Harvest moon and arc minutes (revisited?)

I think I read this here but my searches are fruitless:

The great big orange “harvest moon” we see sometimes (in the fall?) is really no bigger in the sky than the moon ever gets, if you measure it in degrees (or minutes). It’s an optical illusion caused by the atmosphere.

Is this correct? How does this work, and why does it supposedly happen more often in the fall?

I mentioned it to my father, but was unable to explain the process. He also asked if a camera would be fooled by the illusion.

I was told, when I was in high school, that if you bent over and looked at the full moon thru your legs that it would not look bigger, Iv’e tried it and it does seem to work??

Spelling and grammer subject to change without notice.

That’s strange. When I was in High School, I was told that if you turn your back to someone, then bend over with your pants down and your ass in the air, that ** they ** would see the moon from a whole new perspecitve :eek: :smiley:

It’s the mind’s eye, not the camera, that’s being fooled. If you took a photo of the Harvest Moon low to the horizon when it appears bigger (to you) and then another picture when it is higher in the sky, your brain would still be fooled. This is because the eye is comparing the “lower” moon to objects on the horizon. In other words, the eye puts things in proper perspective. But, a “higher” moon lacks such background objects against which the eye can make such a comparison.

The Harvest Moon just gets more attention, and so people think this phenomenon is unique. But, it happens all the time - and not just at full moon, either. Try to notice the moon in different phases, low and high in the sky. Try even looking when its high in the sky and then as its setting. You’ll see it’s just an illusion.

  • Jinx

It’s my understanding there’s a bit more to it that this. People tend to underestimate distances when looking up into the sky (possibly because there are no “background” objects). So when you look at the full moon high in the sky, your brain thinks it’s closer to you than when it’s on the horizon. (Actually it is, by about 1.6%, but that’s not the issue here.) Since the moon appears closer up overhead, your mind adjusts the apparent size to be smaller. See


for fuller details and other explanations.

The other replies are, one way or another, referring to the Moon Illusion - the way the Moon seems to be larger when it’s close to the horizon than when it’s high in the sky.
However, the Harvest Moon is slightly different. That’s usually taken to be the full moon that occurs closest to the autumn equinox. Which is normally whatever full moon falls within September. The conventional explanation for why this should be singled out is the idea that in an agricultural society a full moon acts as a useful extension of the working day during your most time critical part of the year. It’s not what I’d usually use as a cite, but here’s the Farmer’s Almanac’s take on the issue. Quite how far back the name actually extends I don’t know.
I’ve also heard the bit of folklore before that the Harvest Moon is somehow larger, but I don’t quite know where that comes from either.

There is a connection between the seasons and how high the full moon is in the sky, and hence the applicability of the Moon Illusion to it, but it doesn’t line up with the Harvest Moon. In particular, in the northern hemisphere the full moons of winter are high up through most of the night, while those of summer hug the horizon. But since the full moon of September falls between these extremes, it’s neither one nor the other.