National Geographic News
A full moon on a summer night.

A full moon hangs over Vancouver Island, British Columbia.

Photograph by Taylor S. Kennedy, National Geographic Stock

Andrew Fazekas

for National Geographic News

Published January 29, 2010

The biggest full moon of 2010 will rise in the east Friday night, and it'll appear with a bright sidekick: Mars will cozy up just to the left of the supersize moon.

January's full moon is also called the wolf moon, according to Native American tradition associating this month's full moon with wolves howling in the cold midwinter. (Take a moon myths and mysteries quiz.)

The 2010 wolf moon will appear 30 percent brighter and 14 percent larger than any other full moon this year, because our cosmic neighbor will actually be closer to Earth than usual.

The moon will be at its closest perigee—the nearest it gets to our planet during its egg-shaped orbit—for 2010 at 4:04 a.m. ET Saturday, reaching a distance of 221,577 miles (356,593 kilometers) from Earth.

At its farthest from Earth, the moon is said to be at apogee. Perigee and apogee each happen generally once a month, but the moon's wobbly orbit means that the satellite's exact distance at each of those events varies over the year. The moon's phase can also be different during each apogee and perigee.

"This month has the largest full moon of 2010, because it coincides with the special moment when the full moon happens to occur on the same day as it is at perigee," said Marc Jobin, an astronomer at the Montréal Planetarium.

And in a remarkable coincidence, Mars is at opposition Friday—directly opposite to the sun in the sky—so that as the sun sets in the southwest, Mars rises in the northeast.

Around opposition, the red planet gets closest to Earth. This year Mars swung by at just 61 million miles (98 million kilometers) on January 27, and it will still appear remarkably bright during the weekend sky show.

"To the naked eye it will appear as a bright, orange-colored star right next to the full moon—the pair will jump out at you for sure," Jobin said.

Full Moon Illusion

Because this unusually close perigee is happening during a full moon, it is expected to have an effect on Earth's tides.

(Related: "Mysterious Tremors' Strength Ebbs With Tides.")

These effects should be modest, most likely measurable in inches, although perigee tides can be higher if there happens to be a storm surge at the same time.

As for observing the effects of perigee on the moon itself, most casual observers should notice an obvious difference in the moon's apparent size as it rises above the eastern horizon, Jobin said.

That's when an optical illusion usually comes into play that makes any full moon seem larger, since the moon is set against familiar Earthly objects rather than appearing high in the empty sky.

"The combination of the two effects—perigee and moon illusion—will be really be noticeable and spectacular near the horizon," Jobin said.



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