For most of human existence, we had no idea where Mercury really was.
Sure, we could look up at the sun and say, “thataway,” but beyond that, we were pretty much in the dark, literally. If you, for example, tried to draw a picture of Mercury as it appears in space, you would almost certainly be wrong or impossibly lucky.
The reason this happened for most of the time, was that we didn't really have a good system for determining where Mercury was, or would be, at any point in time. As a result, for quite a long time, we weren't even close; we thought it was going in a big circle around Earth, which was a cute idea.
By degrees, we got a little bit better. Copernicus figured out that Mercury wasn't really revolving around us, but instead the Sun. Newton learned (part of!) the system that governs how these planets move and for a short while, we kicked so much ass at astronomy.
"How much ass was trashed, Zala?” you ask. I would love to tell you. In 1781, we discovered Uranus telescopically. That is, we looked at it and said, “Oh. There's a planet there.” Pretty much the same thing happened with most of the other planets. About 40 years after we discovered Uranus, we noticed that it had an awkward kind of wiggle to it. Like Bill Cosby's jowls when he's talking about pudding. It was never “lost” but also never exactly where it should be. This is where the story gets cool (but only if you like math.)
In 1845, a very French person with the very French name of Urbain LeVerrier stood up and said, “Step aside. I'mma gonna math it up in this bitch.” (Translated directly from French.) He looked at a couple numbers...a couple hundred numbers...and hypothesized that there should be a planet beyond Uranus that was pulling on it gravitationally, making it wiggle. And, I hate to ruin the surprise for everyone, but he was right. Using proper observations, math, and good ole' fashioned French elbow grease, LeVerrier predicted the orbit and size of Neptune without ever observing it directly. (Chances are very good he was a virgin.)
I wish that my math skill was adroit enough to explain exactly how hard it was to find Neptune by seeing how much it pulled on Uranus, or even how small the pull was, but it's not. I can, however, explain it in simple terms and let you decide for yourself how hard it was: the sun is 19,764 times more massive than Neptune, yet LeVerrier found Neptune's pull on Uranus. That is how much we kicked ass in the 19th century.
Then, the 20th century came and things got screwy. Already there was a small problem with the orbit of Mercury. You should, if your elementary school was worth a damn, know that the orbits of the planets are not circular, they're elliptical, meaning sometimes they are closer to the sun than at other times. (Note: This is not what causes the seasons.) The earth, for example, is 147 million kilometers away from the sun sometimes, and moves back to the 152 million kilometer point ever year. With Mercury, the problem was not the distance from the sun, but the position in its orbit when it was the closest was inconsistent with Newton's laws by a small amount.
(In case you didn't understand that last sentence, imagine Mercury as the minute hand on a clock. It should be at its closest at 12:00, but instead it was closest at 12:05, to oversimplify and exaggerate the situation.)
When I say that this was a small problem, I mean it was a small problem The discrepancy between where Mercury should be at its closest point and where it was at its closest point was off by 1 degree...8,000 years in the future. In other words, every year, Mercury's point where it was closest to the sun was off by 0.00011944 of a degree. It is something that like this that you think would make an astronomer shrug and say, “Enh. Close enough.”
But not LeVerrier, oh no. Fresh off his stint of saying, “Dude, remember the time I predicted Neptune. Yeah. I totally called that shit,” for many, many years, he stood up, announced that he would once again math it up, and predicted that another planet existed between Mercury and the Sun. He even went so far as to name it Vulcan. The whole thing would have been awesome if it weren't total crap. Enter Einstein.
Einstein comes onto the scene and basically says, “Gravity: Ur doin it wrong.” And then he rofled.
I won't explain, at this time, how Einstein changed the way we think about gravity, but it's enough for now to say that he did. The rules completely changed, and in the new set of rules, Mercury was doing exactly what it should. So what is the lesson that we take away from LeVerrier's story? He encountered, pretty much, the exact same problem, twice, and approached it the exact same way, twice, but he was only right once. As Homer Simpson would say, “The lesson is, never try.”
Not quite. And this shows the two different ways that new science introduced. With Neptune, LeVerrier used existing theories to make predictions about observations. With Mercury, Einstein used correct observations to make new theories.
More on this in Part Two