It turns out that the balls in two popular sports, basketball and tennis, are almost exactly the right size for an Earth-Moon system scale model. This is a practical follow-up to my article Earth-Moon System to Scale.

Take a standard NBA basketball, which has a circumference of 29.5 in (74.9 cm). The diameter of the basketball is then 9.39 in (23.85 cm). If we let this ball represent the Earth, then it turns out that a standard tennis ball, with a diameter of 6.7 cm (2.64 in), is almost exactly the right size to represent the Moon. The appropriate size to represent the Moon is 23.85 ÷ 12742 x 3475 = 6.5 cm, where 12742 km is the diameter of the Earth and 3475 km is the diameter of the Moon. So, the tennis ball is a mere 2 mm too large.

Again taking the basketball for the Earth, the distance between both should be 0.2385 ÷ 12742 x 384400 = 7.20 m, where 384400 km is the distance between the Earth and the Moon. To be precise, 7.20 m is the distance between the *middle* of the balls. To get the distance between the *outside* of the balls, we need to subtract half the diameter of each, which results in 7.20 - (0.2385 + 0.067) ÷ 2 = 7.04 m.

So, if you put a basketball and a tennis ball **7.04 m** (**7.70 yards**) apart, you have an almost perfect scale model of the Earth-Moon system.

As an additional demonstration, you can also show the difference between the shortest and the longest distance in this way. The *average* distance between the Earth and the Moon is 384400 km, but, since the orbit is not a circle, this distance varies between 363104 km and 405696 km. This takes the distance between the balls from 6.64 m (7.26 yards) to 7.44 m (8.14 yards), a difference of 80 cm (31 in).

*If you try any of this and send me a photo, I might add it to this page…*

## Comments

Your picture does not look right but reading everything makes a lot of sense. I think if you have a photo that shows exactly what you did it would look alot better

I agree that the picture doesn't look right, but that's because it's not a real photo. I've just put a basketball and a tennis ball on top of a photo of grass... I count on you to do this in practice and send me the photo! :-)

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