On Wednesday, August 30, you will no doubt hear the mainstream media proclaiming that on that evening we will have an opportunity to witness a “supermoon.” It is a term, or more specifically, a brand, of relatively recent origin. It is not derived from astronomy, but astrology; was first coined by an astrologer, who arbitrarily defined it as “a full moon occurring with the moon at or near (within 90 percent of) its closest approach to Earth in a given orbit (perigee).”
At 12 PM ET on the fifth Wednesday in August, the Moon will actually arrive at perigee, its closest point in its orbit relative to The earth at 221,942 miles (357,181 km) away. And 9 hours and 36 minutes later, the moon will officially be full. Although a full moon theoretically only lasts a moment, that moment is imperceptible to ordinary observation, and for a day or so before and after most people will report seeing almost full moon as “full”: The shaded strip is so narrow and changes in apparent width so slowly that it is difficult for the naked eye to tell whether it is present or which side it is.
And besides his “Super moon” status, this particular full moon will be the second to occur in the month of August, the first occurring on August 1st. As a result, the second full moon on August 30th will also be labeled as a “Blue So, for what it is host, what we will have will be one “Super Blue moon.”
However, unless there is an unusual atmospheric condition such as airborne dust, ash, or smoke, the moon will not appear blue without its normal yellow-white self. Nonetheless, thanks to mainstream media hype, many will likely be looking forward to seeing this big late summer moon.
Related: August Super Blue Moon Guide 2023
If you’re hoping to catch a glimpse of the full moon, our guide to best binoculars can help you find some nice wide angle optics to take in larger areas of the moon’s surface. Or, if you want to take a closer look at the moon’s properties, our guide to best telescope can help you find the equipment you need.
And if you want to take pictures of our natural satellite or the night sky in general, check out our guide on how to photograph the moonas well as ours best cameras for astrophotography and best lenses for astrophotography.
This flood is for you
But there’s also a downside: a full moon nearly coinciding with perigee means that for several days around August 30, the tides will be much higher than normal; low tides will be unusually low while high tides will run unusually high, perhaps even resulting in minor coastal flooding.
Such an extreme tide is known as a perigee spring tide, the word spring comes from the German spring — to “spring up”, and is not – as is often mistaken – a reference to the spring season. Each month spring tides occur when the moon is full and new. At these times the moon and the sun form a line with the Earth, so their tidal effects are added. (The Sun exerts a little less than half of the Moon’s tidal force.) “Neap tides,” on the other hand, occur when the Moon is at first and last quarter, working crosswise with the Sun. At these times the tide is weak.
The tidal force varies as the inverse cube of an object’s distance. On Wednesday, the moon is 14 percent closer to perigee than to apogee. Therefore, it exerts 48 percent more tidal force during spring tide on August 30 than spring tide near apogee two weeks earlier on August 16.
And if a significant storm or hurricane is offshore, working in concert with the already high water levels, the consequences can lead to rough seas, beach erosion and major flooding.
We can only hope that such meteorological conditions do not materialize this year, although it should be noted that the traditional peak of the Atlantic hurricane season comes less than two weeks later, on September 10th.
Supermoon branding “watered down”
For years, astronomers classified a full moon that coincided with perigee as a “perigee full moon.” A term that received little or no fanfare.
Now it seems that every time a full moon coincides with perigee, it is called a “supermoon”. Some newscasters – in an obvious attempt to keep your attention – refer to this event as “rare”, although in fact the moon going full within hours of arriving at perigee is not really that rare an event.
In fact, it occurs on average at an interval of once approximately every 413 days.
After next Wednesday, the next time this happens will be October 17, 2024.
And yet, even the full moon on August 1, which occurred about 11 and a half hours before perigee, as well as next month’s full moon on September 29, which comes almost 33 hours after perigee, are labeled supermoons, apparently because they fall within 90 percent of the moon’s closest approaching the earth. Or in other words, within the top 10 percent of the nearest full moons for a given year.
So now in most years we have not just one but four “supermoons”. Some years it can be as few as two while other years it can be as many as five!
But how “rare” or “super” is it?
Unrealistic expectations: Bigger?
And while Wednesday’s moon will be — as the Observer’s Handbook of the Royal Astronomical Society of Canada suggests — the “biggest full moon of 2023” (14% larger in apparent size compared to a full moon at apogee — it’s furthest from Earth) variation in the moon’s distance is not readily apparent to observers looking at the Moon directly.
So if you go out and look at the moon on Wednesday night expecting to see something special, you’re probably going to be disappointed. There are always many pictures posted on the internet before a “Supermoon”, showing incredibly large full moons, all taken with telephoto lenses, which all indicate that the moon will look fantastically large in the sky.
In fact, without advance knowledge of the full moon’s proximity, it’s likely that most people wouldn’t notice any difference between Wednesday’s full moon and any other full moon. But when the “super moon” concept is proposed, the same people will go outside, look up and declare that the moon make looks much larger than normal; similarly the expression “the emperor’s new clothes” has become an idiom of logical fallacies.
Brighter?
Then there is the matter of the moon’s brightness. Websites say the “supermoon” appears “30 percent brighter than other full moons.” But it actually equals one minimal increase of less than three-tenths of a magnitude; so the moonlight on Wednesday night will not be exceptionally bright.
Still, there are likely those who believe they will see an exceptionally dazzling full moon that night. In June 2013, a friend told me she expected this year’s version of the “supermoon” to look “radically brighter,” “Like with those 3-way bulbs; I thought it would be like turning the moonlight up a notch .”
Instead, the moon’s brightness looked no different compared to previous nights.
The moon illusion
Wednesday’s moon may still seem huge, but for a different reason.
When the perigee is close to the horizon, it can appear absolutely huge. It is then the famous one “moon illusion” combined with reality to create a truly stunning view. For reasons not fully understood by astronomers or psychologists, a low-hanging moon appears incredibly large when hovering near trees, buildings, and other foreground objects.
The fact that the moon will be much closer than usual on Wednesday will only help strengthen this strange effect.
So a perigee moon, either rising in the east at sunset or setting in the west at sunrise, can appear to make the moon appear so close that it almost seems like you can touch it. You can check this yourself by first noting the moonrise and moonset times for your area by going to this US Naval Observatory website.
Don’t overlook Saturn!
A full moon is positioned opposite the sun in the sky. As it turns out, three days before the moon reaches this point in the sky, the planet Saturn comes in opposition to the sun, when it is also opposite the sun in the sky. So on Wednesday night, Saturn will “photobomb” the Moon, which is located about 5 and a half degrees to its upper right.
Saturn is, of course, much farther than our nearest neighbor; it will be 814.6 million miles (1.31 billion km) or 73 light minutes from Earth. The ringed wonder will shine like a quiet yellow-white “star”. The famous rings will be tilted 9 degrees to Earth and are visible in powerful binoculars or small spotting binoculars that magnify at least 25 powers.
And so, regardless of exactly how you perceive Wednesday’s full moon, we here at Space.com wish you all clear, moonlit skies.
Joe Rao serves as an instructor and guest speaker at New York’s Hayden Planetarium. He writes about astronomy for Natural history journalthe The Farmer’s Almanac and other publications.
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