If the Moon looks orange or red tonight, the most likely reason is that it's low in the sky. Near the horizon, moonlight passes through far more of Earth's atmosphere before it reaches you, and the air scatters away the blue part of the light, leaving the orange and red. It's the same effect that colors a sunset. Smoke from wildfires, dust or haze can deepen the color, sometimes even when the Moon is higher up, and a deep copper-red full moon can also mean a total lunar eclipse.
It's low in the sky: the usual reason
The Moon has no color of its own to speak of. It shines by reflected sunlight, and sunlight is a mix of every color. What changes is the air between the Moon and your eyes.
Air molecules scatter light, and they scatter short, blue wavelengths far more strongly than long, red ones. Physicists call this Rayleigh scattering, and it's why the daytime sky is blue: blue sunlight is being bounced around the whole sky. When the Moon is high, its light crosses a fairly thin layer of air and loses only a little blue on the way, so it looks white or pale yellow. When the Moon is near the horizon, its light travels sideways through the atmosphere, through dozens of times as much air as when it's overhead. By the time it arrives, much of the blue has been scattered out of the beam, and what's left looks yellow, orange or deep amber.
That's why the color fades as the Moon climbs. A Moon that comes up like a pumpkin at seven o'clock is usually silvery white by ten. If it's orange and close to the horizon, you need no other explanation.
A low full moon is also the one people notice most. A full moon rises around sunset, just when people are out, and the moon illusion makes it look huge next to trees and rooftops. We explain that trick of perception in why does the moon look bigger.
Wildfire smoke can turn it red, even high up
If the Moon is well above the horizon and still looks orange or red, smoke is the usual suspect. Wildfire smoke is made of very fine particles, and in the right size range they filter out blue and green light efficiently, leaving reds and oranges. A NASA image note on drifting wildfire smoke made the same point about sunsets: when smoke drifts over a region, sunsets there turn noticeably redder and more orange for as long as it lingers.
Smoke travels a long way. It can be lofted high into the atmosphere and carried hundreds or even thousands of miles, so a red Moon can appear on a night when there's no fire anywhere near you and the air at ground level smells fine. If the Moon looks unusually red, an air-quality map or a smoke forecast from your weather service will often show a plume overhead.
Haze, dust and pollution
Not every particle makes the Moon vivid. A JPL scientist writing about red skies noted that pollution and dust in the lower atmosphere tend to subdue the color of a rising or setting Sun, while fine smoke particles, or tiny aerosols thrown high by a major volcanic eruption, can deepen it to an intense red. The same applies to the Moon.
So in practice:
- Summer haze and humidity often give a low Moon a dull yellow or muddy orange look rather than a clear, glowing one.
- Dust, from dry fields at harvest time or a dust storm, can tint it brownish or orange.
- City pollution can make it look dim and yellowish near the horizon.
- Fine smoke or volcanic haze is what produces the really deep reds.
A total lunar eclipse: the copper-red full moon
There's one more possibility, and it's the dramatic one. During a total lunar eclipse, Earth passes directly between the Sun and the full moon, and Earth's shadow covers the Moon completely. The Moon doesn't go black. Instead, a little sunlight is bent through Earth's atmosphere and into the shadow, and the same scattering that reddens sunsets strips out the blue on the way. The Moon glows a faint orange, copper or brick red. People call it a blood moon.
How red it gets depends on what's in Earth's atmosphere at the time, including volcanic aerosols and clouds along the edge of the planet. A lunar eclipse can only happen at full moon, it lasts hours from start to finish, and it's predicted years ahead, so a quick check of an eclipse calendar will tell you whether one is happening tonight. Our guide to the blood moon meaning covers the science and the old stories, and the full moon calendar shows when the next full moons fall.
How to tell which one you're seeing
A quick checklist for tonight:
- Is it near the horizon? If yes, it's almost certainly ordinary atmospheric reddening. Watch it for an hour; it should pale as it rises.
- Is it high and still red? Look for smoke or haze. A hazy sky, a red Sun earlier in the day or an air-quality alert are good clues.
- Is part of it darker, with a curved shadow edge? That's Earth's shadow, so it's a lunar eclipse in progress.
- Is it a thin crescent, glowing orange low in the west after sunset? Same horizon effect, just on a smaller Moon.
For tonight's phase and how much of the Moon is lit, see our moon phase today page.
Does an orange moon mean anything?
As a sky event, no. An orange or red Moon is the atmosphere at work, and it happens every month to anyone who watches the Moon rise. The names attached to some of the most photographed full moons don't change that. The Harvest Moon and the Hunter's Moon have a reputation for glowing orange simply because they're so often watched coming up at dusk, through autumn air that can be dusty from the fields.
Red moons do carry a long history of omens, mostly tied to eclipses, when a suddenly darkened, bloody-looking Moon unsettled people who had no warning of it. Astrologers give meaning to the full moon's sign, or to an eclipse, but not to the color of the light, which depends on your local weather. If a vivid Moon stops you in your tracks on the way home, that's reason enough to look.
Questions readers ask
Is an orange moon a bad sign?
No. An orange Moon is caused by Earth's atmosphere scattering blue light, usually because the Moon is low on the horizon. It's harmless and happens all the time. Deep red can mean smoke in the air, which may matter for air quality but says nothing about luck.
Why is the moon orange and so big tonight?
Both effects happen near the horizon. The orange comes from looking through thick air, and the size comes from the moon illusion, a perceptual effect that makes a low Moon seem enormous. Measure it with a fingertip at arm's length and it's the same size as when it's overhead.
What color is the Moon really?
A dark gray, slightly brownish. The lunar surface reflects only a small share of the sunlight that hits it, a little more than worn asphalt would. It looks bright white overhead only because it's lit by full sunlight against a black sky.
Is every Harvest Moon orange?
No. A Harvest Moon is orange when it's low, like any other full moon, and turns pale as it climbs. Its reputation comes from being watched at moonrise, often through hazy or dusty autumn air.
