Tonight, in a year full of them, we’ll get a rare celestial event. As the sun sets in the west, the full moon will rise in the east, creating striking vistas and in some places warming the moon’s white light to orange or even red. It rarely falls in October, and in fact it hasn’t risen in October since 2009. So how does a harvest moon happen, and why?
- The harvest moon is the first full moon after the autumn equinox, a point where the day and the night are of roughly equal length, which fell on September 22nd this year. In of itself, this is a marker; the days are shortening and the nights are lengthening.
- This has an unusual effect on moonrises. All full moons rise around sunset, but the moon moves eastward across the sky faster than the sun, which delays moonrise, at least to the naked eye, usually by about 50 minutes. But not, as astronomers discovered centuries ago, at the autumn equinox.
- Around the autumn equinox, the moon is so far to the east in the sky, at least in how we observe it from the ground, and the day is short enough that, for a brief day or two, they’re in perfect balance. The moon rises right as the sun sets, creating a striking image.
- The color of the moon, though, isn’t unique; it’s because the moon is close to the horizon. Sunlight has to scatter more through Earth’s atmosphere, filtering out the bluer wavelengths and leaving the moon to reflect the warmer colors. We just rarely see it due to late moonrise. The next full moon will have an even more intense red color thanks to this effect. The blood-like tint is part of why it’s called a hunter’s moon.
So, despite what our ancestors believed, it’s not the end of the world. It’s just a chance to observe the glorious clockwork of the solar system at work, revealing its quirks for us to witness. And, of course, further proof that nature really wants us to get some killer Instagrams. Keep an eye on the sky tonight, and tag us on any awesome harvest moon photos you snap on Instagram.