Puzzler Answer 1/13/97: The Moon and Your Finger

Ray: Everyone knows that from planet Earth, the moon and sun appear to be about the same size, even though we know they're not the same size (from our vantage point they're the same size) hence we can get things like eclipses.

Tom: By the way, who sent this in?

Ray: Tony Coley from Raleigh, North Carolina. Anyway, where was I? The moon can go between us and the sun and block out the sun and if they didn't appear to be the same size we wouldn't have an eclipse. Now, knowing this, you can take the tip of your finger and at arms length you close one eye and you can block out the sun with the tip of your index finger. NO - not that finger! The other finger! Now, I wouldn't try this because according to Tony's mother you can go blind if you do this! However, you go out at night and you hold that same finger up and close one eye and you can't block out the moon. How come?

Here's the answer:

Because during the day, when you look at the sun, your pupil shrinks because of the sunlight. But at night, your pupil dilates to absorb more light, so your pupil actually looks around the tip of your finger.

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