Astronomers, being human, can misunderstand what they see. For example, there was a time when experienced astronomers thought that Venus had a moon. This and some other ideas weren't foolish notions, but they faded away in the light of better technology or new understanding. Here are some of these astronomical phantoms.
A prominent French mathematician named Urbain Le Verrier (1811-1877) was convinced that there was a planet nearer the Sun than Mercury. He named it Vulcan after the Roman god of fire. But since no one had seen it, why did Le Verrier think Vulcan existed?
Years earlier it had been suggested that a new planet might be causing observed deviations in Uranus's orbit. Since Le Verrier's calculations led to the discovery of Neptune, he was ready to apply this to the problem of an anomaly in Mercury's orbit.
In the years after Le Verrier tried to calculate Vulcan's orbit, there were reported sightings of planets transiting the Sun, or seen during a total solar eclipse. However the data didn't produce a convincing orbit for Vulcan, nor did a worldwide search reveal it.
Today we have satellites that monitor the Sun and they haven't spotted Vulcan either. But we wouldn't expect to see such a planet, because although Mercury's orbit doesn't behave as predicted by Newtonian physics, it does behave according to Einstein's theory of relativity. Einstein's theory incorporates the effect of the Sun's gravity gently curving the space around it.
Briefly in 1974, NASA thought Mariner 10 had discovered a moon of Mercury when ultraviolet radiation was detected near Mercury. The source had to be local, because the gas and dust between the stars absorbed ultraviolet radiation. It was also moving at about the right speed for a moon. Eventually they identified it as the star 31 Crateris. Although losing out on finding a moon, it was useful to learn that not all ultraviolet radiation from distant sources was absorbed before reaching the Solar System.
As an April Fool joke in 2011, the NASA website's Featured Image was supposedly of a moon of Mercury newly discovered by the MESSENGER spacecraft.
Neith, moon of Venus
In the 18th century a number of astronomers started reporting seeing a moon of Venus. Yet despite searching, others failed to find it. Eminent Viennese astronomer Maximilian Hell experimented, viewing with his eye at different angles to the telescope. He found that at certain angles there was a small secondary image that was a reflection off the observer's own eye.
The secondary reflection didn't account for all of the sightings, but the idea died down after several notable astronomers searched for the moon and couldn't find it. Nonetheless it was revived a century later by Belgian astronomer Jean-Charles Hozeau, who said there was a second planet which was sometimes in conjunction with Venus. He named it Neith after an Egyptian goddess. The Belgian Academy of Sciences investigated all of the reported sightings - most of them were stars.
Earth's other moon
In the mid-nineteenth century French astronomer Frédéric Petit announced that he'd discovered a second moon of Earth. Other astronomers found his description of the orbit unconvincing, even laughable. Fifteen years later he produced a modified paper that was also not taken seriously. In fact, the only reason for remembering it is its central role in Jules Verne's novel Round the Moon.
Yet many claims followed Petit's, and in the 1950s Clyde Tombaugh, discoverer of Pluto, was asked to investigate. His four-year search found nothing. It is possible for small asteroids to be temporarily in orbit around the Earth, but so far no long-term satellites have been found.
The asteroid Ceres was discovered by Giuseppe Piazzi in 1801 and in the following year, Heinrich Olbers discovered Pallas. Both objects were listed as planets. Olbers predicted that there would be more of them, because he thought they were fragments of a larger planet that had been destroyed.
The idea was credible into the twentieth century, and the hypothetical planet was named Phaeton after the son of Helios the Greek Sun god. Zeus had to destroy him to protect Earth when the boy tried to drive the Sun chariot, and it went out of control.
But it now seems that no planet ever existed in the asteroid belt because Jupiter's gravity would have disrupted planet formation. In any case, the total mass of the belt is only 4% that of the Moon, and the asteroids are too chemically varied to have come from the same body.
I've mentioned that Neptune was discovered through its gravitational influence on Uranus. But after a time it seemed that there were deviations in Neptune's orbit and Uranus was still not on course. Perhaps a Planet X beyond Neptune was influencing them?
Percival Lowell thought so. His scientific credibility had evaporated after his insistence that there were canals on Mars, including vivid detail about the civilization that built them. He was therefore keen to find Planet X. Lowell never did discover it, nor did anyone else. But there was a good outcome. While Clyde Tombaugh was searching for Planet X at Lowell Observatory in Arizona, he discovered Pluto.
Since 1992 theory has matched observation. That's when the orbits of Uranus and Neptune were calculated using a more accurate value for Neptune's mass. It was the value provided by Voyager 2's flyby of the planet. The problem with the orbits was just one of measurement.