Skywatchers in the Land of Living Skies have an extremely unique opportunity this month, as August will host not only two rare full moons in one month, but also an extremely rare astronomical phenomenon.
If you thought a blue moon was rare, try witnessing a super blue moon.
A super blue moon is the combination of two separate phenomena: a rare blue moon occurring at the same time as a supermoon.
The term “blue moon” has nothing to do with the colour of the earth’s natural satellite. The term "blue moon" allegedly originates from a 16th century expression “the moon is blue,” meaning that something is impossible.
A blue moon refers to how frequently the moon appears in the sky, relative to the calendar month. The average month has 30.437 days, and the moon takes 29.5 days to complete its lunar cycle (going from new moon to new moon). Because of this slight difference in the math, there will be times when the same month will have two full moons visible. Put simply, the term “blue moon” refers to two full moons occurring in the same calendar month (as we are witnessing this August).
A supermoon refers to a moon that appears slightly larger and brighter than average, and this is due to the nature of its orbit around the earth. Coined in 1979 by astrologer Richard Nolle, the term supermoon means that the moon is closer to earth in its orbit.
Supermoons occur from the way the moon orbits around the earth. Earth itself is not a perfect sphere, but due to rotation and other factors it’s shaped more like an oval. As a result of this, the moon will have a point in its orbit when it is further away from us (at apogee) and when it is closer to the earth (at perigee). In this process, the distance between the two bodies changes by around 14 per cent, and ranges from 408,000 km at apogee to around 350,000 km at perigee.
This 14 per cent increase may sound like an incredible difference, but it’s still hard to notice with the untrained eye due to the vast distances involved. Technically, however, the moon does appear larger and brighter.
The idiomatic expression “once in a blue moon” is often used to describe a rare but still recurring event, but according to NASA, a blue moon isn’t all that rare as it recurs every 2.5 years on average. For super blue moons, NASA said they occur about once every decade.
The exact moment the moon is defined as a “full moon” is the point that it sits exactly opposite of the sun, at an exact 180-degree separation. This means that the moon is fully engulfed in light and will appear to be at its brightest. According to NASA, the moon should be at peak brightness around 6:37 p.m. (adjusted to CST).
The US Navy has an online sunrise calendar that can be useful to anyone wishing to view the larger than average moonrise, and this can be found at https://aa.usno.navy.mil/data/RS_OneDay.
If the weather doesn’t cooperate, anyone interested in viewing the phenomenon can watch a free live stream recording that’s hosted by the Virtual Telescope Project based out of the Bellatrix Astronomical Observatory in Italy at https://www.virtualtelescope.eu/.
For some tips on how best to photograph the moon, visit the helpful guide on Space.com at https://www.space.com/how-to-photograph-the-moon-camera.
The next supermoon is predicted for Sept. 18, 2024.
For the next super blue moon, observers will have to wait until Jan. 2037.
