What is a Supermoon?

Goddard's Chief Scientist Talks About the 'Supermoon' Phenomenon
http://www.nasa.gov/topics/moonmars/features/supermoon.html:


Dr. Jim Garvin

Credit: NASA/GSFC

Dr. James Garvin, chief scientist at NASA's Goddard Space Flight Center, answers your questions about the 'supermoon' phenomenon.

Question: What is the definition of a supermoon and why is it called that?

'Supermoon' is a situation when the moon is slightly closer to Earth in its orbit than on average, and this effect is most noticeable when it occurs at the same time as a full moon. So, the moon may seem bigger although the difference in its distance from Earth is only a few percent at such times.

It is called a supermoon because this is a very noticeable alignment that at first glance would seem to have an effect. The 'super' in supermoon is really just the appearance of being closer, but unless we were measuring the Earth-Moon distance by laser rangefinders (as we do to track the LRO [Lunar Reconnaissance Orbiter] spacecraft in low lunar orbit and to watch the Earth-Moon distance over years), there is really no difference. The supermoon really attests to the wonderful new wealth of data NASA's LRO mission has returned for the Moon, making several key science questions about our nearest neighbor all the more important.

Are there any adverse effects on Earth because of the close proximity of the moon?

The effects on Earth from a supermoon are minor, and according to the most detailed studies by terrestrial seismologists and volcanologists, the combination of the moon being at its closest to Earth in its orbit, and being in its 'full moon' configuration (relative to the Earth and sun), should not affect the internal energy balance of the Earth since there are lunar tides every day. The Earth has stored a tremendous amount of internal energy within its thin outer shell or crust, and the small differences in the tidal forces exerted by the moon (and sun) are not enough to fundamentally overcome the much larger forces within the planet due to convection (and other aspects of the internal energy balance that drives plate tectonics). Nonetheless, these supermoon times remind us of the effect of our 'Africa-sized' nearest neighbor on our lives, affecting ocean tides and contributing to many cultural aspects of our lives (as a visible aspect of how our planet is part of the solar system and space).


By Bruce McClure and Deborah Byrd in FAQS
HUMAN WORLD | SPACE on Jul 11, 2014 - http://earthsky.org/space/what-is-a-supermoon:

What is a supermoon? We confess: before a few years ago, we in astronomy had never heard that term. To the best of our knowledge, astrologer Richard Nolle coined the term supermoon over 30 years ago. The term has only recently come into popular usage. Nolle has defined a supermoon 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.

That’s a pretty generous definition and allows for many supermoons. By this definition, according to Nolle:

There are 4-6 supermoons a year on average.

What did astronomers call these moons before we called them supermoons? We called them a perigee full moon, or a perigee new moon. Perigee just means “near Earth.”

The moon is full, or opposite Earth from the sun, once each month. It’s new, or more or less between the Earth and sun, once each month. And, every month, as the moon orbits Earth, it comes closest to Earth. That point is called perigee. The moon always swings farthest away once each month; that point is called apogee.

No doubt about it. Supermoon is a catchier term than perigee new moon or perigee full moon.

We first became familiar with the supermoon label in the year 2011 when the media used it to describe the full moon of March 19, 2011. On that date, the full moon aligned with proxigee – the closest perigee of the year – to stage the closest, largest full moon of 2011.


Around each new moon (left) and full moon (right) – when the sun, Earth, and moon are located more or less on a line in space – the range between high and low tides is greatest. These are called spring tides. A supermoon – new or full moon at its closest to Earth – accentuates these tides. Image via physicalgeography.net


Each month, on the day of the new moon, the Earth, moon and sun are aligned, with the moon 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.

The July, August and September extra-close full moons will accentuate the spring tide, giving rise to what’s called a perigean spring tide. If you live along an ocean coastline, watch for high tides caused by these full moons.

Will these high tides cause flooding? Probably not, unless a strong weather system accompanies the perigean spring tide. Still, keep an eye on the weather, because storms do have a large potential to accentuate perigean spring tides.

Dates of closest full supermoons in past and future years. More often than not, the one day of the year that the full moon and perigee align also brings about the year’s closest perigee (also called proxigee). Because the moon has recurring cycles, we can count on the full moon and perigee to come in concert in periods of about one year, one month and 18 days.

Therefore, the full moon and perigee realign in periods of about one year and 48 days. So we can figure the dates of the closest full moons in recent and future years as:

March 19, 2011
May 6, 2012
June 23, 2013
August 10, 2014
September 28, 2015
November 14, 2016
January 2, 2018.

There won’t be a perigee full moon in 2017 because the full moon and perigee won’t realign again (after November 14, 2016) until January 2, 2018.

By the way, some astronomers will call all the full moons listed above proxigee full moons.  But, like you, we’ll have fun just calling ‘em supermoons.