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Who proposed the geocentric view that was accepted and used for more than 18 centuries?



What is the name given to the phenomenon in which the apparent motion of the superior planets sometimes reverses?

Retrograde motion


How did Ptolemy's model explain the retrograde motions of the planets?

Planets orbit in small circles called epicycles, and the epicycle is orbiting the Earth in a larger orbit called a deferent. This caused the planet to have a "looping" motion as seen from Earth.


Scientists today do not accept the Ptolemaic model because:

the work of Tycho and Kepler showed the heliocentric model was more accurate.


The place in a planet's orbit that is closest to the Sun is called



How did Copernicus' model explain the retrograde motions of the planets?

Planets orbit the Sun at different speeds. When an inner, faster-moving planet "passes" a slower outer one, the slower planet appears to move backwards.


According to Copernicus, the retrograde motion for Mars must occur

at opposition, when the earth overtakes Mars and passes between Mars and the Sun.


Kepler's first law worked, where Copernicus' original heliocentric model failed, because Kepler described the orbits as

elliptical, not circular.


Which of the following is a contribution to astronomy made by Galileo?

The astronomical telescope can show us far more detail than the eye can.
The Moon has craters, mountain, valleys, and dark flat areas on its surface.
Jupiter has four moons orbiting it.
Venus appears almost fully lit when it lies on the far side of the Sun.


In Ptolemy’s Earth-centered model for the solar system (not shown), Venus’s phase is never full as viewed from Earth because it always lies between Earth and the Sun. In reality, as Galileo first recognized, Venus is __________.

full whenever it is on the opposite side of the Sun from Earth, although we cannot see the full Venus because it is close to the Sun in the sky


Imagine that Venus is in its full phase today. If we could see it, at what time would the full Venus be highest in the sky?

at noon


When would you expect to see Venus high in the sky at midnight?



In Ptolemy’s Earth-centered model for the solar system (not shown), Venus always stays close to the Sun in the sky and, because it always stays between Earth and the Sun, its phases range only between new and crescent. The following statements are all true and were all observed by Galileo. Which one provides evidence that Venus orbits the Sun and not Earth?

We sometimes see gibbous (nearly but not quite full) Venus.


The force of gravity between two objects:

increases with the masses of the bodies, but decreases with the square of the distances between them


Consider Earth and the Moon. As you should now realize, the gravitational force that Earth exerts on the Moon is equal and opposite to that which the Moon exerts on Earth. Therefore, according to Newton’s second law of motion __________.

the Moon has a larger acceleration than Earth, because it has a smaller mass


What are the key observations made by Galileo that challenged widespread philosophical beliefs about the solar system?

1. Jupiter has orbiting moons.
2. The Moon has mountains, valleys, and craters.
3. Venus goes through a full set of phases.
4. The Sun has sunspots and rotates on its axis.


What conclusion did Kepler eventually come to that revolutionized the heliocentric model of the solar system?

Kepler determined that the planetary orbits are elliptical.


Which observation(s) could you make today that Galileo and Kepler could not have made to confirm that the heliocentric model is correct?

1. Doppler shifts in stellar spectra of nearby stars
2. Stellar parallax in nearby stars
3. Transit of an extrasolar planet


How does orbital speed at aphelion compare to the speed at perihelion?



In what way did Newton improve Kepler's laws?

He discovered the dependence on mass in the third law.


According to Newton, planets orbit in ellipses with what at the two foci?

The center of mass and nothing


If the distance beween the centers of two objects doubles, what happens to the gravitational pull between them if the masses do not change?

The force will decrease to one-fourth its previous size.


If Earth’s orbit around the Sun were to double in size, the new “year” would be

more than 2 current Earth years.