Thousands of years ago, our ancestors noticed that most stars were located in fixed patterns. Those patterns moved slowly east to west during the night and were visible in particular seasons but did not change. Those ancestors named the patterns, now referred to as "constellations", after mythical heroes, animals and other objects. Our ancestors also noticed there was a belt of sky, containing the constellations Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus and Pisces, where five "stars" wandered to and fro. Those were called "wandering stars,” or "planets”—Mercury, Venus, Mars, Jupiter and Saturn. When telescopes became important tools in astronomy, another planet, Uranus, was discovered. When Isaac Newton defined the force now referred to as "gravity", where all bodies in the universe attract one another, he enabled us to calculate how planets move, and in doing so gave us a tool for discovering new ones. Imagine someone dancing with an invisible partner. By watching how the visible partner is moving, it is easy to realize there is an invisible partner, and maybe also estimate how big they are. To use this idea to deduce the presence of unknown planets, we measure precisely how a planet moves around the Sun and determine whether that orbit is being perturbed in any way. If so, what size body could account for those perturbations and where it should be located? Then we look for it. Perturbations of Uranus' orbit led us to the discovery of Neptune. During the last half of the 19th century, observers found there was something really odd about the orbit of Mercury, the closest planet to the Sun. After the perturbations due to known planets, especially Jupiter, were taken out, there was still an unexplained perturbation too big to be ignored. Astronomers, such as Urbain Le Verrier in France, assumed them to be caused by a new, unknown planet. The calculations predicted it had to be orbiting the Sun at a distance of 21 million kilometres and taking only 19 days to do so. Mercury orbits the Sun in a very elliptical path, taking it from 46 million kilometres to 70 million kilometres, and it takes 88 days to complete each orbit. The evidence for the existence of this planet was deemed so strong it was given a name, Vulcan, after the god of fire, with responsibility for volcanoes and such things. The search was on. There were several reports of astronomers seeing Vulcan's black disc crossing the Sun. However, none of the observations could be confirmed. The planet obstinately refused to be conclusively found, no matter how intense the search was and how powerful the telescopes used. Finally, it turned out the explanation of Mercury's weird behaviour was not a planet, it was something much stranger. The scientist whose work was relevant to this particular problem was not Isaac Newton but was Albert Einstein. In his General Theory of Relativity, Einstein proposed that what we perceive as the force of gravity is really one manifestation of the distortion of spacetime by massive objects. Another is a distortion of the passage of time. As we approach a massive object that distortion increases faster and faster. As Mercury's highly elliptical orbit takes it between 46 to 70 million kilometres from the Sun, the distortion of spacetime varies by more than a factor of two. That accounted for those additional perturbations of Mercury's orbit. That strongly supported Einstein's theory. It also removed the evidence for the existence of Vulcan, making it the “planet that never was". • Venus and Saturn lie close together low in the southeast just before dawn, with Mercury much lower in the glow. • Jupiter is low in the west after sunset, with Mars higher, in the southwest. • The Moon will be full on May 12. This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.