A beginner's guide to understanding supermoons. What do perigee, apogee and syzygy mean and how can you spot the next supermoon?
The term supermoon was first used by astrologer Richard Nolle in 1979. He used this word to avoid the slightly less dramatic sounding ‘perigee syzygy’ (pronounced si-zuh-jee). If we break down the meaning of perigee syzygy, we will be able to discover what a supermoon is.
Perigee and apogee
Like the orbits of most celestial objects, the orbit of the Moon is not perfectly circular. The slightly elliptical orbit means the Moon is not always the same distance from the Earth.
When the Moon is furthest away from the Earth, it is said to be at apogee and when it is closest to Earth it is at perigee.
For a supermoon to occur, there are two criteria that need to be met. The first of these is that the Moon must be close to the perigee of that orbit. The Moon orbits the Earth every 27.3 days; therefore, the Moon is at perigee approximately once a month. However, we do not have a supermoon every month, for that to happen we also need syzygy.
Syzygy is the approximate straight-line configuration of three or more celestial bodies. Syzygy between the Sun, Earth and Moon happens twice every 29.5 days, when there is a new moon, and when there is a full moon.
For a supermoon to happen we must have a full moon, or a new moon, whilst the Moon is at perigee. This is not a rare occurrence and happens a few times a year. When there is a new moon or full moon at apogee, it is sometimes called a micromoon.
Usually, only perigean full moons are called a supermoons because the full moon can be observed.
There is no set astronomical definition for how close the Moon must be to perigee for it to be a supermoon. Some people use specific distances to class a Moon as a supermoon, others state the full moon must happen within a certain time of the Moon being at perigee. Most of these definitions, however, agree that a full moon that is roughly 360,000 km or closer to the Earth is a supermoon.
What can we see?
A supermoon will appear up to 30% brighter than a micromoon because more of the light the Moon reflects reaches the Earth. However, this increase in brightness may not be very obvious to observers.
Because the Moon is closer to the Earth during a supermoon the Moon can appear up to 14% larger than a micromoon and up to 7% larger than an average full moon. However, like the increased brightness, this size difference is quite difficult to see by most observers.
It is possible to see this change in size from photographs, if you compare pictures of different full moons taken at the same time, from the same place, with the same camera settings. However, often those incredible pictures of the Moon looking extremely large in the sky are produced using camera techniques that makes the Moon appear bigger than it is.
Often when people see a much larger Moon, it is due to something called the Moon illusion, and not because of a supermoon. The Moon illusion is the name given to an optical illusion that makes the full moon look much bigger when it is close to the horizon than when it is higher in the sky. You can test this is an illusion by rolling a piece of paper into a tube so that it’s the approximate size of the Moon as it appears on the horizon. If you tape this tube in place and then look at the Moon when it is in high in the sky, there will not be a difference in the size.
What makes a supermoon super?
What is a Super Blood Moon?
A lunar eclipse can make the Moon appear a reddish colour, and subsequently it sometimes gets called a blood moon. Sometimes a lunar eclipse occurs at the same time as a supermoon causing a slightly brighter, slightly bigger, slightly red-looking Moon, which people call a Super Blood Moon.
Naming the full moon in each month has become popular and is based on Native American traditions. For example, the January full Moon is often called the Wolf Moon. If the Wolf Moon occurs whilst at perigee, it might be called the Super Wolf Moon. If a lunar eclipse happened at the same time, as it did in January 2019, that Moon may be named the Super Blood Wolf Moon.
Do supermoons affect the weather and tides?
High tides that occur during the full or new moon are called spring tides and are slightly higher than normal high tides. A spring tide that coincides with the Moon close to perigee is called a perigean spring tide. These tides are an average of 5cm higher than normal spring tides.
These slightly higher tides sometimes get sensationalised leading to suggestions that a supermoon will cause extreme flooding and other natural disasters. However, all that is going to happen when there is a supermoon is that the Moon may appear bigger and brighter and the tides may be a little higher.
When to see supermoons
As previously stated, different people calculate when a supermoon will be differently. These upcoming full moons will all be 360,000 km or closer to the Earth as calculated by Time and Date.
14 June 2022
13 July 2022
31 August 2023
18 September 2024
17 October 2024
About the author: Elspeth Lewis is an Education Presenter at the National Space Centre.