The phantom moon of Venus, 1645-1768

With the invention of the telescope around 1600 astronomers saw a new world in the sky. They saw mountains on the Moon, moons around Jupiter and Saturn, and a few astronomers believed they saw a moon orbiting Venus. That moon became a problem for astronomers because they only saw it occasionally, separated by many years. The moon was reportedly seen in Italy, France, England, Germany and Denmark between 1645 and 1768. Thereafter it disappeared from the sky. The most obvious explanation was, of course, that the moon never existed. In this paper we detail the observations and how they were assessed. The last reports about this phantom moon of Venus came from the observatory in Copenhagen between 1761 and 1768. In this paper we focus especially on these observations. Observations elsewhere are treated in Kragh (2008). We shall argue that the alleged Venus moon detections were not constructions in the brain, influenced by astronomers' expectations that Venus, like the Earth, Jupiter and Saturn, ought to have a companion. Most astronomers who thought they saw the moon had no preconceived ideas about a Venusian moon. We shall show that from the late 1760s it became generally accepted that the so-called 'moon of Venus' was a ghost image in the telescope, a reflection of Venus in the lens' surfaces.