Narrator: Listen to part of a lecture in an astronomy class.
Professor: OK. Today, I’m going to tell you about how a famous object in the Solar System got its name. Let’s start with Kepler. Remember from the reading Kepler’s First Law?… Uh, planetary orbits are in the shapes of…
Female Student: Ellipses.
Professor: Right. An ellipse is basically an oval, uh, a stretched-out circle. How much it’s stretched is called the eccentricity. A circle is actually an ellipse with zero eccentricity, and if the eccentricity is just say 2 percent, the circle is stretched just a bit. But if you push the eccentricity towards 100 percent, the ellipse gets really stretched out and skinny.
Um, how eccentric do you think the Earth’s orbit is?
Male Student: I’d say like 1 or 2 percent.
Professor: Good, it’s between 1 and 2 percent. That’s a very good thing for us, by the way, because if we were on a really eccentric orbit, it’d be a lot hotter in the summer, like oceans boiling, and in the winter they might freeze completely solid.
Mercury has a kind of elliptical orbit, 20 percent or so. But there are other objects in the Solar System with even more elliptical orbits. Know what they are?
Male Student: Meteors?
Professor: Well… not necessarily. Meteors are usually just little chunks of rock that enter the Earth’s atmosphere and burn up. But out in the Solar System, they’re mostly just, uh, randomly kicked around. The things I’m thinking about are bigger—not as big as a planet, but several miles across—and more importantly they follow more regular orbits, just... eccentric ones. They swoop in near the Sun and go back out again…
Female Student: Comets.
Professor: Yes, comets. Comets travel in pretty eccentric orbits. Now… when comets get in close to the Sun, what do they do?
Male Student: They grow a tail.
Professor: Exactly, they grow tails. Think of comets as dirty snowballs. Radiation from the Sun boils off some water and other stuff, and that’s what makes up the tail.
All right, here’s the story of a famous comet. It wasn’t first seen by the person it’s named after. In fact, this comet was seen back in 240 BC by Chinese astronomers, who called comets “broom stars.” The tail is the broom, the brush part.
So then this comet gets seen in 164 BC by astronomers in Babylon, and every 75 or 76 years after that it’s in Chinese records. But no one seemed to know it was the same comet. For one thing, it looked different every time. Sometimes it was close to the Earth, sometimes it was pretty far away. And there are a lot of other comets in the meantime, all kinds of broom stars.
Then in 1607 Kepler himself even observes this particular comet, but he doesn’t connect the dots. He’s figuring out his laws of planetary orbits, which were just observational, by the way, just patterns. He never proved why the orbits were ellipses. He just said, planets move in elliptical orbits, that’s what we see but who knows why. So Kepler’s Laws weren’t fundamental laws of nature.
Next comes Isaac Newton. He publishes the Principia in 1687, which lays out the laws of motion and gravitation—these are the fundamental laws of nature that give rise to Kepler’s Laws, or maybe I should say Kepler’s Patterns. So in the Principia, Newton also explains how to calculate cometary orbits from observations, because comets follow Newton’s Laws along with planets and everything else.
Finally, Newton’s buddy Edmond Halley applies the Principia method to a bunch of historical records of comets—
Female Student: Halley’s Comet.
Professor: Yes! Hold that thought. Halley calculates all these cometary orbits using Newton’s Laws, and he finds that the comet Kepler saw in 1607, and another one seen in 1531 and a third one in 1682, all three comets have the same size orbit, the same shape orbit, the same orientation in the sky. The numbers that describe the orbit are called orbital elements, and these three comets all have the same orbital elements.
So Halley says, these are all the same comet, reappearing every 75 to 76 years. And he predicts this comet will come back in late 1758. He dies in 1742, but sure enough, in December 1758 the comet shows back up.
That’s why we call it Halley’s Comet—not because Halley was the first one to ever see it. People had been seeing it for hundreds and hundreds of years. But Halley was the first one to make use of scientific laws, he was the first one to prove it was all the same comet, and he even predicted its return.
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What is Kepler’s First Law? |
Detail. This is the first question the professor poses the class. |
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| ✗ | A A fundamental law of nature |
Later the professor specifically says that Kepler’s Laws are not fundamental. She even calls them Kepler’s Patterns. |
| ✗ | B A prediction of when a comet would return |
Halley made the prediction of when a comet would return, using Newton’s Laws. |
| ✗ | C A description of how comet tails form |
The professor describes how comet tails form, but she does not associate that process with Kepler’s First Law. |
| ✓ | D A statement about the shapes of planetary orbits |
Correct. After asking what Kepler’s First Law is, the professor prompts the class: “Planetary orbits are in the shapes of…” A student responds, “Ellipses,” and the professor says, “Right.” The point that planets move in elliptical orbits is repeated later. |
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Why does the professor explain Edmond Halley’s work to the class? |
Gist-Purpose. Edmond Halley’s work is explained toward the end of the lecture. Halley identifies three separate observations as one comet and predicts its return. As a result, the comet is named for him. |
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| ✗ | A To highlight how long in the historical record Halley’s Comet has been observed |
Halley’s work comes toward the end of a long historical record of sightings of the comet. But the professor explains Halley’s work without emphasizing the length of this historical record. |
| ✓ | B To explain how Halley’s Comet got its name |
Correct. The professor begins by saying: “Today, I’m going to tell you about how a famous object in the Solar System got its name.” This object is Halley’s Comet. Toward the end of the lecture, the professor explains Halley’s work and then says: “That’s why we call it Halley’s Comet.” |
| ✗ | C To walk through detailed calculations that use Newton’s Laws |
The professor emphasizes that Newton’s Laws made Halley’s work (his calculations) possible, but she never demonstrates any actual calculations to the class. |
| ✗ | D To emphasize the eccentricity of cometary orbits |
Midway through the lecture, the professor describes comets as objects that “travel in pretty eccentric orbits.” But this is not why she explains Halley’s work later on. In that explanation, she never emphasizes how eccentric cometary orbits are. |
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How does the professor introduce comets to the class? |
Organization. The professor talks about elliptical orbits in general, mentions that Earth’s orbit is not very elliptical, and then asks the class which objects in the Solar System have very elliptical orbits. The correct response, given by a student, is comets. The professor then goes on to describe comets further. |
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| ✓ | A By asking which objects have a certain kind of orbit |
Correct. To introduce comets, the professor asks which objects have “even more elliptical orbits” than Mercury. |
| ✗ | B By discussing the composition of comets |
The professor describes comets as “dirty snowballs” after introducing comets. |
| ✗ | C By drawing attention to meteors first |
The professor does not bring up meteors or try to draw attention to them. A student gives “meteors” as an incorrect response, so the professor has to describe them briefly and draw a contrast to the objects she’s thinking of (comets). But this was not her original intent. |
| ✗ | D By describing the effects of a hypothetical orbit of the Earth |
The professor mentions the “oceans boiling” that might happen if the Earth were in a very eccentric orbit (hypothetically). But this mention is an aside. The professor says “by the way” as she makes this aside. |
Narrator: Listen again to part of the lecture. Then answer the question.
Professor: So Kepler’s Laws weren’t fundamental laws of nature. Next comes Isaac Newton. He publishes the Principia in 1687, which lays out the laws of motion and gravitation—these are the fundamental laws of nature that give rise to Kepler’s Laws, or maybe I should say Kepler’s Patterns.
Narrator: Why does the professor say this:
Professor:…or maybe I should say Kepler’s Patterns.
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Why does the professor say this? |
Function of What Is Said. Look for the purpose of the professor’s specific words. By saying “Kepler’s Patterns,” the professor is emphasizing that Kepler’s Laws are not like Newton’s Laws, which are “fundamental laws of nature.” |
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| ✗ | A To argue that Kepler’s Laws are essentially flawed |
The professor is not arguing that the laws are wrong, or flawed. Rather, they are just not fundamental, in the way that Newton’s Laws are. |
| ✗ | B To point out how comets do not obey Kepler’s Laws |
The professor never says that comets do not obey any of Kepler’s Laws. According to the professor, comets follow elliptical orbits, just as Kepler’s First Law says that planets do. |
| ✗ | C To suggest how Kepler’s Laws may be applied more broadly |
By calling Kepler’s Laws “patterns,” the professor is not trying to apply these laws more broadly. Rather, she is indicating that these laws are not fundamental laws. |
| ✓ | D To contrast Kepler’s Laws with more fundamental laws of nature |
Correct. The professor changes the name to “Kepler’s Patterns” to highlight the contrast between these “patterns” and “fundamental laws of nature,” which Kepler’s Laws are not. |
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According to the professor, why did no one before Halley know that various historical observations of Halley’s Comet were of the same comet? Choose 2 answers. |
Detail. As the professor describes various observations of Halley’s Comet through history, she says “But no one seemed to know it was the same comet.” She immediately gives two reasons. |
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| ✓ | a The comet looked different every time it was observed. |
Correct. The professor says: “For one thing, it looked different every time. Sometimes it was close to the Earth, sometimes it was pretty far away.” |
| ✗ | b Whenever the comet was observed, its orbit had a different size and shape. |
The professor does not cite this reason. In fact, Halley found that the comet’s orbit each time had the same size and shape. That’s why he claimed that the same comet had been observed each time. |
| ✗ | c The historical observations were not precise enough. |
The professor never describes how precise or not the historical observations were. |
| ✓ | d There were many other observations of other comets. |
Correct. The professor also says: “And there are a lot of other comets in the meantime, all kinds of broom stars.” |
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What does the professor imply about Halley’s Comet? |
Inference. The professor discusses the comet itself in the latter part of the lecture. |
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| ✗ | A Newton could not have identified the comet first. |
The professor never implies that Newton could not have identified the comet first. In fact, Newton provided the method that Halley used to identify the comet. So, if anything, Newton probably could have identified the comet first, in theory. |
| ✗ | B The comet has a more eccentric orbit than most other comets. |
The professor never discusses how eccentric the orbit of Halley’s comet is. Nor does she compare its eccentricity to that of any other comets. All you can assume is that Halley’s Comet has an eccentric orbit. After all, it is a comet, and according to the professor, comets have “pretty eccentric orbits.” |
| ✓ | C No other comet has the same orbital elements. |
Correct. If another comet had the same orbital elements (the numbers describing the orbit) as Halley’s Comet, then Halley could not have made his claim. He could not have argued that the various observations were all of the same single comet. The professor implies that Halley’s Comet (as well as every other comet) has a unique set of orbital elements describing its orbit. |
| ✗ | D Halley’s Comet was not observed before 240 BC. |
The professor describes the first known and recorded observation, which occurred in 240 BC. But you can’t infer that before that time, no one had ever observed the comet at all. |