On the night of 30/31 August 2023, we experienced the Blue Supermoon that has been so widely discussed over the preceding months. From my vantage point, in the Northwest Province of South Africa, it happened at 3:35 on the morning of 31 August 2023. It was a beautiful, cloudless night and the view was magnificent. I decided to write a short note on what exactly the term “Blue Supermoon” means. Before I get to that, the first question that came up in my mind was, who exactly are the “we” who had the privilege of experiencing this phenomenon. Was this visible all over the earth at the same time, or how exactly does our view of the moon from earth works?
The first thing we need clear up is the date - 30/31 August. That is simply because of the International Date Line. At the exact time of the full moon, it will still be 30 August in some countries, while it will already be 31 August in others. Now, do we all see the full moon at the same time? We see the moon as a FULL moon when the side of the moon facing earth is completely illuminated by the sun from the earth’s angle. This occurs when the earth, moon and sun is aligned, with the earth in the middle (when the moon is in the middle, we have new moon). If the earth is located at such an angle that it blocks some of the sunlight, it casts a shadow on the moon, which is called a “lunar eclipse”. This is why a lunar eclipse can only occur during a full moon, but I digress. We did not experience a lunar eclipse on 30/31 August. On the contrary, we experienced the moon at its brightest since July 2022. The answer to my first question is a dubious one. It is full moon at the same moment everywhere (moment, not time, since time depends on your time zone). The Blue Supermoon occurred at 11:35 am on 31 August in Sydney, 3:35 am on 31 August in Johannesburg and 9:35 pm on 30 August in New York. This is the exact same moment, but across different time zones. At this moment in time, the moon was also below the horizon for roughly one half of the earth, which means they couldn’t see it at the that exact moment. However, a lunar phase is technically recognised for about 2 days (24 hours on both sides of the actual instant of the event), which means everybody got to see the Blue Supermoon on 30/31 August. Just another interesting and related fact. We all see the same phases of the moon at the same time; however, we see it from different angles. In other words, a waxing or waning moon that appears “right side up” in the Southern Hemisphere, will appear upside down in the Northern Hemisphere. This is off course not applicable when it is full moon, as a circle will remain a circle from all angles.
What is a Blue Moon? A Blue Moon is the term given to the second full moon when it happens that two full moons occur in one month. (More specifically, this is called a monthly blue moon. There is also a seasonal blue moon, but more about this on another day). This can happen, because the time between full moons is exactly 29.53 days. This means that any month that has 30 or 31 days (all the months except February) can theoretically have a Blue Moon. This happens on average every two or three years and, because this is a relatively rare occurrence, it has given rise to the saying: “Once in a Blue Moon”. The last Blue Moon occurred on 22 August 2021 and the next one will occur on 19 August 2024, after that on 31 May 2026 and then again on 31 December 2028 (I bet this one is going to be the centre of many discussions and wonderful celebrations).
What is a Supermoon? The orbit of the moon around the earth is elliptical, shaped like an egg, and not round. Which means that the moon is not always the same distance from the earth. During every lunar month, there is a point or moment when the moon is at is closest distance to the earth for that specific orbit (the orbit is also not exactly the same from one month to the next). This is called perigee, when the moon is on average 363 300 km from earth. The furthest point of the orbit is called apogee, when the moon is on average 405 500 km away. If the full moon occurs when the moon is within 90% of its nearest point to the earth, it is called a Supermoon. The full moon can appear up to 14% larger when it occurs at perigee than when it occurs at apogee. The last Supermoon occurred on 12 July 2022 and the next one will occur on 18 September 2024.
What makes the occurrence of 30/31 August 2023 so special? The Blue Moon and the Supermoon events coincided, resulting in the much talked about Blue Supermoon. A Blue Supermoon last occurred in December 2018 and the next one will occur in 2037. It is very interesting that 2037 will indeed have two Blue Supermoons. How is that possible? Very interesting… and we will leave that for another discussion.
Dr. Hélen Prinsloo
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