By BRUCE McCLURE

The Full Moon falling on March 19 ushers in the third Full Moon after the December 21 solstice, and it will be a big one.

In North America, we commonly call this Full Moon the Sap Moon, Crow Moon or Worm Moon. This year, the March Full Moon gives us the closest and largest Full Moon of 2011.

This upcoming Full Moon will coincide with lunar perigee, the Moon's closest approach to Earth for the month. At perigee on March 19, the Moon will be less than 222,000 miles away. Contrast this to lunar apogee, the Moon's farthest distance, on March 6, when it was over 252,000 miles distant.

As a rule of thumb, the year's closest perigee takes place on the one day of the year that the Full Moon and perigee align. March 19 won't only feature the closest Full Moon of the year, but the Moon at its nearest for all of 2011.

Closest Full Moons recur in cycles of 14 lunar months. That's because 14 returns to Full Moon almost exactly equal 15 returns to perigee. The lunar month refers to the period of time between successive Full Moons: 29.53059 days. The anomalistic month is the period between successive perigees: 27.55455 days. Hence:

14 x 29.53059 = 413.428 days 15 x 27.55455 = 413.318 days

This period of time is about equal to 1 year 1 month and 18 days. In 2012, the closest Full Moon will occur on May 5.

Enjoy the Sap Moon, the closest and largest Full Moon of 2011, as it lights up the nighttime from dusk till dawn.

Astronomer Bruce McClure of Norwood writes for EarthSky, an astronomy information and entertainment service at EarthSky.org.