SHAFTSBURY — The waning gibbous moon over Southern Vermont might seem especially large and vivid this week, and clearly visible in the daytime sky. This is due to the last celestial hurrah of 2024 — the last in a series of consecutive supermoons.
Hugh Crowl, who teaches astronomy at Bennington College, explained.
“The moon has been slightly larger and brighter this time of the month because it is slightly closer to us. This is a relatively small effect. At its closest, the moon’s angular size is something like 13 to 14% larger than it is at its furthest/smallest — so, about 7% larger than the moon’s ‘average’ angular size,” said Crowl. “This, in turn, makes the whole moon a bit brighter — something like 27 to 30% brighter at its brightest/closest than it is at its faintest/furthest. Note that the Moon’s distance changes throughout its orbit and, when it’s at its closest when it is also full — as was close to true this month — the effect of the larger moon is particularly notable.”
In general, our visibility of the moon changes throughout the month, with the moon appearing illuminated in phases.
According to the National Aeronautics and Space Administration (NASA), there are eight phases of the lunar calendar, and each one illustrates a phase of illumination.
The eight phases are the new moon, the waxing crescent, the first quarter, the waxing gibbous, the full moon, the waning gibbous, the third quarter and the waning crescent — and, the cycle of new moon to new moon repeats every 29.5 days (this is different from the 27.3 days it takes for the moon to complete one orbit around the earth).
Additionally, the perceived size of the moon changes.
First of all, the moon’s orbit is not circular; it is elliptical. If the moon in its “earth orbit” is physically closer to the earth, it will appear larger. At its closest point, the moon is in “perigee” and is roughly 223,693 miles from earth If the moon in its “earth orbit” is further from the earth, it will appear smaller. At its furthest point, the moon is in “apogee” and is roughly 251,966 miles from earth.
Secondly, the size of the rising or setting moon (the moon near the horizon) often appears larger than a risen moon (the moon when it is high in the sky). This is due to an optical illusion, and the difference is known as the “moon illusion.” Our brains perceive the moon as larger, but measurements from photographs show that it is the same size near the horizon as it is when it is high in the sky.
“There is something of an optical illusion when the moon is close to the horizon,” confirmed Crowl. “For reasons that are not totally understood, our brain interprets the moon closer to the horizon as bigger than when we see it high in the sky. To be clear, the angular size of the moon doesn’t change, but it sure does look big when we see it rising over the Green Mountains.”
As for the brilliantly visible moon during the daylight hours, Crowl added, “The moon is always up for approximately half of any 24-hour period. There are times when the moon is up from sunset to sunrise (when the moon is full), times when the moon is up between noon and midnight (when the moon is first quarter), and many other times in between. We often don’t notice the moon when the sun is up because of how bright the sun is, but if you’re particularly observant, you may see it at lots of different times of the day throughout the year.”
The phenomena of the supermoon can be explained by NASA.
“When the moon is at its closest point to earth during a full moon phase, that’s a ‘supermoon.’ Supermoons only happen three to four times a year, and always appear consecutively. Throughout most of Earth’s orbit around the sun, perigee and the full moon do not overlap.”
Coming on the heels of the sturgeon supermoon in August, the harvest supermoon in September, the hunter’s supermoon in October, and the beaver supermoon on Nov. 15, the waning gibbous moon that we can now observe over Southern Vermont is roughly 14,000 miles closer to our planet than normal, which in turn makes it appear to be about 14% larger and 30% brighter than usual.
The waning gibbous phase lasts for only about seven days before passing to the third quarter phase. The percentage of illumination will continue to diminish with each day. On Wednesday, Nov. 20, illumination was estimated to be at 73%. Thursday’s waning gibbous illumination will be at 63%, and Friday’s waning gibbous illumination will be at 54%. On Saturday, we will welcome the third quarter moon.
In astronomy, this waning gibbous moon phase represents the passing of the streak of the four consecutive supermoons of 2024.
In astrology, this waning gibbous moon phase is said to represent a time of reflection, gratitude, sharing and letting go — a perfect reminder for us as we move into the week of Thanksgiving, the holiday season and the end of 2024.
We will not see a supermoon again until October of 2025.