Why does the moon change color in the fall?
Why does the moon change color in the fall? asks Alex Dickinson, a student in Holtsville, NY.
It just wouldn’t be fall without a huge Halloween moon glowing orange at the horizon, rising above a spooky landscape of black tree limbs and piled-up leaves.
We call the moons of autumn harvest moons, but the official Harvest Moon rose on September 26th. The Harvest Moon is the full moon closest to the autumnal equinox, which fell on September 23rd this year. The Hravest Moon was probably named by farmers. Long before there were huge, gas-powered harvesters with blinding headlights, there was the bright fall moon, lighting the fields as the work of crop-gathering stretched into the night.
The romantic image of the autumn moon even inspired a famous song: “Shine On Harvest Moon,” written more than 100 years ago, remained popular for much of the 20th century. (For a clip of comedic actors Laurel and Hardy performing the song, visit
www.hamienet.com/midi12542_Shine-on-Harvest-Moon.html. )
The moon is a rocky gray-and-brown ball, lit up by brilliant sunlight. But the color we on Earth see depends on where the moon is located in the sky. No matter what the time of year, as the moon first peeks over the horizon, it may appear yellow, orange, or nearly red. Gradually, as the Earth turns eastward and the moon rises higher in the sky, the color pales to white.
The color deepens and fades because of how the Earth’s atmosphere plays with streaming-in moonlight. The bottom layer of air is the thickest — more full of gas molecules, dust, and pollutants. So as the moon begins to rise, we are looking at it through a heavy blanket of air, extending from us to the horizon.
Moonlight is white light. But white light is made of a hidden rainbow of colors — red, orange, yellow, green, blue, violet. As white moonlight travels through the thick air near the ground, gas molecules and tiny particles scatter bluer light out the sides of the beam into the rest of the sky. What’s left behind is mostly red, orange, and yellow light. So the moon’s face near the horizon is significantly reddened.
But as the moon rises higher, we see it through thinner and thinner air. More moonlight reaches our eyes unscathed, with more blues left in the beam. Since we are able to see more of the entire spectrum of moonlight, we see a whiter moon.
Why do fall moons seem particularly big and orange? In mid-northern latitudes, autumn is the time when the moon’s path across the sky reaches its minimum or shallowest angle. During most of the year, the moon rises about 50 minutes later each night. But near the time of the fall equinox, the time shortens to about 30 minutes. That means more light for farmers harvesting after sunset in the Northern hemisphere. Since the moon traces a curve nearer to the horizon, it also looks bigger and redder, creating the perfect Halloween moon to decorate the autumn sky.
