Sunday, July 10, 2011

Happy Anniversary, Neptune!

According to the calculations presented in this blog post, Neptune will complete its first orbit around the Sun since it was discovered in 1846 on Monday July 11 at about 1:50 PM in the afternoon Pacific Daylight Time (21:48 UTC, plus or minus 15 minutes). There are actually several dates floating around, the most common being July 12. However, that date is calculated based on Neptune's position around the Sun, while the July 11 date is calculated based on Neptune's position relative to the Solar System's center of mass, or barycenter. Normally the barycenter can be approximated as being within the Sun, but when you're calculating orbits that range over dozens of decades you need to take into account the fact that it actually is periodically outside the Sun, which was surprising  for me. The blog post I linked to has a nifty picture that shows how the barycenter moved relative to the Sun over the last half-century or so. The post itself is a bit long, but not too technical, and the author does an excellent job of explaining his decisions and calculations which is why I'm going with his date for the event. In any case, even if it turns out to be incorrect, what's a day or two off in a orbit lasting 164.79 Earth-years?

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