How Common are Supermoons?

Astronomical targets

Certainly, “Supermoon” is a term that confuses many people across the world, but not anymore! Here it is explained: In simple terms, a Supermoon occurs when the moon is at the closest to the Earth and at the same time, it’s in full moon phase.  These two conditions must be met to see it. It is something like a quite unusual a few times in a life time event.

According to astronomers, the moon takes approximately 27.321 days to completely revolve around the earth. Astronomers call this time a sidereal period or a sidereal month. This is just the easy part of it. There are more complicated scenarios you must pay attention to know when a Supermoon appears.

Elliptical orbit changes distance between the Earth and moon

Just like all other orbits, the Moon’s orbit is elliptical. It’s not completely circular as most people think. This has an implication that there are particular times when the moon will be closer to the Earth as compared to other times.

The time period when the moon is at the furthest distance from Earth, is called apogee and when its closest to Earth is called perigee. The difference between the apogee and the perigee in terms of distance is 50 000 km. Therefore, the size of a full moon when observed from Earth varies quite significally. A full moon at perigee can be up to 10 % larger than if it occurred at apogee.

Watch: What is A Supermoon video

Moon’s orbit varies because of gravitational forces

It’s also important to understand that the Moon’s orbit also varies as a result of gravitational force from the sun and the adjacent planets. This gravitational pull pushes the moon slightly out of it’s position. This is yet another reason why the apogee and perigee distances vary. As a result, a Supermoon witnessed during November 2016 was the closest and the biggest since 1948. The next such occurrence will happen again in 2034.

Regardless of what the exact distances are, apparently as the Moon orbits around Earth, the Moon is closest to Earth once every 27.321 days and at furthest from Earth once every 27.321 days.

However, considering that the Moon takes roughly 8.8504 years to orbit the Earth in a lifetime cycle, in a period that astronomers call precession, the apogee and Perigee are destined to change.

The apogee and perigee do not always occur at definite points in the Moon’s orbit and as a result, not at the same time per orbit. Therefore, it implies that the Moon is at its closest to Earth every 27.554 days (Anomalistic period) as opposed to every 27.321 days.

Much as we get a full moon every 27.321 days, it’s not obvious that the moon will be exactly at the same position in the sky with respect to the sun after every 27.321 days.

The reason is simple. Just like the Moon revolves around the earth, the Earth also orbits around the sun. The difference is that the moon takes more than two more days to get to the same position as it has to move a little further. It actually goes through this cycle in about 29.530 days, which is also known as a synodic period or month.

How often a Supermoon occurs?

Moon distance chart (click to enlarge). Credit: Lasunncty CC BY-SA 4.0 Wiki

You may be asking yourself how often a Supermoon occurs. Keep in mind that to see it, two conditions already mentioned, must be met. First, the Moon must be full and that happens every 29.530 days. Secondly, the Moon must be at its closest to Earth, which happens every 27.554 days. Of course, it may take time for these criteria to be met but it eventually happens every 411.776 days.

Normally, when a full moon is observed, the Sun, Earth, and Moon appear to be arranged along an imaginary line. This kind of an alignment of celestial bodies is referred to as a syzygy by astronomers. Therefore, in the circumstances when we have the Sun-Earth-Moon syzygy and the moon is at its furthest from Earth, we have a Supermoon!

Why a Supermoon is the best moment to observe moon with a telescope?

Generally, when a Supermoon occurs it is a brilliant and magnificent sight in the sky worth to watch, especially if you have a telescope. It is also the best possible moment to observe the moon with a telescope as the moon is closest to the Earth, making it more clearly visible, and even more details can be seen also using amateur telescopes! For example, Celestron NexStar 4 SE is a perfect telescope for watching moon, and planets. You could also use astronomy binoculars for moon gazing!

Hopefully, this was helpful article and answered to your question. Read more about the celestial event and find more interesting articles from our astronomy blog. If you have any questions or comments about the topic, please leave a reply below!


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