THIS month’s full moon, which graced the world’s skies last Dec. 3, was a supermoon. A supermoon is a full moon that is larger than average.
If you missed that supermoon, do not worry. Although it was the only supermoon of the year, it was actually the first in a triad of supermoons!
The full moons of Jan. 2 and Jan. 31 of next year will also be supermoons.
But how super is a supermoon, anyway? Does the occurrence of a supermoon mean anything for us here on Earth? Why do supermoons happen in the first place?
The term ‘supermoon’ is came from astrology. In astronomy, the preferred term for a supermoon is perigee-syzygy or, specifically, the perigee-syzygy of the Earth-Moon-Sun system.
Quite a mouthful, is it not? It is therefore no wonder why the astrological term ‘supermoon’ stuck and is now used by so many.
So, why do supermoons happen? In other words, why don’t all full moons have the same size in the sky?
The answer to these questions actually explain the scientific term perigee-syzygy.
The Moon does not always appear the same size in the sky because it is not always the same distance from the Earth. The Moon’s distance changes because it follows an elliptical path around the Earth.
As the Moon goes around the Earth once a month, there is a point where it is nearest to the Earth, a point where it is farthest, and the rest of the path is everything in between. When the Moon is closest to Earth, it is said to be in perigee. When it is farthest, it is said to be in apogee.
As seen from the Earth, a Moon in perigee looks larger because it is closer. When the Moon is at perigee, it can appear up to 14 percent bigger in our skies compared to a Moon at apogee.
A supermoon is therefore just a full moon that falls near perigee.
A full moon happens when the Sun, Earth, and Moon are aligned. Astronomers call such an alignment a ‘syzygy.’ Hence the term perigee-syzygy.
A supermoon is not very large compared to an average full moon. It can, however, be significantly brighter. A supermoon is around 20 percent brighter than an average full moon.
How big is an average full moon, anyway? Try holding one clenched fist at arms length. Next, let your pinky finger stick out. Look at the width of your pinky finger when held at that distance from your eyes. That is how big an average full moon is in the sky.
Not so big, right? When a full moon is near the horizon, visually close to trees, houses, and mountains, it appears big because of the famous ‘Moon Illusion’, which is a topic for another article.
Now imagine a moon around 10 percent bigger than your pinky held at arm’s length. That is how big a supermoon is.
Although that does not sound so impressive, a supermoon is nevertheless a good occasion to look at the Moon and marvel at the fact that science has made us understand the workings of that piece of rock in the sky that we were able to send representatives of our species to walk there.
As an added bonus, the supermoon of Jan. 31 next year is both a blue moon and a blood moon!
It is a ‘blue moon’ because it is the second full moon of that month.
The Moon does not turn blue during a blue moon. The ‘blue’ in blue moon probably came from the old word ‘belewe,’ which means “betrayal.” Blue moons were probably considered “betrayals” by people who had to take note that there was an extra full moon in the season, which meant the harvest moon will come later.
Furthermore, blue moons are not a rare occurrence, contrary to what the phrase “once in a blue moon” suggests. They occur ever two to three years.
The full moon of Jan. 31 will be a blood moon because it will occur during a lunar eclipse. A lunar eclipse happens when the Moon enters the shadow of the Earth. When it enters the darkest part of the Earth’s shadow, it turns reddish in color, leading to the term ‘blood moon.’
If there is a supermoon you would not want to miss, it will be the one on Jan. 31.
However, supermoon or not, looking up at the Moon, stars, and planets is something I hope all of us will keep on doing to remind us of our place in this amazing universe.
Pecier Decierdo is the resident physicist and astronomer of The Mind Museum.
