alternatehistory.com

Take a look at this...quite interesting.
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In the winter of 1612-1613, Jupiter began to align with Neptune from Earth’s point of view. The alignment was so complete that on January 4, 1613, Jupiter’s disk actually blocked (occulted) Neptune’s. Galileo, having discovered four moons around Jupiter in January 1610, was still observing Jupiter three years later. He made careful drawings of Jupiter, its moons, and any background stars in his telescope’s field of view. Upon comparing the background stars in Galileo’s drawings to the positions Neptune would have had that winter, astronomers have concluded that Galileo drew Neptune as a background ‘star’ in drawings he made on December 28, 1612, and on January 27 and 28, 1613.

Galileo’s simple telescope was not powerful enough to resolve Neptune into a disk. (You need a telescope at least 10-12 inches in diameter to do this). In order to recognize it as a planet, Galileo would have needed to see Neptune change position against background stars. Since it orbits about 30 times as far from the Sun as Earth does, Neptune takes 146 years to go around the Sun once. As a result, its motion against the background stars is harder to notice. Once a year, Earth comes around to Neptune’s side of the Sun. This makes Neptune seem to slow down, stop, and reverse direction against the background stars. (This is called ‘retrograde’ motion.) As it turns out, in December 1612, Earth was just coming around to Neptune’s side of the Sun, and Neptune was virtually stationary and about to begin retrograde motion. Neptune’s motion against the background stars would have been all but unobservable in December 1612.

By January 1613, however, Neptune was in full retrograde motion. On January 27 and 28, Galileo did notice that one of his background stars had slightly changed position compared to another. According to University of Melbourne physicist David Jamieson, this indicates that Galileo knew he had found a new planet. However, we see no sign that he attempted a second observation of that mysterious star, or that he reported the finding of a new planet. Thus Galileo, first to see Neptune, does not get credit for discovering it.
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Suppose that Galileo does some more observations of this. I can't remember if he predates Kepler, but he knows immediately that he's discovered another planet. He calls it Medicus and publishes it in 1616 after three more years of observations. When people look at him in disbelief, he just has them go over to his telescope and tells them to look for themselves.

Assume he does not know enough to compute its orbit accurately, and assume that it is too faint for him to see that it's blue.

Continue from there.
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