As some folk may recall from their grade school science classes, irregularities in the orbit of Uranus led to the discovery of Neptune. Again, according to those long ago science classes, Neptune was considered to not quite explain the irregularities in the orbit of Uranus and, further, seemed to have irregularities of its own which led, after nearly a century, to the discovery of Pluto.
Well, that tale is not entirely accurate.
Meanwhile, there were irregularities in the orbit of Mercury. Specifically, the “perihelion precession” did not match that predicted by Newton’s Law of Universal Gravitation and his three laws of motion.
To understand what this means, planets orbit around the sun* very close to elipses. (There are complications which we need not worry about here.) The nearest approach is the perihelion. The farthest point is the aphelion. You can draw a line from the perihelion, through the Sun to the aphelion. However, what actually happens is that every orbit this line shifts slightly as in the following picture (both the shift and the eccentricity of the orbit are greatly exaggerated for clarity):
The shift in perihelion from orbit to orbit wasn’t quite what Newton’s laws said it should be. The difference was extremely small–43 arcseconds (0.0119 degrees) per century–but it was enough to show that Mercury’s orbit did not match what Newton’s Laws said it should. Something was going on.
The presumption soon was made that a previously undiscovered planet, inside that of the orbit of Mercury, was affecting the orbit of Mercury and could explain the discrepancy. This hypothetical planet was named “Vulcan” after the Roman blacksmith of the gods. On March 29, 1959, Edmond Modeste Lescarbault described in a letter to French Mathematician Urbain Le Verrier seeing in transit across the sun an object that could be the new planet. Le verrier became convinced that Lescarbault saw the supposed and previously undiscovered planet. On January 2, 1860, he announced the discovery of the planet Vulcan.
There were, of course, doubts about the reality of Lescarbault’s observations. Other’s, however, reported their own observations of this supposed planet. And while controversy remained, there certainly had to be something out there as, after all, Mercury’s orbit continued to fail to match what Newton’s Laws predicted for it and Newton’s Laws were far and away the most thoroughly tested and validated laws in all of science. Had they not successfully led to the discovery of Neptune (1846, 14 years before the “discovery” of Vulcan)? It was “settled science.”
It was not until 1915 when a Swiss Patent Clerk (oh, you know where this is going, don’t you?) expanded on his previous work in Special Relativity which resolved the observation that the speed of light is constant no matter what the relative motion of the source and the observer and the effects that had on time and distance. This new General Relativity led to a new theory of gravity related to a curvature in space-time. And when the perihelion precession of Mercury was calculated in this new theory it was found to match within the limits of measurement precision (and remains so to this day).
The lessons from this tale are many, but I’d like to point out two:
- The science is never settled.
- Expectations can drive observations. Often we see what we expect to see, which may, or may not, be what is.