Copyright 2007 Starry Mirror and Glen Ward
Online Astronomy Newspaper
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THE VENUSIAN MYSTERY MOON - DECEMBER 20
BRIDGEPORT, WV (S-M) - Astronomers today know that Venus has no sizable moons, but in the early days of the telescope, no one could say for sure, and some even claimed to have seen the moon of Venus. The question was not definitively settled until the early 1800's.
Famed French-Italian astronomer Cassini made the first reported sighting of the mystery moon in 1686 while observing from Paris. He wrote "While observing on August 18, 1686 at 4:15 in the morning, looking at Venus with a telescope of 34 feet focal length, I saw at a distance of 3/4 of her diameter, eastward, a luminous appearance, of a shape not well-defined, which seemed to have the same phase with Venus... The diameter was one quarter that of Venus."
The moon remained an open question for half a century. On November 3, 1740, astronomer James Short saw the mysterious moon again with a 16 inch reflector. He reported the object's diameter as about 1/3 that of Venus, and said it showed the same phase as the planet. Short even went so far as to use another telescope to observe the object, to ensure that it was not an abberation in the instruments. This was the one time Short saw the object; he never found it again.
Astronomer Jacques Montaigne seemed to have confirmed the moon's existence in 1761 by observing it four times. On May 3,4,7, and 11 he claimed to have seen a crescent 1/4 the size of Venus about 20 arc-seconds from the planet. Each night, the moon appeared to have moved, as if in orbit around Venus. For a while, the mystery appeared to be solved.
However, large new telescopes were soon to come into use, and famous observers like William Herschel could find no trace of the moon. In addition, Venus passed in front of the Sun in 1761, and there were few reports of any object accompanying the planet. Eventually, belief in the Venusian moon died out.
In the late 1800's, astronomer Paul Stroobant analyzed the data and concluded that many of the observations of a Venusian "moon" were in fact of stars, and that the rest were probably abberations in the observers' telescopes. Venus is one of the brightest objects in the sky, and can easily induce spurious reflections inside a telescope. It took improved telescopes and skepticism on the part of astronomers to disprove the theory of a Venusian satellite. - GW