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Closest full moon of 2011 on March 19 – high tides – Saturn

Today – March 19 – features the closest full moon of 2011. Some are calling it a supermoon. As a result, you might look for higher-than-usual tides along the ocean coastlines throughout the world.

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

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

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 point to Earth for this month. At perigee today, the moon lies only 356,575 kilometers (221,575 miles) away. Earlier this month, on March 6, the moon swung to apogee – its farthest point for the month. At that time, the moon was 406,583 kilometers (252,639 miles) distant.

How frequently do full moon and the year’s closest perigee coincide? The moon has a closest point to Earth every month. It has a closest point to Earth every year. But it’s common for the year’s closest perigee to come during the month that full moon and perigee most closely align. That is certainly the case for March 2011. In March 2011, the moon reaches the crest of its full phase within about an hour of perigee.

The 2011 full moon perigee is the moon’s closest encounter with Earth since December 12, 2008. The moon won’t come this close again until November 14, 2016. 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. More often than not, the full moon perigee presents the closest perigee of the year.

From the years 2010 to 2016, inclusive, the full moon perigee stages the closest perigee of the year. Finally, in 2017, the new moon perigee will be the closest perigee of the year. It’s either the full moon or new moon perigee that gives us the year’s closest perigee, though it’s more commonly the full moon.

The full moon and perigee coincide 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
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 May 6, 2012, because the 14th full moon after today’s full moon will fall on that date.

Will the tides be higher than usual? Yes, all full moons bring higher-than-usual tides, and perigee full moons bring the highest (and lowest) tides of all. Each month, on the day of the full moon, the moon, Earth and sun are aligned, with Earth in between. This line up creates 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 (full moon) spring tides all the more.

If you live along a coastline, watch for high tides caused by the March 19 perigee full moon – or supermoon – over the next several days. Will the high tides cause flooding? Probably not, unless a strong weather system moves into the coastline where you are. Still, keep an eye on the weather, because storms do have a large potential to accentuate high spring tides.

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