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22 Cards in this Set

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  • Back
Kepler's first law states that the orbit of each planet is an ellipse with the Sun at one focus. Which of the following statements describe a characteristic of the solar system that is explained by Kepler's first law?
Earth is slightly closer to the Sun on one side of its orbit than on the other side.
The Sun is located slightly off-center from the middle of each planet's orbit.
(None of the planets has a perfectly circular orbit, which means that all planets (including Earth) are closer to the Sun on one side of their orbit than on the other. The Sun's off-center position arises because it is located at a focus of each planet's elliptical orbit, rather than at the center of the ellipse.)
Kepler's second law states that as a planet orbits the Sun, it sweeps out equal areas in equal times. Which of the following statements describe a characteristic of the solar system that is explained by Kepler's second law?
Pluto moves faster when it is closer to the Sun than when it is farther from the Sun

The same ideas holds for any object orbiting the Sun: An object must move faster when it is closer to the Sun and slower when it is farther from the Sun.
Kepler's third law states that a planet's orbital period, p, is related to its average (semimajor axis) orbital distance, a, according to the mathematical relationship . Which of the following statements describe a characteristic of the solar system that is explained by Kepler's third law?
Inner planets orbit the Sun at higher speed than outer planets.
Venus orbits the Sun faster than Earth orbits the Sun.

From the relationship , it follows that planets closer to the Sun must orbit at higher average speeds than planets farther from the Sun. For example, Venus must orbit the Sun faster than Earth, because Venus is closer to the Sun.
According to Kepler’s second law, as a planet or other object moves around its orbit, it sweeps out equal __________ in equal __________.

areas / times
Although Kepler wrote his laws specifically to describe the orbits of the planets around the Sun, they apply more generally. Kepler's second law tells us that as an object moves around its orbit, it sweeps out equal areas in equal times. Because all the areas shown here are equal, the time it takes the comet to travel each segment must also be the same.
Explanation of how fast a comet travels
From Parts A and B, you know that the comet takes the same time to cover each of the four segments shown, but that it travels greater distances in the segments that are closer to the Sun. Therefore, its speed must also be faster when it is closer to the Sun. In other words, the fact that that the comet sweeps out equal areas in equal times implies that its orbital speed is faster when it is nearer to the Sun and slower when it is farther away.
orbital periods
You correctly ranked the planets according to how long they take to complete an orbit, which is what we call the orbital period. Note that the pattern is one of the ideas that are part of Kepler’s third law: Planets with larger average orbital distances have longer orbital periods.
orbits
You correctly ranked the planets according to how long they take to complete an orbit, which is what we call the orbital period. Note that the pattern is one of the ideas that are part of Kepler’s third law: Planets with larger average orbital distances have longer orbital periods.
circumpolar stars
Circumpolar stars are the stars at any particular location (latitude) that remain about the horizon at all times, making circles around the north (or south) celestial pole. For example, at latitude 40°N, all stars within 40° of the north celestial pole are circumpolar. The photo here shows star tracks over a period of several hours; the stars for which we can see complete circles above the horizon are circumpolar.
According to the Earth-centered model, the known planets rise in the east and set in the west each day because __________.
each of these planets circles Earth each day from east to west
Note that this is quite different from the case in the Sun-centered model, in which the daily motion of planets is a consequence of Earth’s daily rotation. That is, the Earth-centered model could in principle allow for planets circling Earth in any direction, and therefore moving across the sky in any direction.
Consider the hypothetical observation “a planet beyond Saturn rises in west, sets in east.” This observation is not consistent with a Sun-centered model, because in this model __________.
the rise and set of all objects depends only on Earth’s rotation
We never see a crescent Jupiter from Earth because Jupiter __________.
Is farther than Earth from the Sun

An object must come between Earth and the Sun for us to see it in a crescent phase, which is why we see crescents only for Mercury, Venus, and the Moon.
Suppose the planet Uranus were much brighter in the sky, so that it was as easily visible to the naked eye as Jupiter or Saturn. Which one of the following statements would most likely be true in that case?
A week would have eight days instead of seven.

A week has seven days because seven naked-eye objects appear to move among the stars: the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. If Uranus had been an eighth object visibly moving among the stars, a week very likely would have eight days.
How does a 12-month lunar calendar differ from our 12-month solar calendar?
It has about 11 fewer days
Which of the following best describes a set of conditions under which archaeoastronomers would conclude that an ancient structure was used for astronomical purposes?
The structure has holes in the ceiling that allow viewing the passage of constellations that figure prominently in the culture's folklore, and many other structures built by the same culture have ceiling holes placed in the same way.
How did the Ptolemaic model explain the apparent retrograde motion of the planets?
The planets moved along small circles that moved on larger circles around the Earth.

This created a "loop-the-loop" motion that made the planets in the model appear to sometimes go backward as viewed from Earth.
Earth is farthest from the Sun in July and closest to the Sun in January. During which Northern Hemisphere season is Earth moving fastest in its orbit?
Kepler's second law tells us that planets move fastest when they are nearest to the Sun. Since this in January for Earth, it is Northern Hemisphere winter.
According to Kepler's third law (p2 = a3), how does a planet's mass affect its orbit around the Sun?
A planet's mass has no effect on its orbit around the Sun.

Kepler's third law makes no allowance for planetary mass, and in fact the planet's mass has virtually no effect on its orbit of the Sun. (The Sun's mass has a major effect, however.)
All the following statements are true. Which one follows directly from Kepler's third law (p2 = a3)?
Venus orbits the Sun at a slower average speed than Mercury.
Suppose a comet orbits the Sun on a highly eccentric orbit with an average (semimajor axis) distance of 1 AU. How long does it take to complete each orbit, and how do we know?
1 year, which we know from Kepler's third law.

Kepler's third law tells us that any object with the same average distance as Earth will orbit in the same time of 1 year.
Galileo challenged the idea that objects in the heavens were perfect by _________.
observing sunspots on the Sun and mountains on the Moon

Both the Sun and Moon had been generally assumed to have "perfect" surfaces.
Galileo observed all of the following. Which observation offered direct proof of a planet orbiting the Sun?
Galileo's observed that Venus goes through all the phases, which cannot be explained unless Venus is orbiting the Sun. (In the Ptolemaic system, Venus's phases vary only from new to crescent and back.)
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

Newton’s second law of motion, F=ma, means that for a particular force F, the product mass x acceleration must always be the same. Therefore if mass is larger, acceleration must be smaller, and vice versa.