Is biggest, closest full moon on June 23, 2013 a supermoon? | Tonight | EarthSky

Yes, many will call the June 2013 full moon a supermoon. The upcoming full moon on June 23, 2013, will not only be the closest and largest full moon of the year. It’ll also present the moon’s closest encounter with Earth for all of 2013.

This year’s closest and largest full moon will occur on June 23 at precisely 11:32 Universal Time. At United States’ time zones, that means the moon will turn full on June 23 at 7:32 a.m. EDT, 6:32 a.m. CDT, 5:32 a.m. MDT and 4:32 a.m. PDT. We astronomers call this sort of close full moon a perigee full moon. The word perigee describes the moon’s closest point to Earth for a given month. Two years ago, when the closest and largest full moon fell on March 19, 2011, many used a term we’d never heard before: supermoon. Last year, we heard this term again to describe the year’s closest full moon on May 6, 2012.

This year, we also hear the term supermoon referring to the year’s closest full moon on June 23, 2013. What does supermoon mean exactly? And how special is the June 23, 2013 supermoon?

The supermoon of March 19, 2011 (right), compared to an average moon of December 20, 2010 (left). Will you be able to notice with your eye alone that tonight's full moon is bigger or brighter than usual? Astronomers say no, but it'll be fun to stand outside under tonight's full moon and know the moon is closer than it has been since May 6, 2012. Image Credit: Marco Langbroek, the Netherlands, via Wikimedia Commons.

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The word supermoon didn’t come from astronomy. Instead, it came from astrology. 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, the June 2013 full moon lines up much more closely with perigee – the moon’s closest point to Earth – than Nolle’s original definition. According to Guy Ottewell’s Astronomical Calendar 2013, the 2013 June full moon falls only 22 minutes after the moon reaches perigee, the moon’s closest point to Earth for this month and year. At perigee, the moon lies only 356,991 kilometers (221,824 miles) away. Two weeks from now, on July 7, the moon will swing out to apogee – its farthest point for the month and year – at 406,490 kilometers (252,581 miles) distant.

Day and night sides of Earth at instant of June 23 full moon

Day and night sides of Earth at instant of full moon (2013 June 23 at 11:32 Universal Time). In North America, the full moon is setting in the west at sunrise. From eastern Asia, it’s rising in the east at sunset. The full moon resides close to zenith – straight overhead – as seen from the Samoan islands in the central South Pacific Ocean. Image credit: Earth and Moon Viewer

In fact, June 2013 presents the moon’s closest encounter with Earth until August 10, 2014, at which time the moon will be a scant 5 kilometers closer to Earth. The full moon will come even closer to Earth on September 28, 2015 (356,877 kilometers) and closer yet on November 14, 2016 (356,509 kilometers). November 2016 will feature the closest full moon until November 25, 2034! Maybe this helps you see that supermoons – while interesting – are fairly routine astronomical events.

Even the proximity of full moon with perigee isn’t all that rare. The extra-close moon in all of these years – 2011, 2012, 2013, 2014, 2015 and 2016 – finds the full moon taking place at or nearly the same hour as lunar perigee. More often than not, the closest perigee of the year comes on the one day of the year that the full moon and perigee most closely coincide. (See table below.)

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 days15 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 August 10, 2014, because the 14th full moon after the 2013 June 23 full moon will fall on that date.

Moon closest to Earth

YearDateDistance
2011March 19356,575 km
2012May 6356,955 km
2013June 23356,991 km
2014August 10356,896 km
2015September 28356,877 km
2016November 14356,509 km

Looking further into the future, 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 June 23 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.

Each full moon has its own name. Here’s a list.

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 June 23, 2013 is the closest and largest full moon of this year. Some will call it a supermoon.

Understanding the full moon

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Moon image at top of post: Alice Popkorn

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