Why does the Moon turn red during a lunar eclipse?

Thursday night, the Moon turns red. Not a filter, not a camera trick, the real Moon over your real house. The full Moon dims, slides into something enormous, and glows like an ember. No trick.

And like everything else this month, exactly on schedule. Welcome back to Watt & Why: The Eclipse Wave, Episode 8, the finale! Two weeks ago, the sky went dark in the middle of the day, and half of you needed special glasses and a lucky map. Thursday night, the sky answers with its gentlest big show: a Blood Moon, and this time everybody's invited.

Remember the lineup from the big day? Sun, Moon, Earth—the Moon in the middle, casting a shadow so small it was a dot racing across the map. You had to stand in that dot to see anything. Now flip it.

Two weeks later, the Moon has swung around to the other side, and it's Earth in the middle. Earth is nearly four times wider than the Moon, so its shadow is enormous. Out at the Moon's distance, it's still almost three Moons wide. No aiming, no thin path.

If the Moon is over your sky, you're in. Half the planet gets a seat. And because you're looking at the Moon, not the Sun, no glasses! After a month of warnings, for once, just look up.

But here's the question this episode exists for: Moonlight is bounced sunlight, so if Earth blocks the Sun, the Moon should simply go dark. Lights out, a hole in the sky. It doesn't, because Earth wears a thin coat of air. When sunlight grazes the edge of our planet, the air bends it inward, and it bends the red light best.

The blue gets scattered away—that's literally why your daytime sky is blue. The red curves on into the shadow and lands on the Moon. Now think about what that light is: it's every sunrise and every sunset happening on Earth right now, all of them at once, bent together, shining on the Moon. When you see a Blood Moon, you're seeing the whole world's sunsets from the outside.

So Thursday's eclipse: the Moon crosses the shadow a little off-center, so it doesn't go all the way in. 93% of its width does—nearly total! One bright sliver stays lit along the edge, and the deep side glows that quiet copper red. And watch the bite as it grows—its edge is curved.

That curve is Earth, our planet's round shape stamped onto the Moon in shadow. People spotted that curve more than 2,000 years ago and knew the Earth was round long before anyone could fly up and check. So when do you look? Here's the lovely thing about a lunar eclipse: unlike the solar one, there's no racing shadow and no lucky towns.

Everyone on the night side sees the same moment—you just read it off different clocks. The deepest moment comes at 4:12 Universal Time. For the Americas, that's Thursday evening: 9:12 in Los Angeles, just past midnight in New York, 1:00 a.m. in São Paulo with the Moon almost straight overhead.

Europe and Africa, you're the before-dawn crew: 5:12 in London, 6:12 in Madrid with the Moon low in the west. Set an alarm, it's worth it! Asia and Australia, the Moon's below your horizon this time, so grab the livestream. And unlike Totality's two frantic minutes, this show runs on and on.

From first bite to last, more than three hours—hours, not minutes! Equipment list: your eyes, enough. Binoculars, a nice bonus—you can watch the color creep across, crater by crater. Cocoa: optional, but recommended.

Every solar rule from this month—the glasses, the ISO stamps, the thin path, the countdown panic—none of it applies. You are watching a shadow fall on rock a quarter-million miles away. It is perfectly, wonderfully, boringly safe. Pick a window with a view of the Moon, stay a while.

And if clouds win, the livestream never has clouds. And notice when this is happening: 2 weeks after the solar eclipse. That's not luck, that's the season. Remember the doorways?

Eclipses travel in pairs. On August 12th, the new Moon crossed in front of us and the Moon's shadow touched Earth. Now the full Moon swings around the back, and Earth's shadow touches the Moon. Crossing to crossing, two weeks apart—a shadow for a shadow.

Fair trade. And that is the wave. Eight weeks ago, the eclipse was just a date on a calendar. Look what you can read now: Why the Sun vanished, how to watch without hurting your eyes, the minute-by-minute of totality, the eclipse that proved Einstein right...

...the day we all saw it, why the sky waits for its seasons, the luckiest fit in the sky, and Thursday night, a Moon the color of sunset. The sky runs on clockwork, and now you can read it when the Moon turns red. So remember, Thursday night, August 27th into Friday morning, the red is every sunrise and sunset on Earth bent onto the Moon. And you need nothing but a window, your eyes, and a little patience.

That's The Eclipse Wave complete! Thank you for watching the sky with me. Subscribe if you'd like more, because the sky is never, ever done with us. Thursday night, look up.

See you on Watt & Why!

Transcript of the episode, lightly cleaned for readability.