There are only three days left to plan any parties you might want to throw in honor of Neptune (that's right, Pluto, we've moved on). As you can read in our current issue, it was discovered 160 years ago this Saturday:

"German astronomer Johann Galle first observed the planet Neptune on [September 23]. That might not sound too remarkable, but it was the first time an object in the solar system had been discovered using the law of gravity. Years earlier, astronomers noticed that Uranus had an irregular orbit and hypothesized that a new planet was causing Uranus to veer off its otherwise expected path. Using predictions from other astronomers regarding where the orbit would hit its "roadblock," and thus where the planet was positioned, British astronomer James Challis beat Galle to the punch, discovering Neptune in August of 1846. The problem? Challis' sky charts were so shoddy that he didn't realize what he was looking at. So when Galle began his search a month later (using a better star chart), he found Neptune within one degree of its predicted position. By the next evening, the object had moved, confirming that it was indeed a planet, not a star.

It was fortunate that both Challis and Galle observed the planet when they did, because within 30 years, Neptune was far away from where the original calculations had placed it. Of course, today we can accurately predict where Neptune will be. Five years from now, on August 10, 2011, the planet will have finally returned to the position at which it was first discovered, having made a complete revolution around the Sun."