I knew this, but only because I recall the approximate the number of miles to the moon, and the circumference of the earth. Those near my age might also remember that a fast rocket ship takes three days to get to the moon. -via reddit

My problem wasn't knowing the ratio, it was translating 9.5 times the basketball circumference into linear distance. 23 feet or 7.1 meters is the answer. 29.5 inches around a basketball. 4.1 inches around a tennis ball gives a ratio of 8. Moon is 6790 miles around, Earth is 25,000 giving a ratio of 3.7 . As a double check, I know the angular width of the Moon from the Earth is 0.5 degrees, and the other way around is 2 degrees, giving a ratio of 4.

In other words, the tennis ball is too small! (or the basketball is too large)
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The correct answer is actually "undefined" because that tennis ball is too large to fit on that scale.
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