At the end of every year I stop and reflect on how we define a year. On Earth it takes us a little over three-hundred and sixty-five days to make a complete orbit of the Sun, but our planet is in constant motion, so the decision to mark the year’s end in the middle of winter seemed to me, when I was young, strangely arbitrary. And in fact many cultures celebrate the “new year” at different times—in most cases because the lunar calendar is at least as widely used as the solar calendar.
Because I’m interested in astronomy and, when I look at the night skies, mostly look for the planets, our closest neighbors, I also consider how years are measured just in other parts of our solar system. Mercury spins around the Sun in a matter of Earth months so that sometimes it’s the closest planet to us, while objects at the more distant reaches take centuries just to complete a single orbit.
Something I don’t think I’ve ever thought about until just recently, though, is how fast each planet moves. I’m not talking about rotation, although a “day” in each part of the solar system also varies wildly, with Mercury taking almost as long to turn on its axis as it takes to orbit the sun, Earth and Mars having roughly twenty-four and twenty-five hour days, and Jupiter rotating once every thirteen hours. The speed of individual orbits is something entirely different, and, not surprisingly, the closer planets travel faster, slung around in their ellipses by the sun’s gravity, while the others move more slowly.
Mercury travels around the Sun at 107,082 miles per hour.
Venus travels around the Sun at 78,337 miles per hour.
Earth travels around the Sun at 66,615 miles per hour.
Mars travels around the Sun at 53,853 miles per hour.
Jupiter travels around the Sun at 29,236 miles per hour. Notice the significant drop there, which is not surprising. Mars is approximately 146 million miles from the Sun. Jupiter averages 460 million miles from the Sun—so distant it takes roughly forty-three minutes for light to reach it.
Saturn travels around the Sun at 21,675 miles per hour.
Uranus travels around the Sun at 15,233 miles per hour, which is comparatively slow but still pretty fast for Uranus.
Neptune travels around the Sun at 12,146 miles per hour but is still so far out it takes more than one-hundred and sixty-three of our years to make one orbit.
As the old year slips into the new I can’t stop thinking about all the worlds we know of in constant motion.