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One Big Reason Life on Earth Exists
A TED-Ed lesson on the 'Big Whack' — the ancient collision that formed our Moon and tilted Earth's axis. Discover how that 23.5° tilt, kept steady by the Moon, gives us our seasons.
How it works
- Press play to start the video.
- Videos 5 minutes or longer pause once or twice for a quick check. Get it right and you're on your way. If not, you'll see a friendly explanation and the video simply keeps going. Short videos skip the checks.
- At the end there's a 3-question test. Score 60% or more to complete the lesson, and 80% or more to master it. You can always watch again and try for a higher score.