Newton’s laws enabled the scientists to understand and decode the movement of objects in the solar system. It worked every time, everywhere. Everywhere except Mercury. Something strange was going on with our solar system’s innermost planet.

It turns out that every time the Mercury orbits around the Sun, its ellipse precesses ever so slightly. So far so good, no problem there.  Albeit, when scientists tried to calculate how quickly this should precess and then they observed how fast this procession was, the numbers did not match. There was a discrepancy of 43 seconds of arc per century (one second of arc=1/3600 degrees). This is small, very small. Nevertheless, it is still a discrepancy.

Newton’s law could not explain this discrepancy and nobody knew what was cause it. Until 1859. Urbain-Jean  Joseph Le Verrier, the man who discovered Neptune and sought out opportunities to extend his knowledge, thought he had found the solution to the Mercury’s anomaly.

“….a planet, or if one prefers a group of smaller planets circling in the vicinity of Mercury’s orbit, would be capable of producing the anomalous perturbation felt by the latter planet… According to this hypothesis, the mass sought should exist inside the orbit of Mercury.”

A planet, or a belt or asteroids inside the orbit of Mercury. It sounded exciting.

And indeed, in March 1859, Edmond Modeste Lescarbault, a French doctor and an amateur astronomer, discovered an object transiting the Sun in a position that was coincided with Le Verrier’s theory of an intramercurial planet. A planet fever hit the scientific community and the popular press. By no later than February 1860, the solar system’s newest planet had a name.


Awesome! Vulcan discovery was celebrated around the world, Le Verrier was a true scientific hero, and the scientific community thought had solved Mercury’s discrepancy anomaly. Alas, not for long. You see, there were doubters. Professional and amateur astronomers who watched the sky, at the same time, saw nothing. No object was crossing the Sun.

So, Vulcan dwindled into a mostly forgotten embarrassment and the mystery of the movement of Mercury remained until a young man in Switzerland started to think about something else entirely. His name was Albert Einstein and he was thinking about how fast gravity travels from point A to point B, from Sun, say, to Earth.

In 1916, Einstein published his general theory of relativity. Gravity is not a force like other forces, says Einstein, but a consequence of the fact that space-time is not flat. Space-time is curved, or “warped” by the distribution of mass and energy in it. Matter-energy together tell space-time what shape to be and space-time tells matter how to move. The sun with its great mass creates a dent in space time and Mercury, “so firmly embraced by our star’s gravitational field, it lies deep within that solar gravity well.” Einstein “captured in all the abstract majesty of his mathematics” that the orbit of Mercury follows that four dimensional curve, away from the Newtonian theory of gravity.

It was the end of Vulcan illusion.

In The Hunt for Vulcan, Thomas Levenson, a professor at the Massachusetts Institute of Technology, tells us a lot more than the forgotten story of Vulcan.  He has written an engaging, fascinating and fast-paced book, a delightful story for anyone interested in science and/or history of science. .

Image Credit: Wikipedia