Today is special: it is now one full Neptunian year since this giant planet was discovered in 1846!

So today is Neptune's birthday! Um. Well, kinda. Yeah, as usual, stuff like this gets complicated. I realized this anniversary was coming up about a year ago, and contacted an old friend about it: Kelly Beatty, editor at Sky and Telescope magazine, who then contacted astronomers John Westfall and Roger Sinnott. We had some fun email exchanges about all this! I think I have a good grip on this now, so let me explain.

The short form First, to celebrate a birthday, you need the birthdate. That's the first complication. Neptune was discovered on September 23, 1846 by astronomer Johann Galle using star charts by Johann Encke, and they are generally given credit for it. However, that date of September 23 is a bit dicey! Galle and Encke report that they found Neptune on 9/23 at 12:00:15 "Berlin M.T.", according to Westfall. But they reckoned the day starting at noon! And since they're using Berlin mean time, you have to account for the longitude of Berlin with respect to 0° longitude on Earth. According to Westfall, once you do all that, you get a discovery time of September 23 at 23:06:40. Worse yet, there may be some imprecision in the exact time the astronomers reported the discovery, although Galle reported the time to a fraction of a second

. Westfall reports that might be as much as 1.2 hours, preferring a discovery time of September 24, 1846 at 00:15 GMT. Who's right? Turns out, it doesn't matter much, since we only need to know the time to within a few hours to get the right date for the birthday. Still, Westfall appears to have looked at this pretty hard, so you know what? Good enough. I'll use his numbers.

A year by any other name would take as long OK, so we have the birthdate. Now, how long is a Neptune year? Yeah, well, that turns out not to be so easy to answer either! There are lots of ways to measure a year

. And worse, Neptune's year isn't constant; the gravity of Uranus tugs on Neptune, accelerating it, changing its period around the Sun. The effect is small, but measurable, and in fact it was Neptune's effect on Uranus that allowed astronomers to find it in the first place! So the time it takes Neptune to circle the Sun once changes over time. Arg. But there's a way to cut through that: instead of trying to figure out Neptune's exact period and adding it to the discovery date, we can ask when Neptune returns to the same position in the sky where it was when it was discovered, call that one Neptunian year, and be done with it. Doing that based on the Neptune's position as seen from Earth is complicated (of course) and is biased. After all Neptune is orbiting the Sun, not Earth.

In fact, it's not that simple (stop me if you've heard that before). Neptune actually orbits the solar system's barycenter, its center of mass. You might think that would be the center of the Sun, but Jupiter is big enough to pull on the Sun a bit, making the entire solar system off-center (think of it like an adult and child holding hands and spinning around; the mass of the kid pulls the grownup a bit off-center as they circle each other -- or just look at the animation here and let yourself get dizzy). The other planets contribute as well. This makes things a lot harder to figure out, and this is getting ridiculous as it is.

[Note added after I finished this article but before it got posted: Tammy Plotner at Universe Today does in fact go into the barycenter argument, and correctly concludes that yesterday was the barycentric Neptunian birthday. It comes down to a matter of preference, I think.]

So instead, let's simplify (yay!), pick a coordinate system based on the center of the Sun, get the coordinates of Neptune when it was discovered, and then figure out when it returns to those same coordinates. We can use Neptune's heliocentric longitude to do this.

Heliocentric longitude and latitude are like their counterparts on Earth, except measured from the center of the Sun. And instead of using Earth's equator as we do for long and lat on Earth, for the heliocentric coordinates we use the Earth's orbit around the Sun! That defines a plane on the sky just like the Earth's equator does on the Earth's surface. And just like 0° longitude on Earth is arbitrary (it passes through Greenwich, England, where the coordinate system was defined), heliocentric longitude has its zero point as the position on the sky of Earth's vernal equinox, where the Earth's orbit intersects the projection of the Earth's equator on the sky. Yeah, I know. This makes my head hurt sometimes too. I think what's most frightening about all this is that I understand it. Still, the diagram here (click to embiggen) shows the layout. All you really need to know is that the yellow line there points to 0° heliocentric longitude in the sky. Think of it as a benchmark. Halfway around the sky is 180° longitude, and so on. So imagine you were at the center of the Sun (wearing sunblock 10 ^12 ) in 1846 and looking at Neptune at the moment humans on Earth discovered it. You ask yourself, "How far east is Neptune at this moment, as measured from 0°?" The number you get, if you're using Westfall's time and date of discovery, is 329° 05’ 51.5”. OK, cool. Then you wait about 165 years. At some point, Neptune will have that exact same longitude again. When is that date? Drum roll please...

July 12, 2011, at 18:38 GMT

Aha! So why not use that? If you live in the US, at 14:38 Eastern time, Neptune will have completed one circuit around the Sun since it was discovered, according to its own calendar. Phew! And as you can see, if we're off by a couple of hours either way, the date stays the same for us here in the States. I'm good with that. Oh -- by the way, we can use these numbers to then ask how long Neptune's year is. The amount of time between these two dates is 60,191.8 days, or 164.8 Earth years. On Neptune, I'd only be a little over 3 months old.

This too shall pass I almost hesitate to mention that as seen from Earth, Neptune will pass this same spot in the sky not once but five times! That's because the Earth is moving around the Sun as well, and our perspective changes. Explaining it here will make both our heads explode, so I'll just link to a video of Brian Cox talking about it

. He does a great job. But if you're keeping track at home, according to Westfall Neptune has already passed this position in the sky as seen from Earth three times (in April and July 2010, and in February in 2011) and will again twice more this year (in October and November). But at this point I'm done. July 12 is good enough for me.

Bismillach, NO! So we can finally, safely, say "Happy Birthday, Neptune!" today. Right? Right? Well... I'd hate to point it out, but it's been shown that Galileo actually saw Neptune on December 28, 1612, and thought it was a star. He observed it again a month later. Had he noticed that particular "star" had moved, we'd be doing even more math here. Too bad, though. If Galileo had been able to figure out what he saw, why, he'd have been famous! But I think we can ignore that, since Galileo didn't know he had seen it. Galle gets the credit, so his date is the one we'll use. Yay! So here we are, one full Neptunian revolution after it was first found. It's kind of a fun thing to think about today; how even something as trying to figure out the orbit of a major planet can be such a pain in the butt. You can try to observe Neptune itself, too

, if it's clear where you are and you have binoculars. And if you're wondering how I'm going to celebrate, why, I'm going to sit back and eat a Krabby Patty

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[embed width="610"]http://www.youtube.com/watch?v=61_jG2aa7GY[/embed]

Credits: Neptune: NASA; barycenter animation and ecliptic diagram: Wikipedia.

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