December’s Big Bright Moon

It’s closer than you think

Friday’s full moon, dubbed the Cold Moon, the Long Moon or the Yule Moon, appears the biggest and brightest in 15 years, and it’s no optical illusion.

The moon’s orbit around Earth is elliptical, varying each month from its closest, perigee, to its farthest, apogee. This month the moon reaches perigee four hours after full moon, when its center will be a little less than 221,600 miles from earth’s center. In contrast, the moon reaches apogee December 26 and will be almost 253,000 miles from Earth center to center. So Friday’s full moon will be more than 31,000 miles — almost 14 percent — closer than its farthest point.

Over a year, perigee can vary by more than five percent, and this month marks the closest perigee of the year. February marked this year’s farthest perigee, when the moon was a bit more than 230,000 miles away. Looking ahead, the moon reaches its farthest perigee of 2009 on April 2, when it will be a little less than 230,000 miles away, more than three percent difference from this month.

Not only is the moon’s orbit around Earth elliptical — eccentric in astronomy-ese — so, too, is Earth’s orbit around the sun. Despite our cold weather, we are nearing our closest point to the sun, perihelion, which falls on January 4. While still a few weeks away, we, and the moon, are some three million miles — more than three percent — closer to the sun than at aphelion, our farthest point, July 4.

All this adds up to more sunlight reaching the moon, making it visibly brighter. With no celestial reference points to gauge the full moon month to month, it’s harder to see the difference in size. So consider taking a picture of Friday’s full moon and then another of full moon on April 19 next year.