In the early 17th century, Johannes Kepler discovered that the paths of the planets around the Sun are ellipses and not the circles beloved of the Greeks. In the late 17th century, Isaac Newton explained this. An ellipse is the path of a body experiencing a force of gravity that weakens according to an inverse square law—one that is four times as weak at twice the distance, nine times as weak at three times the distance, and so on.
Newton’s law of gravity is “universal”, which means that every mass pulls on every other mass. Consequently, each planet is tugged not only by the gravity of the Sun but by the gravity of every other planet. This causes a planet’s elliptical path to gradually change its orientation in space, or “precess”. In the 19th century, however, astronomers discovered something odd about Mercury, the closest planet to the Sun. Even after its dance to the tune of the other planets had been taken into account, there was a tiny bit of precession left unaccounted for.
To explain the “anomalous procession of the perihelion of Mercury”, the French astronomer Urbain Le Verrier predicted the existence of a blisteringly hot world closer to the Sun that was tugging on Mercury. For 50 years, there were intermittent claims to have seen “Vulcan”. But no such planet existed (though the name Vulcan survives today as the home world of Star Trek’s Mr Spock). On 18th November 1915, however, Albert Einstein gave a lecture to the Prussian Academy of Sciences in Berlin in which he explained the anomalous motion of Mercury with a revolutionary new theory of gravity.