answers Instructor's for Midterm 1 EPSC 171A Spring 08. Text in brackets is for your further information, but it's not expected that you will have said this in your answers. Earth & Planetary Sciences 171A Spring 2008 FIRST MIDTERM EXAM February 14, 2008 Answer all questions. Write directly on the exam sheets (extra paper will be available for anybody who wants it). The exam starts 11:40 AM and concludes 12:55 PM.
PART 1 Fill in the Blanks Fill in the blanks with a word, number or short phrase. Each question is worth 2 points. 1-1 1-2 1-3 1-4 1-5 The principal element in the Earth's core is presumed to be ___iron_______. By far the two most abundant elements in the Sun are _hydrogen and helium_. __January__________ is the month in which the Earth is closest to the Sun. The most abundant element in the Earth is _oxygen___. The atmospheric component most directly indicative of life on Earth is _oxygen_ . [NB Strictly speaking the answer should be molecular oxygen, O2, as distinct from combined oxygen, as in CO2 or silicates. Just saying "oxygen" is good enough, however.] When the Sun runs out of fuel in its core, the next stage in its evolution will be to the type of star called a ___red giant__. The region of the celestial sphere close to the ecliptic is called __the Zodiac__ . The relatively bright star close to the north celestial pole is called __Polaris__ . The diameter of the Moon is approximately _one quarter__ the diameter of the Earth. [NB Insert a fraction.] The Sun has lived through about __half____ of its projected life as a "main sequence" star. [NB Insert a fraction.] PART 2 Short-Answer Questions These questions can be satisfactorily answered in one sentence or maybe just a few words. Write your answers under each question in the space provided. Each question is worth 3 points.
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What is the main source of energy in the Sun? The main energy source is nuclear energy, specifically the energy liberated in the fusion of four protons (hydrogen nuclei) to make one 4He (helium nucleus).
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What is the "solar neutrino problem"? Several generations of neutrino detectors [a rather difficult apparatus to build and operate] found only a small fraction [about a third] as many neutrinos as are predicted from the known rate of energy generation in the Sun. [Most people now believe that the solution to this problem is that neutrinos spontaneously change from one kind to another kind, one which is not detected by the apparatus.]
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In the paradigm of "plate tectonics", what is a plate? A plate is a rigid, brittle shell (like an eggshell) on the surface of the Earth, about 70100 km vertically thick (down to a region in the mantle where the rock is soft and allows the upper shell to slide relative to the underlying mantle), and in horizontal extent anywhere from a hundred km or so up to the size of the Pacific Ocean. The various plates on the surface of the Earth move relative to each other.
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What is the "music of the spheres"? Classical philosophers [like Pythagoras] thought that planets were embedded on concentric, independently moving spheres. These spheres were thought to make harmonious sounds as they moved.
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What does it mean to say that we are entering the "Age of Aquarius"? The vernal equinox, one of the two points of intersection of the celestial equator and the ecliptic [equivalently, the point on the Earth's/Sun's orbit where days and nights are of equal length] moves [precesses] across the sky. In classical times it was in the constellation Aires, subsequently it has moved through the constellation Pisces, and it is now entering the constellation Aquarius.
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Why don't we have eclipses every month? We would, if the plane of the Moon's orbit around the Earth coincided with the plane of the Earth's orbit around the Sun (the ecliptic). But the Moon's orbit is inclined [by 5.5o] to the ecliptic, and we get eclipses only at the relatively rare occasions when the Moon is crossing the ecliptic at the same time that Earth, Moon and Sun lie along a straight line. [See Figure 1.21 in the text, also discussed in class.]
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What is parallax? Parallax is the phenomenon that a relatively nearby object is seen at slightly different angles, relative to distant objects, when viewed from different positions [along a line perpendicular to the line of sight]. [Nearby stars show a parallax, i.e. different
apparent position on the celestial sphere, when viewed in different seasons, i.e. from different parts of the Earth's orbit.] [Parallax different appearance in different eyes is also how we primates judge distance.] 2-8 List the classical planets (the ones visible to the naked eye): Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. 2-9 What's the difference between the Gregorian calendar and the Julian calendar? The difference is how often we have leap years. The Julian calendar has a leap day every fourth year. This would be precisely correct if the (tropical) year were precisely 365.2500 days. The year is actually a bit shorter [365.242 days], and to account for this the Gregorian calendar leaves out an occasional leap day. [When the Gregorian calendar was adopted in 1582, ten days were omitted from the calendar to get the seasons back in synch with the classical era, but that was a one-off deal that's not a big concern anymore.] 2-10 What's the difference between a sidereal day and a solar day? A sidereal day is the time for the Earth to complete one revolution relative to the "fixed" (i.e. distant) stars. But during this one day the Earth moves a little distance in its orbit around the Sun, so it has to turn a little more to repeat the same orientation relative to the Sun. Thus the solar day is a tad longer than the sidereal day. [By 3.9 minutes, if you want to get technical. Since the mean solar day is, by definition, 24 hours, the sidereal day is only 23 hours 56.1 minutes.] PART 3 Essay Questions Each question should be answered in coherent sentences, organized into a paragraph or two or three. All the questions are given below. Use the blank sheet at the end, or the backs of the question sheets, for your answers. Additional paper is available if needed. Each question is worth 10 points. Note: I do not expect that your answers will be as extensive as those below 3-1 Describe the essential features of the prevailing view of planetary motions in classical Greek-Roman culture, as expressed in Ptolemy's Almagest. Explain which aspects of this cosmogony are evidently based on "philosophical" assumptions and which seem to reflect the need to reconcile philosophical expectations with observation. It was assumed that the planets, including the Sun, moved in circles around the Earth, mostly near a common path in the sky (the zodiac). This was basically a philosophical position: It was held that the heavens were perfect and unchanging, so that the motion of the planets must be along a perfect path, i.e. a circle. Additionally, it was also generally thought that the planets were mounted on solid spheres that revolved around the Earth, like the sphere of the stars (the celestial sphere), but at different rates.
While philosophically attractive, this scheme was manifestly unable to account for actual observations of how planets traveled through the sky, relative to the celestial sphere. To "save the appearances", the complication was introduced that the planets moved on epicycles, which are circles whose centers moved around the Earth on other circles. In detail, sometimes the planetary epicycles were themselves imagined to move on epicycles, or to involve circles not quite centered on the Earth. The model had enough free parameters that it could actually give a pretty good rendering of planetary movements across the sky, albeit only in a very system complicated constructed to save the philosophical notion of circular motion in the heavens. 3-2 Upon examining the planet Venus in his telescope, Galileo made observations that were important in discriminating between the rival Ptolemaic and Copernican cosmogonies. Describe the observations and explain how they distinguished the two theories. Is Galileo's interpretation still valid today, or has it subsequently been discredited? On the basis of naked eye observations, prior to the introduction of the telescope, it could be seen that Venus was sometimes brighter and sometimes less bright, but it appeared only as a point of light, with no spatial detail visible. With a telescope, Venus could be seen as an extended object, and it was observed to exhibit phases, just like the Moon, which corresponded to a spherical object, half-illuminated by the Sun, but with only part of the illuminated hemisphere visible from Earth. It was not the existence of phases that was the big deal: Both the Ptolemaic (geocentric) and the Copernican (heliocentric) models predicted phases. But the two models predicted different patterns of phases. In the Ptolemaic model, Venus moved on an epicycle whose center was on the circumference of another circle centered on Earth, but Venus was always closer to the Earth than the Sun was. In this model you would never see more of the illuminated half of Venus than you could at maximum elongation (angular distance from the Sun), less than half full. In the Copernican model, following crescent phase and maximum elongation the visible part of Venus would continue to grow, waxing gibbous, as it moved to the other side of the Sun, eventually reaching "full Venus", then waning gibbous, etc. Galileo observed that Venus exhibited gibbous phases and the "full Venus" phase, and moreover it was full when it was at its smallest size. There was no way this could happen in the Ptolemaic picture, but it followed naturally from the Copernican view: When Venus approached full phase it was on the other side of the Sun from the Earth! Galileo was not right about everything he said, but then and now his observations of the phases of Venus, once you understand them, are conclusive evidence against the Ptolemaic world-view. 3-3 Describe the most important ways in which the Earth and the Moon are different. Describe the most important ways in which they are similar. The answer depends on what you think "important" means, but here are major points stressed in class and in the text.
For one thing, the Moon is a lot smaller than the Earth (about 1/80 by mass). Its chemistry is different: It has no atmosphere or oceans, and evidently never did, i.e. it lacks the volatile elements/compounds that make an atmosphere/ocean. There are other chemical differences, even if less obvious, e.g. the Moon has much less Fe than the Earth. In contrast to the active geological processes happening on the Earth, the Moon is basically a dead planet: Practically nothing happens there except things driven by external influence, e.g. sunlight or impacting materials from outer space. The Earth is still making new rocks today; on the Moon the youngest rocks are around 3 billion years old. The lack of internal/tectonic activity on the Moon is thought to be a consequence of its smaller size, which led to its cooling off more quickly and more thoroughly than the Earth. But when you start comparing the Earth and the Moon with other things in the solar system, especially with the Sun itself, they don't look so different from each other. The fundamental similarity in composition is that in both planets the chemistry is dominated by a single anion, oxygen, and several cations, notably silicon, magnesium, iron, calcium and aluminum. Put these together and in both planets what you get is "rock". That's certainly different from the Sun, which is all gas. [And, as we will see later, that's also very different from the gas-giant planets and their mostly icy satellites.] The proportions of the non-volatile elements in the Earth and Moon are also very similar, indicating commonality in their origin and processing. And, of course, they're both in the same part of the solar system and they're essentially the same age, again indicating common factors in their origins. 3-4 Describe Isaac Newton's contributions to our understanding of planetary motion. We could probably carry on at great length about Newton's contributions here, but a couple of points stand out. Arguably his greatest contribution was that he showed that the motion of the planets could be explained by one simple interaction, the gravitational force between Sun and planets, and that this gravitational interact...