High tide alert! Closest full moon of 2011 on March 19

By Bruce McClure

Today – March 19 – features the closest full moon of 2011. Look for higher-than-usual tides along the ocean coastlines throughout the world.

Because the full moon shines opposite the sun in our sky, you’ll see the moon beaming all night tonight from dusk till dawn. This extra-close full moon will usher in large tides along the ocean shorelines for the next several days, especially if these high tides are accompanied by strong onshore winds.

Understanding the full moon

Incidentally, the bright point of light in the vicinity of tonight’s moon is the planet Saturn, the 6th planet outward from the sun.

Why is this moon so close? The reason is that the 2011 March full moon falls on the same date as perigee – the moon’s closest approach to Earth for this month. At perigee today, the moon lies only 356,575 kilometers away. Earlier this month, the moon swung to apogee – its farthest point for the month – on March 6. At that time, the moon was 406,583 kilometers distant.

Why will the tides be higher than usual? At every full moon, the Earth is more or less between the moon and sun. Each month, on the day of the full moon, the moon and sun line up to create wide-ranging tides, known as spring tides. High spring tides climb up especially high, and on the same day low tides plunge especially low.

Today’s extra-close full moon accentuates these monthly spring tides all the more.

Looking for a tide almanac? EarthSky Recommends…

The full moon and lunar perigee coincide in cycles of 14 lunar (or 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
15 x 27.55455 days = 413.318 days

This time period is equal to about 1 year, 1 month, and 18 days. Full moon and perigee will realign again on May 6, 2012, because the 14th full moon after today’s full moon will fall on that date.

So, if you live along a coastline, watch for high tides over the next several days. And keep an eye on the weather, because storms have a large potential to accentuate the tides.