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Astrologer Richard Nolle of the website astropro.com takes credit for coining the term supermoon. In 1979, he defined it as:
…a new or full moon which occurs with the moon at or near (within 90% of) its closest approach to Earth in a given orbit (perigee). In short, Earth, moon and sun are all in a line, with moon in its nearest approach to Earth.
By this definition, according to Nolle:
There are 4-6 supermoons a year on average.
That doesn’t sound very special, does it? In fact, tonight’s full moon lines up much more closely with perigee – the moon’s closest point to Earth – than Nolle’s original definition. The 2012 May full moon falls within an hour of perigee. At perigee, the moon lies only 356,955 kilometers (221,802 miles) away. Later this month, on May 19, the moon will swing out to apogee – its farthest point for the month – at 406,448 kilometers (252,555 miles) distant. So you can see tonight’s moon really is at its closest.
In fact, May 2012 presents the moon’s closest encounter with Earth since March 19, 2011, at which time the moon was a scant 380 kilometers closer to Earth. The moon won’t come as close as tonight’s extra-close moon until August 10, 2014 – although in 2013 the moon at its closest (June 23, 2013) will lie only 36 kilometers farther away than the closest moon in 2012. (See table below) Maybe this helps you see that supermoons – while interesting – are fairly routine astronomical events.
Even the proximity of full moon with perigee in today’s moon isn’t all that rare. The extra-close moon in all of these years – 2011, 2012, 2013, 2014 and 2015 – finds the full moon taking place within an hour or so of lunar perigee. More often than not, the closest perigee of the year comes on the day that the full moon and perigee coincide.
By the way, according to U.S. clocks, the full moon falls this evening at precisely at 10:35 p.m. Central Daylight Time. This same full moon falls tomorrow (Sunday, May 6) at 3:35 Universal Time (UT) – the standard time at the prime meridian of 0o longitude, or, for example, in Greenwich, England.
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
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 June 23, 2013, because the 14th full moon after today’s full moon will fall on that date.
Moon closest to Earth
|2011||March 19||356,575 km|
|2012||May 6||356,955 km|
|2013||June 23||356,991 km|
|2014||August 10||356,896 km|
|2015||September 28||356,877 km|
Looking further into the future, the perigee full moon on November 14, 2016 (356,509 km) will even be closer than the one on March 19, 2011 (356,575 km). The perigee full moon will come closer than 356,500 kilometers for the first time in the 21st century on November 25, 2034 (356,446 km). The closest moon of the 21st century will fall on December 6, 2052 (356,421 km).
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 May 5-6 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.
As a result, if you live near a coast, you’ll want to be on the lookout for higher-than-usual tides.
Because the moon – as always – shines opposite the sun in our sky at full moon, 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.
Bottom line: The full moon of May 5 (or 6), 2012 is the closest and largest full moon of this year. Some will call it a supermoon.