How Common are Supermoons?

On Monday 14 November 2016 the skies over the UK and Ireland will be graced by a beautiful bright full moon. Also on this this day there will be a “supermoon”. This is a term I have a love-hate relationship with. It excites and intrigues people and encourages them to go outside and gaze in wonder at the beautiful sight of our neighbouring world. However the expression supermoon is almost meaningless and often linked to superstition. It is perhaps unsurprising that the term supermoon was coined by an astrologer in the twentieth century.


Not so super, but still a wonderful sight. (Image credit: Michael Gil, CC BY 2.0)

Not so super, but any full moon still a wonderful sight. (Image credit: Michael Gil, CC BY 2.0)

What then is a supermoon? It’s when the Moon is simultaneously at its closest to Earth and in its full moon phase. You might think “Is that it? Surely that happens all the time?” Actually it doesn’t. It is an uncommon event but not rare. Let me try to show why.
The Moon completes an orbit around the Earth in roughly 27.321 days. Astronomers call this the sidereal period (or a sidereal month).
That’s easy to understand. Let us start to add some complications.
The Moon’s orbit (like all orbits) is not perfectly circular but elliptical (shaped like a squashed circle). This means sometimes it will be closer to the Earth than other times. When the Moon is furthest away, it is said to be at its apogee and when it is closest to Earth it is said to be at its perigee. There’s a difference of about 50 000 km between the apogee and perigee distances. This means that the size of a full moon seen from Earth does vary; a full moon at perigee can be up to 10% larger than if it occurred at apogee. Just to add a little more complication, the Moon’s orbit is not fixed and “perfect”; the gravitational pull of the Sun and our neighbouring planets pull on the Moon nudging it slightly out of position. This means the apogee and perigee distances vary slightly.This why the November 2016 supermoon is the closest and therefore biggest supermoon since 1948 and until 2034.
Whatever the exact distances, it seems obvious then as it goes around our planet, the Moon is at perigee once every 27.321 days and at apogee once every 27.321 days.
Alas, it is not quite so simple. The Moon’s entire orbit moves around the Earth in a cycle lasting about 8.8504 years. This is called precession. This means that apogee and perigee do not occur at fixed points in the Moon’s orbit and therefore not at the same time per orbit. The Moon is actually at perigee every 27.554 days (called the “Anomalistic period”, a great name for a rock band) rather than every 27.321 days.
What this means is that the Moon is at its closest to Earth, one of the two “ingredients” for a supermoon, every 27.554 days. The other ingredient we need is a full moon.
How often then do we get a full moon? “Every 27.321 days!” I hear you say….but once again it is not so simple. Observe the Moon one night, then wait exactly 27.321 days. You ought to expect the Moon to be at exactly the same position in the sky with respect to the Sun. But it won’t be! As the Moon orbits the Earth, the Earth is also moving in orbit around the Sun. This means the Moon must take a little longer to move a little further before getting to the same position. It actually takes more than two days to “catch up”, going through this cycle in about 29.530 days. This is called the synodic period or synodic month and is the basis of the standard 30 day month.
What about supermoons then? How often do we get one? I’m glad you asked, remember we get a supermoon when the Moon is both full (every 29.530 days) and at perigee (every 27.554 days). When these happen at once takes a little working out but it’s possible to get an answer showing that both criteria for a supermoon are met simultaneously every 411.776 days (a little bit under 14 months).
When we observe a full moon, the Sun, Earth and Moon are arranged along an imaginary line. Astronomers call an alignment of celestial bodies like this a syzygy, and when we have an Sun-Earth-Moon syzygy with the Moon at perigee we have a supermoon.
On Monday 14 November 2016 the actual supermoon occurs at 13:52 GMT. This is when the Moon is full and at its closest to Earth. The Moon will not rise until later (shortly after 5pm here in Armagh), so we will not see the Moon until it is a couple of hours past the supermoon but it will still be a bright and wonderful sight in the sky! It will look especially impressive when it is still low in the sky (the infamous “Moon Illusion“) Go out and see it for yourself!
(Article by Colin Johnston, Science Education Director)