Supermoon Illusion

A true to life representation of the supermoon size vs the average full moon
A true to life representation of the supermoon size vs the average full moon

Step outside on On Monday the 14th November just after sunset and look to the north east and you will see a large moon rising (assuming it is clear). Looks huge, right? Much bigger than normal? Lots of reports in the media about this massive supermoon and how the full moon won’t appear this big again until 2034. But what exactly is a supermoon and is it rare?.

The moon orbits the earth, but it doesn’t orbit in a perfect circle. It’s orbit is elliptical. There is a point in the moon’s orbit when it is closest to earth (perigee) and another point when it is furthest away (apogee). When a full moon coincides with the perigee we experience a supermoon. In this instance, the full moon and the closest point in the orbit line up very closely meaning that November’s full moon is over 14% bigger and 30% brighter than a full moon that occurs when it is at apogee. This is the largest the full moon will appear for almost 70 years. When compared to a full moon at its average distance from earth, the moon will appear 7 % larger and 16 % brighter.

Adding to the spectacle, is a phenomenon known as the moon illusion. Simply put, when the moon has just risen, we tend to see it close to trees, buildings, hills and the horizon. We have a frame of reference for it and so the moon looks bigger than it actually is. The same moon, later on in the evening, when it is high up overhead will appear much smaller. In fact, if you were to measure the actual size of the moon when it is overhead and when it is close to the horizon, you would discover that they are the same.

As for the rarity of a supermoon. It’s not quite as rare as a blue moon, a supermoon (depending on definition) occurs once every 13 or 14 months, whereas a blue moon (the 3rd of 4 full moons that occur in a quarter of a year) happens every 2 or 3 years. If you miss this supermoon, not to worry, The supermoon of January 2018 will be at a distance of 356,605 km, a mere 82 km further away that this one.

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