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Earth-moon distance and the diameters of the planets

June 15, 2014

A few days ago Mike sent me this:

Earth-moon distance and planetary diameters

I was surprised to see so many people calling BS on this–it’s simple enough to double-check. So I did. Here are the results.

Mean radii in km, from Wikipedia:

  • Mercury – 2400
  • Venus – 6100
  • Mars – 3400
  • Jupiter – 69,900
  • Saturn – 58,200
  • Uranus – 25,400
  • Neptune – 24,600
  • Total – 190,000

Doubled, to convert to diameters – 380,000 km

Average Earth-moon distance, also from Wikipedia: 384,000 km.

Yep, this checks out. With the proviso that the Earth-moon distance actually varies from 363,000 to 405,000 km, so sometimes you’d have to leave out Mars and Venus, and other times you’d have to clone them to fill the extra space.

If you want a remarkable coincidence, the moon formed maybe only 10,000 miles from Earth and has been gradually receding ever since. So we are living in the tiny slice of Earth history when the moon is at just the right distance to appear the same relative size as the sun, and thus produce total eclipses as we know them. Annular eclipses have only been around for a few tens of millions of years, and in another few tens of millions of years, they’re all we’ll ever get, because the moon will be too distant to completely block the sun.

Anyway, after I sent Mike this reply, he said, “That is a whole lot of awesome, which clearly ought to be a 10MA post”. And now it is.

UPDATE October 13, 2014: A much more detailed explanation of the end of total solar eclipses in the distant future can be found on this page, under the “Final Totality” heading.

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