Answer to Question 1
Galileo was the first person to observe the sky carefully through telescopes and apply his observations to the main theoretical problem of the day-the place of Earth. What Galileo saw through his telescopes was so amazing that he rushed a small book into print,Sidereus Nuncius(The Starry Messenger). In that book, he reported two major discoveries about the Solar System. First, the Moon was not perfect. It had mountains and valleys on its surface, and Galileo used the shadows to calculate the height of the mountains. Aristotle's philosophy held that the Moon was perfect, but Galileo showed that it was not only imperfect but was even a world like Earth. Second, Galileo's telescope revealed four new planets circling Jupiter, planets that we know today as the Galilean moons of Jupiter.
Answer to Question 2
In Copernicus's model, Earth moves faster along its orbit than the planets that lie farther from the Sun. Consequently, Earth periodically overtakes and passes these planets. Imagine that you are a runner on a track moving along an inside lane. Runners well ahead of you appear to be moving forward relative to background scenery. As you overtake and pass slower runners in outside lanes, they fall behind, seeming to move backward for a few moments relative to the scenery. The same thing happens as Earth passes a planet such as Mars. Although Mars moves steadily along its orbit, as seen from Earth it seems to slow to a stop and move westward (retrograde) relative to the background stars as Earth passes it. Because the planets' orbits do not lie in precisely the same plane, a planet does not resume its eastward motion in precisely the same path it followed earlier. Instead, it describes a loop with a shape depending on the angle between the two orbital planes.
The Starry Night
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