The Harvest Moon
Blog vol 3. 21. The Harvest Moon.
On October 9th, we enjoyed the full moon that is traditionally called the “Harvest” moon. It occurs around the Autumnal Equinox, so either in September or October, and it happens at harvest time in the northern hemisphere.
What I marvel at is the size of the harvest moon; it appears twice as large when it starts to come up over the horizon. Sorry to burst your bubble or remove a little magic from your lives, but it really is no bigger, it just looks that way.
Scientists have scratched their heads on this one.
There is the long-known Ponzo illusion, where we have converging perspective lines and two horizontal lines above each other, the one further back looks longer even though they are actually the same length.
This goes a long way to explaining our larger moon. When the moon is near the horizon, our brain perceives it as closer to us and adjusts the perceived size. There are many ways to tell that this is happening. One way is to take a picture of the moon on the horizon and then with the same settings take a shot of it up in the sky, and you will find that the moon is the same size. Another way, which is less technical, is to just turn around and bend over and peek between your legs, upside down, and a smaller moon ensues.
What is really cool, is that astronauts out in space still experience this same moon illusion, even though there are no background clues to help make it happen. (Read more about this moon illusion here). My best guess is that we have learned perspective patterns that just kick in whatever the circumstances.
The Harvest moon is also named for its time of rising. Usually, there is a ½ hour difference from day to day on the time of moonrise, but the Harvest moon rises at about the same time every evening. It gives the perception that we have more days of full moon at this time of year to help in the harvesting.
The reason for this timing is the angle from the elliptic, which is the path that the planets and the moon follow as we track them in the night sky. During September and October, we have a shallower angle in the east for this path resulting in less variation in the time of moonrise.
All this to say that it is a great time of year to go out and enjoy a moon on full display.
Til next week,
the good doctor
