Study Session is a review of a test we presented to everyone at a party ages and ages ago. Follow along with the whole test here.

2. How many planets would they count on July 17, 1844? 

A. 5

B. 7

C. 8

D. 9

The number of planets has historically been way more fluid than you'd think, but I'm happy to report that on July 17, 1844 scientists In The Know would steadfastly put the number at SEVEN. The question was kind of a low blow, though, because it isn't like July 16th or July 18th held any astronomical surprises, it's just a date hanging out somewhere in time.

People have always been interested in planets; I know we're all city folk now, but if you ever take a close look at the night sky you'll notice that some bits of light all move together (the stars), while a few others seem to stumble around haphazardly (the planets). The word 'planet' comes from the Greek πλανήτης (planētēs), which means wanderer. BECAUSE THEY WANDER AROUND.

The Babylonians and ancient Greeks counted seven planets; the five "classical planets" that you can see with the naked eye along with the Sun and the Moon. This held sway for a loooong time in the Western world, you can see this reflected in the names of the days of the week. Don't worry, we just bought a book about The History of Time so we'll be teaching a wicked class on weekday names and months and hours and all of that sometime soon.

It was long thought that the planets rolled around in celestial spheres, big ol' rings in the sky made out of crystal, or of a magical fifth element known as quintessence. These spheres were generally thought of to be hard and impermeable, with each planet being isolated from all the others. Religious scholars in the Middle Ages added a place called the empyrean heavens outside of the last sphere, that being the place where God and everyone else hung out.

Their most important contribution to astronomical study are amazing drawings of how these spheres were laid out. Check 'em out.

In 1543 Copernicus published De revolutionibus orbium coelestium, in which he proposed the heliocentric view of the universe, i.e. the Sun's in the middle, not the Earth. Out of fear of religious-inspired reprisal, the book is dedicated to the Pope and contains a nice disclaimer to the effect of "well I mean, don't really think I'm saying the Earth isn't in the center, I'm just saying, you know, that this thing is better at predicting stuff. Not that it's real. Just maths, you know." Funnily enough Copernicus didn't even add it, some random guy did who was tasked with getting the book to the printer.

The heliocentric view of the universe knocked us back down to 5 planets, since the Sun and Moon were out of the running.

We stumbled across Uranus in 1781, the great thing about it being that it was there all along. Totally visible to the naked eye, same as everything else, it was just a little bit dim.

William Herschel, who discovered it, was also given the chance to name it. He attempted to go with "Georgium Sidus," but everyone thought that was pretty dumb and boring and then a Swede went ahead and named it Uranus for him. That's 7!

Sidenote: for a while we went planet-crazy, labeling everything we could see a planet. Didn't matter if it was as big as a baseball, it went around the god damned sun. Tiny rocks, little bits of dust, whatever. This granted us Vesta, Ceres, Juno and a bunch more. We had something like 11 planets rocking our solar system. Eventually that was realized to be a little too free of a definition and we went along with Had To Be Kinda Big.

1846 brought us up to 8 with Neptune, which has the claim to fame of being discovered solely by mathematical calculation. You must read about the battle to name it. Hint: Naming planets after yourself is tacky.

It turned out that math predicted another planet, which was to become known as PLANET X. In 1930 we finally figured out that Pluto was out there, wildly overestimated its size, and we were up to 9 planets.

in 2006 things came to a frightful, battle-ridden astronomy-brawling head. Tons of almost-Pluto-sized objects were being discovered out and about in the far reaches of the Solar System and we needed to do something about it.

And what to do but cast Pluto into an astronmical underworld, no longer a planet, now relegated to dwarf planet status, forever lonely in its officially decidedly-less-sexy role as 134340 Pluto! That's 8 planets, folks, and no number of online petitions can help fix this one.

To those living in Brooklyn: Unlike the rest of Earth, I'm pretty sure we only have 3 or 4 planets and a dozen and a half stars, with half of them realistically being very slow planes.