Fifty years ago a ship in the Indian Ocean reported strange balls of coloured light hovering low in the night sky. The SS Strathnaver was sailing from Australia to London when the second officer, Mr Vint, looked out to the horizon with binoculars and saw an orange globe. This changed into two red spheres, and finally metamorphosed into a green body that eventually sank below the horizon and disappeared.

It might sound like a scene from Close Encounters of the Third Kind, but it was the setting of Venus and Jupiter close together on the horizon. They changed shapes and colours thanks to a trick of the atmosphere.

Celestial bodies close to the horizon shine through the greatest thickness of atmosphere seen from our viewpoint. This can distort the light like a prism, teasing it out into the colours of the rainbow – orange, red and green, almost like traffic lights. The other colours are usually lost or hidden in the atmosphere.

In fact, Venus can be quite spooky on its own. In 1969 Governor Jimmy Carter, before he became president, saw a n apparent UFO outside a restaurant after sunset. He described the object as bright and changing colour from blue to red, then back to white. “We were trying to figure out what in the world it could be, and then it receded into the distance,” he recalled. But it was Venus.