the Moon of Venus, 1672-1892

In 1672, Giovanni Domenico Cassini, one of the prominent astronomers of the time, noticed a small companion close to Venus. Did Venus have a satellite? Cassini decided not to announce his observation, but when he saw it again 14 years later, he entered the observation in his journal. The object was estimated to have about one-quarter the diameter of Venus, and it showed the same phase as Venus.
The object was later seen by other astronomers: James Short in 1740, Andreas Mayer in 1759, and Joseph Louis Lagrange in 1761. (Lagrange announced that the orbital plane of the satellite was perpendicular to the ecliptic.) During 1761, the object was seen a total of 18 times by five observers. The observations of Scheuten on June 6, 1761 was especially interesting. He saw Venus in transit across the Sun's disk, accompanied by a smaller dark spot on one side that followed Venus in its transit. However, Samuel Dunn at Chelsea, England, who also watched that transit, did not see the additional spot. In 1764, there were 8 observations by two observers. Other observers failed to find the satellite.
Now the astronomical world was faced with a controversy. Several observers had reported seeing the satellite while several others had failed to find it in spite of determined efforts. In 1766, the director of the Vienna observatory, Father Hell, published a treatise in which he declared that all observations of the satellite were optical illusions. He believed the image of Venus is so bright that it is reflected in the eye, back into the telescope, creating a secondary image at a smaller scale.

Others published treatises declaring that the observations were real. J. H. Lambert of Germany published orbital elements of the satellite in Berliner Astronomischer Jarhbuch 1777:

• mean distance, 66.5 Venus radii;
• orbital period, 11 days, 3 hours; and,
• inclination to ecliptic, 64°.

It was hoped that the satellite could be seen during the transit of Venus in front of the Sun on June 1, 1777. In retrospect, it is clear that Lambert made a mistake in calculating these orbital elements. At 66.5 Venus radii, the distance from Venus is about the same as ourMoon's distance from the Earth. This does not fit with the orbital period of 11 days, which is about one-third of the orbital period of our Moon. (The mass of Venus is slightly smaller than the mass of the Earth.)In 1768 , Christian Horrebow made one more observation of the satellite from Copenhagen. There were also three searches, including one made by one of the greatest astronomers of all time, William Herschel. All three of them failed to find any satellite. Quite late in the game, F. Schorr from Germany tried to make a case for the satellite in a book published in 1875.
In 1884, M. Hozeau, former director of the Royal Observatory of Brussels, suggested a different hypothesis. By analysing available observations, Hozeau concluded that the moon appeared close to Venus approximately every 2.96 years. Hozeau suggested that this was a separate planet, with a 283-day orbit around the Sun that placed it in conjunction with Venus every 1,080 days. Hozeau also named the new planet Neith, after the mysterious goddess of Sais, whose veil no mortal raised.
In 1887, three years after Hozeau had revived interest in the subject, the Belgian Academy of Sciences published a long paper in which each and every reported observation was investigated in detail. Several observations of the satellite were really stars seen in the vicinity of Venus. Roedkier's observations "checked out" especially well -- he had been fooled, in succession, by Chi Orionis, M Tauri, 71 Orionis, and Nu Geminorum. James Short had really seen a star somewhat fainter than 8th magnitude. All observations by Le Verrier and Montaigne could be similarly explained. Lambert's orbital calculations were demolished. The very last observation, by Horrebow in 1768, could be ascribed to Theta Librae.
After this paper was published, only one more observation was reported, by a man who had earlier made a search for the satellite of Venus but failed to find it. On August 13, 1892, Edward Emerson Barnard recorded a 7th magnitude object near Venus. There is no star in the position recorded by Barnard, and Barnard's eyesight was notoriously excellent. We still don't know what he saw. Was it an asteroid that had not been charted? Or was it a short-lived nova that nobody else happened to see?