Sky Show Dec. 12: Biggest, Brightest Full Moon of 2008
for National Geographic News
|Updated December 12, 2008|
Don't expect to spot an Apollo lunar lander. But on December 12, 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 December 12's'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. (Get more moon facts.)
"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." (See map.)
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."
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