A harvest ‘supermoon’ this week and a new comet
On Monday night, we officially have our Harvest Moon for 2025 over Butler, and it’s also our first of three “supermoons.” I’m totally OK calling it a Harvest Moon, but I’m far from being in love with the “supermoon” moniker. I’ll defend my case on that later.
If it seems that the Harvest Moon is late this year, that’s true. Most folks assume that any full moon anytime in September is the Harvest Moon, but technically, that’s not the case.
The semiofficial definition of a Harvest Moon is the full moon that is closest to the date of the autumnal equinox, otherwise known as the first day of astronomical autumn. The fall equinox this year was Sept. 22, 14 days before the full moon on this coming Monday night. The full moon last month was Sept. 7, 15 days from the equinox. So just by a nose, our October full moon wins. Honestly, who really cares?
Astronomically, the Harvest Moon is special because of the unique celestial mechanics of this time of year. The moon migrates eastward among the stars each night because of its 27.3-day orbit around Earth. Because of that, the moon usually rises about 30 to 60 minutes later each night. Around the time of the autumnal equinox though, the moon rises only about 20 minutes later each night, making it seem that the full moon is lasting longer.
Harvest Moons really don’t look all that different from any other full moon. You may have seen a picture of a big orange Harvest Moon rising. Full moons the rest of the year appear more or less just as large. No matter what phase the moon is in, it sports that orange-red color when it rises or sets because when it’s close to the horizon, we see it through the thicker and dustier layers of Earth’s atmosphere. That scatters away all but the reddish components of the white light of the moon. When the moon is higher, we do see it through a thinner layer of our atmosphere and the scattering effect goes away.
The reason full moons look much larger when they rise or set is simply an optical illusion. Prove that to yourself by holding a nickel out at arm’s length when the full moon is rising and again when it’s at its highest around midnight, and you’ll see that your nickel will cover exactly the same amount of the moon.
Calling the moon on Monday night a supermoon is a bit of a stretch in my book. By definition from an astrologer — and not an astronomer — a supermoon is any full moon that’s a little closer to Earth than average. Since the moon’s orbit around the Earth is not a perfect circle — but rather an ellipse or oval — the moon’s distance from Earth can vary in its orbit, but not all that much.
I’m not going to get into all the numbers here, except to say that when you have a “supermoon” the moon is only 7% larger and only 14% brighter. It’s not that big of a deal. You can hardly tell the difference.
Call me a curmudgeon but the term “supermoon” is more hype than anything else. Calling it a Harvest Moon is just fine with me.
C/2025 R2 SWAN is the formal name of a comet that was discovered last month. I’m always leery about talking about comets, especially new ones, because they can be so unpredictable. They may not turn out to be as bright as predicted, or the sun could totally break up this dirty cosmic snowball.
Assuming that doesn’t happen, look for Comet SWAN this week in the very low southwestern sky, just above the horizon near the bright star Antares as you can see in the diagram. It’s going to be tough to spot, but you may catch it with binoculars, and possibly with the naked eye, in the countryside.
As October continues, it’ll appear a little higher in the sky and may be easier to see. Stay tuned! Currently, the comet is about 37 million miles away and traveling at a speed of just under 110,000 mph.
Early in the morning, not far from the Big Dipper, another comet is brightening. It’s Comet Lemmon, best seen before morning twilight. It could become visible to the naked eye in mid-October. I’ll have more on that next week in Starwatch.
Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul. He is also the author of “Stars: a Month by Month Tour of the Constellations,” published by Adventure Publications and available at bookstores and at adventurepublications.net. Contact him at mikewlynch@comcast.net.
