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How to measure distance with your thumb.

Stretch out your arm, thumb up, and sight a target with one eye. Now switch eyes: the thumb jumps sideways. Estimate how many metres it jumped at the target, using something like a 7 m wide house, and multiply by 10. The factor is geometry: arm 60 to 70 cm, eyes 6 to 7 cm apart, roughly 1:10.

Useful: Calibrate yourself once on a known distance; depending on your build, your factor is between 7 and 12.

Source: de.wikipedia.org

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