Geographic News 12 Dec 08;
Don't expect to spot an Apollo lunar lander. But tonight, weather permitting, sky-watchers around the world will see the biggest and brightest full moon of 2008.
Although a full moon happens every month, the one that rises tomorrow will appear about 30 percent brighter and 14 percent larger than the other full moons seen so far this year.
That's because our cosmic neighbor will be much closer than usual. The moon will be at its closest perigee—the nearest it gets to Earth during its egg-shaped orbit around our planet.
At its farthest from Earth, the moon is said to be at apogee. (Find out more about tonight's perigee and watch a moon-facts video in National Geographic News's space blog, Breaking Orbit.)
Perigee and apogee each happen generally once a month, but the moon's wobbly orbit means that its exact distance at each of those events varies over the year.
The moon's phase can also be different during each apogee and perigee.
"Typically we don't have the full moon phase and perigee coinciding at the same time, so that makes this event particularly special," said Ed Krupp, director of the Griffith Observatory in Los Angeles, California.
What's more, tomorrow's event will be the closest lunar perigee since 1993, at 221,560 miles (356,566 kilometers) from Earth.
The moon's farthest apogee for the year will occur a couple weeks later on December 26, when the natural satellite will be 252,650 miles (406,601 kilometers) from Earth.
Because this unusually close perigee is happening during a full moon, it is expected to have an effect on Earth's tides.
"While high tides happen each month when the sun, Earth, and the moon are aligned, there is going to be an enhanced effect, with the moon being the closest it's been in more than a decade," said Ben Burress, staff astronomer at the Chabot Space and Science Center in Oakland, California.
"This would result in extra-large tides in regions that are susceptible to them, like Nova Scotia's Bay of Fundy."
Features in the Bay of Fundy create a sloshing wave action that, in the bay's funneled and tapered basin, give rise to vast tidal ranges.
But even in such places, the effects of perigee are often modest, in most cases measurable in inches. But perigee tides can be higher if there happens to be a storm surge at the same time.
Observing the effects of perigee on the moon itself can be a bit trickier. Most casual observers may only notice a difference in the moon's brightness, Burress said.
The moon's apparent larger size might be most noticeable as it rises above the horizon at sunset.
That's when an optical illusion usually comes into play that makes the full moon seem larger—set against familiar Earthly objects—than when it's higher in the empty sky.
"This combination of the moon illusion and close perigee gives sky-watchers a chance to see the biggest and fullest moonrise possible," Burress said.
What makes this event particularly nice, the Griffith Observatory's Krupp added, is that everyone around the world can witness it without the need for special equipment, just clear skies.
"If you are charmed by the idea of seeing the biggest and brightest full moon visible in 15 years, be ready to go outside at sunset and watch for the rising moon in the east," he said.
"Or stay up all night and watch as the moon rides through the overhead skies—either way it will be a beautiful sight."