Coincidences, statisticians tell us, only seem strange because we’re bad at estimating all the possibilities. With so many objects in the solar system (let alone the universe), they’re inevitable.

For example, 18th century astronomers noted a pattern in the planets’ orbits: In terms of Earth’s distance from the sun (an “Astronomical Unit”), each planet obeys this rule: distance from sun = 0.4 + x/10 where x is 0, 3, 6, 12, 24, etc. (each number, except for 3, is double the previous).

Mars, the fourth planet, should lie 0.4 + 12/10 = 1.6 AU from the sun—pretty close to its actual distance of 1.52 AU. Not only did this sequence work well (except for Neptune), it also suggested an unknown planet in the gap between Mars and Jupiter — exactly where the first asteroid, Ceres, was discovered.

Here’s another coincidence: Despite being 39% wider than Mercury, and almost twice as massive, Mars has almost identical surface gravity: A person who weighed 68.2 pounds on Mars would weigh only 3 ounces less on Mercury.

Mars’s day is only 40 minutes longer than Earth’s, and the tilt of its axis is within two degrees of both Earth’s and Saturn’s.

On any given day, Venus will lie in the same place in the sky as it did on that same day eight years prior, because, coincidentally, it completes almost 13 orbits for every eight of earth’s orbits.

My favorite cosmic coincidence? There are almost as many AUs in a light year as inches in a mile (63,241 vs. 63,360, respectively, a mere 0.2% difference).

Put another way: In a scale model of the solar system in which the earth and sun are one inch apart, the nearest star (Proxima Centauri) will be 4.2 miles away.

Next column: A mighty hero with a wimpy constellation.

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Chris Anderson manages the College of Southern Idaho’s Centennial Observatory in Twin Falls. He can be reached at 732-6663 or canderson@csi.edu.


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