How super was it?

1:1 Size comparison of two moonrises

The headlines yelled at us, "Don't miss the largest full moon of 2024!" and "How to see this year's super Hunter's Moon!" Did they work? Did stories like those make you want to go out on the evening of October 17th to see the super-duper Hunter's Moon?

I've been seeing supermoon headlines since at least 2011, possibly longer than that. Somehow or other, the media picked up on the phenomenon and never let go. What is so darned fascinating about a supermoon? I seriously don't get it! A supermoon is not noticeably larger than the average moon. Anyone can plainly see that for themselves. Can't the news jockeys hear their readers yawning loudly and scrolling on by when they see another meaningless supermoon headline? 

Well, I suppose there's a sucker born every minute, and that's what keeps the supermoon clickbait alive, year in, year out.

You gotta admit it: A supermoon is a hard sell. It's kind of like The Emperor's New Clothes. But let's assume that we give in and go out to look at a supermoon. What is it that we're looking for? That humungous moon hitting our eye like a big pizza pie?

Here's the scoop (as if you hadn't read it a thousand times before). The moon's (roughly) 30-day orbit around the earth is not a perfect circle. It's a slight ellipse. So, it has a farthest distance (apogee) and closest distance (perigee) from earth. The moon's maximum distance is something like 260,000 miles, and minimum distance is about 225,000 miles. Given its orbital period, the moon travels the 35,000-mile difference between apogee and perigee in 15 days, give or take. That means the moon moves either further or closer to us (depending on which side of its orbit it's on) by approximately 2,400 miles per day.

Now here is where the media's sell gets problematic. Full moon  — that is, the astronomically speaking full moon  — falls at a particular time of day, on a particular day of the month. When the moon is precisely 180 degrees from the sun, the side of the moon facing us should appear 100% illuminated, or full. When you see the sun dropping below the western horizon and the moon just beginning to rise above the eastern horizon, you can say that the moon is full. But the exact moment when full moon occurs could be well before or well after moonrise. By the same token, perigee occurs at the instant the moon reaches its closest distance to earth in its orbit, which also may or may not occur exactly at moonrise. The supermoon, then, is when the full moon is rising while being as close as possible to its nearest distance to us. Yet, the supermoon headline stories never take into account that moonrise is at a different time for each of their readers, depending on their geophysical location. 

To simplify time differences around the world, astronomers (and aviators) use a time system known as "universal time" (or UT, or UTC; aviators call it "zulu time"). The reference point for UTC happens to be, for historical reasons, Greenwich, England, a small area of London and home of the Royal Observatory. In UTC, time is referenced from what is called the Prime Meridian, which passes over Greenwich. All the official time-keeping around the world uses an hourly offset from Greenwich. For instance, here in Arizona, my time offset is 7 hours from Greenwich. Thus, I must add 7 hours to my local time to get UTC, or the current time displayed on the great clock of Elizabeth Tower.

If you look up times for astronomical events (such as full moon), most almanacs will give you the time in UTC. They leave it up to you to subtract or add the difference from your local time zone. As an example, let's consider the supermoon on October 17, 2024.

The 2024 Observer's Handbook (from the Royal Astronomical Society of Canada) states that the Hunter's Moon (full moon in October) occurred at "11:26 UT" on the 17th. And perigee occurred at "01:00 UT."

For me, using the almanac, full moon occurred at 04:26 AM (Mountain Standard Time; Arizona stays on Standard Time all year and doesn't switch to Daylight Savings Time) on the 17th. And perigee occurred at 6:00 PM on the 16th. But most of the news stories I read about the supermoon simply told me to look at the full moon on the evening of October 17th. That's nearly 24 hours after perigee, and 14 hours after full moon! To get the "supermoon effect" I technically should have watched the moonrise on October 16th, instead of waiting till the night of the 17th! Better yet, I should have gone out early in the morning of the 17th and watched the super Hunter's Moon as it set in the west while the sun was rising in the east.

Since many of the readers of these supermoon articles were likely going to rely on their wall calendars to remind them which day would be full moon, they, too, probably went outside and watched the full moon rise on the evening of the 17th, but unbeknownst to them, they missed seeing the "largest full moon of 2024"! 

What?! I want my money back, USA Today!!!!

But not to worry. What did I say a few paragraphs ago? The moon moves further away from us by 2,400 miles per day. So instead of 225,000 miles (give or take), the moon we observed on the evening of October 17th was still a very close 227,400 miles away. That's not bad. It was still larger than a typical full moon, which would be 240,000 miles away.

There's another gotcha here, though! And that is our inability to compare sizes of the moon by using only our memory of the previous full moon we observed, 29.5 days ago, if we even bothered to look at it then. Maybe we have to compare it to our memory of the full moon from two or three months ago. This is very complicated because, believe it or not, most people (casual observers of moonrises) cannot recall how large the previous full moon was. And I'll throw that challenge up right here and now! 

Take a US quarter-dollar coin (with a diameter of 0.95-inch) and hold it up in front of your eye, as if trying to compare the quarter's diameter to that of the moon in the distance. Now tell me, how far away do you think you'd have to hold that quarter from your eye so that its diameter exactly equals the apparent size of the full moon? Would you need to hold the quarter at arm's length? Closer? Farther? What would you guess? Okay. Hopefully, you've made your guess by now because (spoiler alert) here's the answer: 9 feet. That's right! A quarter at 9 feet is practically the same size as the average full moon! I'll bet you didn't expect it to be so small, right?

The only way to appreciate the size difference between a given supermoon and a regular old moon is to take a picture, preferably with a zoom lens, say between 200- and 300-mm focal length. Take a picture of an average moon, then, using the same camera and lens, take a picture of a supermoon. Then count the pixels across the diameters of the moon in your two pictures. 

Yes, that's what I did for you already, at the top of this page! Click the image to get a larger view. If you want, when the larger picture opens, right-click and choose "Open in new tab" to get the full size. 

Keep in mind that even the casual observer watching a moonrise will appreciate its beauty, to the point of being moved spiritually. And thanks to the "moon illusion" (look it up), you will always think that the moon on the horizon looks larger than it does when it's high overhead. But you can use the same method I used here to prove to yourself that, yes, the horizon moon and the zenith moon (6 hours later) are not appreciably different in size. It is an illusion. Yet, it's an illusion that you can't do anything about. When you watch the moon rising, you will think it looks larger than it really is.

Maybe we should call this the "supermoon illusion." Because the news media will continue to push these supermoons, and each time when you go out to look at them, you'll fall for the illusion. You'll think to yourself, "Wow! It actually does look bigger!" 

If you enjoy that sort of thing, then maybe you don't want to take pictures and be disappointed by your direct comparison. "Twenty-five pixels? Is that all? How in the world can that be 'super different'?" 

You must admit, though, that if you're only wishing for an emotionally or spiritually uplifting view of a moonrise over the swaying palm trees or picturesque urban skyscrapers, then there is no need to wait for a supermoon. The rising full moon always looks super to folks like us!

To reinforce the importance of observing a supermoon at the correct time by using an almanac rather than relying on news articles that were written by non-astronomers, consider the following picture. You may or may not recall last month's headline hype over the Harvest Moon (September's full moon), which also was a supermoon, and while rising, it underwent a partial eclipse. I took a picture of that one, too, using the same camera and lens I used for the pictures at the top of this page. Since I waited until moonrise on October 17th to shoot the largest supermoon of the year, it was actually smaller than the supermoon in September! (Note: Since the moon was being eclipsed, I had to use a drawing tool to complete the circle and make the comparison easier.)