Super Moon - March 19, 2011



My picture of the moon from last night

Yesterday featured the closest full moon of 2011. Some called it a supermoon because the moon shines opposite the sun in our sky at full moon, you saw the moon beaming all night tonight from dusk until dawn. This extra-close full moon likely caused large tides along the ocean shorelines especially if these high tides are accompanied by strong onshore winds.

Why is this moon so close? The reason is that the 03/2011full moon falls on the same date as perigee (moon’s closest point to Earth for the month). At perigee today, the moon lies only 356,575 km (221,575 mi) away. On March 6, the moon swung to apogee – its farthest point for the month. At that time, the moon was 406,583 km (252,639 mi) distant.
3/19/11 presents the moon’s closest encounter with Earth since 12/12/08. The moon won’t come this close again until 11/14/16. The extra-close moon in all of these years – 2008, 2011 and 2016 – finds the full moon occurring on the same date as lunar perigee.

How often does the full moon coincide with perigee? Closest full moons recur in cycles of 14 lunar (synodic) months, because 14 lunar months almost exactly equal 15 returns to perigee. A lunar month refers to the time period between successive full moons, a mean period of 29.53059 days. An anomalistic month refers to successive returns to perigee, a period of 27.55455 days. Hence:

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

This time period is equal to about 1 year, 1 month, and 18 days. The full moon and perigee will realign again on 5/6/12, because the 14th full moon after today’s full moon will fall on that date.