September’s full Moon, called the Harvest Moon, refers to the full Moon that comes closest to the first day of autumn. Depending on the year, the Harvest Moon can fall before or after the autumnal equinox which arrives annually on or near September 22. This year, the Harvest Moon reaches full phase on September 19 at 4:13 am PDT.
Before the advent of artificial lighting, our ancestors were acutely aware of the daylight hours waning more rapidly around the autumnal equinox – the time when the Sun rises due east and sets due west – than at any other time of year. But back then, people also understood lunar behavior, harvesting by the light of the Moon.
The Harvest Moon will appear as a large orange full moon low above the eastern horizon after 6:40 pm on September 18, followed by sunset at 7:15 pm in the west. The orange color of a moon near the horizon is a true physical effect. When looking toward the horizon, we are actually looking through a greater thickness of Earth’s atmosphere than when looking directly overhead. The atmosphere scatters blue light (the reason the sky looks blue). The thickness of the atmosphere in the direction of a horizon scatters blue light most effectively, but it lets red light pass through. So a moon near the horizon takes on a yellow, orange or reddish hue.
The bigger-than-usual size of a moon seen near the horizon is a trick your eyes play on you, called “the moon illusion.” The illusion is a matter of perception, a trick of the brain, which perceives the Moon when seen overhead as closer than the Moon seen at the horizon. When an object is perceived to be nearer, the brain may compensate by making it look smaller to us. Likewise, an object thought to be farther away will be seen as larger.
Each full Moon during the year has been named throughout the years; next month’s full Moon is the Hunter’s Moon, and it will come this year on October 18. Enjoy!
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Now showing in OMSI’s Kendall Planetarium: Perfect Little Planet and Secrets of the Sun