According to The Bad Astronomer, it's Neptune's first birthday of discovery.
Wait... what does that mean?
It means that it's been 164.8 years (one Neptune orbit) since it was discovered on September 23, 1846.
Mostly. There's a lot of complications. Neptune doesn't orbit the sun; like everything else in the solar system, it orbits the barycenter - the center of mass. Due to Jupiter and Saturn, the barycenter isn't exactly in the middle of the sun.
There's also other stuff. Like, we aren't sure the exact hour it was discovered. Its orbit isn't quite regular due to the gravitational tugs of Uranus and Pluto and Jupiter. And there's competing coordinate systems to choose from.
Also, Galileo. He spotted Neptune in 1612 (the greatest pre-discovery yet known) but thought it was a star because his telescope had limited resolving power.
But today, or thereabouts, is one Neptunian year since its official discovery. I'm going to celebrate by spotting it tonight.
Launch Report 2017-2 - LDRS-36
2 weeks ago