Quiz Questions Flashcards
(12 cards)
Why are the fixed stars so important?
Since they keep the same positions with respect to one another they are reference points by which to mark the position of the other celestial objects.
How does the moons path through the Sky relate to the ecliptic?
. The moon follows closely to the ecliptic line which is the suns path through the stars and constellations so its always in rotation.it also moves through the constellations of the zodiac, but is slightly tilted with respect to the ecliptic. Sometimes the moon crosses above the ecliptic and sometime it passes below.
If the moon is directly overhead and the western half of its face is lighted, where would you expect to see the sun?
Setting on the western horizon.
What condition must be met in order to have a solar eclipse?
The sun and moon must be on the same node (the point where the moon’s path crosses the ecliptic).
Eratosthenes assumed that the rays of the Sun are parallel to one another by the time they reach Earth. Why was this assumption justified? (Describe any observations/measurements we used to come to this conclusion, and how they helped us come to this conclusion.)
For the rays of the sun to appear parallel to one another by the time they arrive at Earth, the Sun must be VERY far away. We decided the Sun must be very far away using the distance between Alexandria and Syene (800 km), the length of a shadow in Alexandria at noon on the summer solstice (12.6 cm for a 1 m gnomon), and the observation that ships disappear over the horizon before they become too small to see.
The curve of the Earth and the position of the Sun causes there to be no shadows in Syene at noon on the summer solstice, and very short shadows in Alexandria. The only way for this to be true is for the Earth to be significantly curved and the Sun very far away, or for the Earth to be very big and the Sun much closer. The observation that ships disappear over the horizon after only 20 km of travel tells us that the Earth curves significantly between Alexandria and Syene. Therefore, the Sun must be VERY far away to produce only a 12.6 cm shadow over the 800 km between Syene and Alexandria.
What was Ptolemy’s eccentric, and what two observations was it meant to account for?
The eccentric is a distance at which the Earth is offset from the center of the solar or planetary orbits. It was meant to account for the different number of days in the seasons, and it was meant to account for the fact that retrogrades of Mars, Jupiter, and Saturn are not evenly spread across the constellations. They happen more often in one part of the sky than another.
Explain how Ptolemy was able to account for the observed change in speed and direction of the planets while sticking to the idea of perfect circular orbits at constant speed.
Ptolemy split all planetary orbits into 2 separate orbits, a deferent (the larger circle centered on the Earth), and an epicycle (a smaller circle moving along the deferent). By adjusting the size and speed of the deferent and epicycle he could account for the normal motion of the planets as well as the periodic retrogrades. Since the speed of the deferent and epicycle were fixed, the speeds were technically constant while giving the change in speed we observe in the sky.
What was Ptolemy’s equant, and what observation was it meant to account for?
The equant is another distance offset from the center of a planetary orbit that is opposite the eccentric. Originally, Ptolemy had each planet’s epicycle moving around the center of the deferent at a constant speed. This did not account for the difference in size of the retrogrades. Instead, he kept the epicycle moving around the deferent, but made the speed constant around the equant. This would mean the planet was moving faster on one part of the circle than another and accounted for the different sizes of the retrogrades.
What were the 3 coincidences in Ptolemy’s model that led Copernicus to develop his heliocentric model of the cosmos?
1) The period of the epicycles for Mars, Jupiter, and Saturn had to be 365 days even though Mars, Jupiter, and Saturn don’t appear attached to the Sun.
2) The retrogrades of Mars, Jupiter, and Saturn happen at different times, with different frequencies, and have different durations. However, half-way through the retrograde of each of the outer planets, the sun is always in opposition (on the opposite side of the Earth).
3) The eccentric distances for Mars, Jupiter, and Saturn are all the same.
What 3 motions did Copernicus give to the Earth, and what observations did they explain?
1) He gave Earth a 24 hour rotation on its axis. This accounts for the day-night cycle.
2) He gave Earth a 365 day rotation around the Sun. This accounts for the retrograde motion of the planets.
3) He gave Earth a precession around its spin axis. This accounts for the very slow shift in the positions of the stars over many thousands of years.
How does giving Earth a 365 day rotation around the sun explain retrograde without epicycles?
Retrograde motion is due to Earth moving faster than the outer planets. Retrograde motion occurs when Earth catches up to the planet and “laps” it in its orbit about the Sun.
How does swapping the position of the Earth and Sun explain why Mercury and Venus are always attached to the Sun in the sky?
If Mercury and Venus orbit closer to the Sun than Earth, we will always have to look inward, toward the Sun to see them. They can never be on the opposite side of the Earth than the Sun.
