Sep 27 - The motion of the planets. Flashcards
Mercury’s strange tendencies
visible infrequently, and only just after sunset or just before sunrise because it is so close to the Sun
Venus’ strange tendencies
often shines brightly in the early evening in the west or before dawn in the east
Jupiter strange tendencies
when it is visible at night, is the brightest object in the sky besides the Moon and Venus
Mars strange tendencies
often recognizable by its reddish color, though you should check a star chart to make sure you aren’t looking at a bright red star
Why was planetary motion so hard to explain?
Planets behave like all other objects in the sky: Earth’s rotation makes them appear to rise in the east and set in the west
However, vary in both speed and direction
In particular, while the planets usually move through the constellations in the same eastward direction (relative to the stars) as the Sun and Moon, planets occasionally reverse course, moving westward through the zodiac
AKA apparent retrograde motion
Why did the ancient Greeks reject the real explanation for planetary motion?
Inability to detect staller parallax: the slight back-and-forth shifting of star positions that occurs as we view the stars from different positions in Earth’s orbit of the Sun.
Greeks believed that all stars lie on the same celestial sphere, they expected to see stellar parallax in a slightly different way
If Earth orbited the Sun, they reasoned, at different times of year we would be closer to different parts of the celestial sphere and would notice changes in the angular separation of stars.
They concluded…
1. Earth orbits the Sun, but the stars are so far away that stellar parallax is undetectable to the naked eye.
2. There is no stellar parallax because Earth remains stationary at the center of the universe.
How did the Greeks explain planetary motion?
The Greek Geocentric Model
Asserted that all heavenly objects move in perfect circles at constant speeds and therefore must reside on huge spheres encircling Earth
