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ormation. The second way we try to keep numbers simple is to use a consistent set of units—the metric International System of Units, or SI (from the French Système International d’Unités). The metric system is summarized in Appendix D (see Example 1.2). 16 Chapter 1 Science and the Universe: A Brief Tour Watch this bri... |
human being on our planet had a net worth of $79.2 billion. Some might say this is an astronomical sum of money. Express this amount in scientific notation. Solution $79.2 billion can be written $79,200,000,000. Expressed in scientific notation it becomes $7.92 × 1010 Getting Familiar with a Light-Year How many kilome... |
” you ask, “I won’t know what’s actually happening there for another 500 years?” But this isn’t the most helpful way to think about the situation. For astronomers, now is when the light reaches us here on Earth. There is no way for us to know anything about that star (or other object) until its light reaches us. 18 Cha... |
had equipment to receive radio or television signals, or came close enough to see the lights of our cities at night, she would soon find signs that this watery planet has sentient life. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 1 Science and the Universe: A Brief Tour 19 F... |
one of the minor eruptions that occurs on the surface of our star. If the Sun were reduced to the size of a basketball, Earth would be a small apple seed about 30 meters from the ball. It takes Earth 1 year (3 × 107 seconds) to go around the Sun at our distance; to make it around, we must travel at approximately 110,0... |
later in the book. The other stars look faint only because they are so very far away. If we continue our basketball analogy, Proxima Centauri, the nearest star beyond the Sun, which is 4.3 light-years away, would be almost 7000 kilometers from the basketball. When you look up at a star-filled sky on a clear night, all... |
call interstellar dust. This gas and dust collect into enormous clouds in many places in the Galaxy, becoming the raw material for future generations of stars. Figure 1.10 shows an image of the disk of the Galaxy as seen from our vantage point. 22 Chapter 1 Science and the Universe: A Brief Tour Figure 1.10. Milky Way... |
/content/col11992/1.8 Chapter 1 Science and the Universe: A Brief Tour 23 1.11). Some of the largest of the star clusters that astronomers have cataloged contain hundreds of thousands of stars and take up volumes of space hundreds of light-years across. Figure 1.11. Star Cluster. This large star cluster is known by its... |
is one of the 88 sections into which astronomers divide the sky, each named after a prominent star pattern within it.) Beyond this Sagittarius dwarf galaxy lie two other small galaxies, about 160,000 light-years away. First recorded by Magellan’s crew as he sailed around the world, these are called the Magellanic Clou... |
cknowledgment: Cambridge Astronomical Survey Unit) Some of the clusters themselves form into larger groups called superclusters. The Local Group is part of a supercluster of galaxies, called the Virgo Supercluster, which stretches over a diameter of 110 million light- years. We are just beginning to explore the structu... |
In the interstellar gas of the Galaxy, there is about one atom in every cubic centimeter. Intergalactic space is filled so sparsely that to find one atom, on average, we must search through a cubic meter of space. Most of the universe is fantastically empty; places that are dense, such as the human body, are tremendou... |
size of the nucleus itself. This is why we say that even solid matter is mostly space. The typical atom is far emptier than the solar system out to Neptune. (The distance from Earth to the Sun, for example, is only 100 times the size of the Sun.) This is one reason atoms are not like miniature solar systems. Remarkabl... |
10, and the oldest rocks we can date on Earth go back to the third week in September (Figure 1.15). This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 1 Science and the Universe: A Brief Tour 29 Figure 1.15. Charting Cosmic Time. On a cosmic calendar, where the time since the Big B... |
Guide to the Solar System. 3rd ed. Workman, 2005. This volume for beginners is a colorfully illustrated voyage among the planets. Sagan, Carl. Cosmos. Ballantine, 2013 [1980]. This tome presents a classic overview of astronomy by an astronomer who had a true gift for explaining things clearly. (You can also check out ... |
can select images by world, feature name, date, or catalog number, and download images in a number of popular formats. However, only NASA mission images are included. Note the Photojournal Search option on the menu at the top of the homepage to access ways to search their archives. Videos Cosmic Voyage: www.youtube.co... |
about this on your own, you can check later in the chapter for some suggested answers.) Today, few people really spend much time looking at the night sky. In ancient days, before electric lights robbed so many people of the beauty of the sky, the stars and planets were an important aspect of everyone’s daily life. All... |
. From the sea or a flat prairie, it is easy to see the horizon as a circle around you, but from most places where people live today, the horizon is at least partially hidden by mountains, trees, buildings, or smog. Figure 2.2. The Sky around Us. The horizon is where the sky meets the ground; an observer’s zenith is th... |
objects in the sky. There is even a special theater, called a planetarium, in which we project a simulation of the stars and planets onto a white dome. As the celestial sphere rotates, the objects on it maintain their positions with respect to one another. A grouping of stars such as the Big Dipper has the same shape ... |
the equator). First of all, notice that Earth’s axis is pointing at the celestial poles, so these two points in the sky do not appear to turn. If you stood at the North Pole of Earth, for example, you would see the north celestial pole overhead, at your zenith. The celestial equator, 90° from the celestial poles, woul... |
are always above the horizon, day and night. This part of the sky is called the north circumpolar zone. For observers in the continental United States, the Big Dipper, Little Dipper, and Cassiopeia are examples of star groups in the north circumpolar zone. On the other hand, stars within 38° of the south celestial pol... |
the Sun as being located at some position on the hypothetical celestial sphere. When the Sun rises—that is, when the rotation of Earth carries the Sun above the horizon—sunlight is scattered by the molecules of our atmosphere, filling our sky with light and hiding the stars above the horizon. For thousands of years, a... |
Sun, the Sun will appear to have completed one circuit of the sky along the ecliptic. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 2 Observing the Sky: The Birth of Astronomy 37 Figure 2.6. Constellations on the Ecliptic. As Earth revolves around the Sun, we sit on “platform ... |
that they orbit the Sun “on their side.” Figure 2.7. The Celestial Tilt. The celestial equator is tilted by 23.5° to the ecliptic. As a result, North Americans and Europeans see the Sun north of the celestial equator and high in our sky in June, and south of the celestial equator and low in the sky in December. This O... |
formula: speed = distance time This is true whether the motion is measured in kilometers per hour or degrees per hour; we just need to use consistent units. As an example, let’s say you notice the bright star Sirius due south from your observing location in the Northern Hemisphere. You note the time, and then later, y... |
term “zodiac” is the same as that of the word “zoo” and means a collection of animals; many of the patterns of stars within the zodiac belt reminded the ancients of animals, such as a fish or a goat.) How the planets appear to move in the sky as the months pass is a combination of their actual motions plus the motion ... |
called Betelgeuse (pronounced “Beetel-juice”). The bright blue star below the belt is his foot and is called Rigel. (credit a: modification of work by Johannes Hevelius; b: modification of work by Matthew Spinelli) Today, we use the term constellation to mean one of 88 sectors into which we divide the sky, much as the... |
Earth called precession Describe Ptolemy’s geocentric system of planetary motion Let us now look briefly back into history. Much of modern Western civilization is derived in one way or another from the ideas of the ancient Greeks and Romans, and this is true in astronomy as well. However, many other ancient cultures a... |
a rich variety of philosophical and religious symbolism. At least 2000 years before Columbus, educated people in the eastern Mediterranean region knew Earth was round. Belief in a spherical Earth may have stemmed from the time of Pythagoras, a philosopher and mathematician who lived 2500 years ago. He believed circles... |
We Know Earth Is Round? feature for more ideas on proving Earth is round.) One Greek thinker, Aristarchus of Samos (310–230 BCE), even suggested that Earth was moving around the Sun, but Aristotle and most of the ancient Greek scholars rejected this idea. One of the reasons for their conclusion was the thought that if... |
around the curvature of Earth. Finally, the ship disappears under the horizon. 2. The International Space Station circles Earth once every 90 minutes or so. Photographs taken from the shuttle and other satellites show that Earth is round from every perspective. 3. Suppose you made a friend in each time zone of Earth. ... |
stars—an idea we will use in our discussion of how telescopes work.) Eratosthenes was told that on the first day of summer at Syene, Egypt (near modern Aswan), sunlight struck the bottom of a vertical well at noon. This indicated that the Sun was directly over the well—meaning that Syene was on a direct line from the ... |
his unit of distance. If it was the common Olympic stadium, his result is about 20% too large. According to another interpretation, he used a stadium equal to 46 Chapter 2 Observing the Sky: The Birth of Astronomy about 1/6 kilometer, in which case his figure was within 1% of the correct value of 40,000 kilometers. Ev... |
gravity tries to topple it (Figure 2.12). This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 2 Observing the Sky: The Birth of Astronomy 47 Figure 2.12. Precession. Just as the axis of a rapidly spinning top wobbles slowly in a circle, so the axis of Earth wobbles in a 26,000-year ... |
’s orbital revolution. As we watch the planets from our vantage point on the moving Earth, it is a little like watching a car race while you are competing in it. Sometimes opponents’ cars pass you, but at other times you pass them, making them appear to move backward for a while with respect to you. 48 Chapter 2 Observ... |
because the Greeks believed that celestial motions had to be circles, Ptolemy had to construct his model using circles alone. To do it, he needed dozens of circles, some moving around other circles, in a complex This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 2 Observing the Sky... |
match the observed motions of the planets, Ptolemy had to center the deferent circles, not on Earth, but at points some distance from Earth. In addition, he introduced uniform circular motion around yet another axis, called the equant point. All of these considerably complicated his scheme. It is a tribute to the geni... |
the key to understanding what we can expect from life. The Beginnings of Astrology Astrology began in Babylonia about two and half millennia ago. The Babylonians, believing the planets and their motions influenced the fortunes of kings and nations, used their knowledge of astronomy to guide their rulers. When the Baby... |
“sign,” they are asking for your “sun sign”—which zodiac sign the Sun was in at the moment you were born. However, more than 2000 years have passed since the signs received their names from the constellations. Because of precession, the constellations of the zodiac slide westward along the ecliptic, going once around ... |
: The Birth of Astronomy Astrology Today Astrologers today use the same basic principles laid down by Ptolemy nearly 2000 years ago. They cast horoscopes (a process much simplified by the development of appropriate computer programs) and suggest interpretations. Sun sign astrology (which you read in the newspapers and ... |
on human personality? This is the kind of question that can be tested using the scientific method (see Testing Astrology). The results of hundreds of tests are all the same: there is no evidence that natal astrology has any predictive power, even in a statistical sense. Why, then, do people often seem to have anecdote... |
negative: the birth dates of leaders in all fields tested have been found to be distributed randomly among all the signs. Sun sign astrology does not predict anything about a person’s future occupation or strong personality traits. In a fine example of such a test, two statisticians examined the reenlistment records o... |
the solar system Explain the Copernican model of planetary motion and describe evidence or arguments in favor of it Describe Galileo’s discoveries concerning the study of motion and forces Explain how Galileo’s discoveries tilted the balance of evidence in favor of the Copernican model Astronomy made no major advances... |
published in the first edition of De Revolutionibus Orbium Coelestium. Notice the word Sol for “Sun” in the middle. (credit: Nicolai Copernici) Copernicus described his ideas in detail in his book De Revolutionibus Orbium Coelestium (On the Revolution of Celestial Orbs), published in 1543, the year of his death. By th... |
was able to work out the correct general picture of the solar system. He placed the planets, starting nearest the Sun, in the correct order: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. Further, he deduced that the nearer a planet is to the Sun, the greater its orbital speed. With his theory, he was able to expla... |
, as it turned out. The cold fusion theory soon went down in flames. How would we look at Copernicus’ model today? When a new hypothesis or theory is proposed in science, it must first be checked for consistency with what is already known. Copernicus’ heliocentric idea passes this test, for it allows planetary position... |
greatest contributions were in the field of mechanics, the study of motion and the actions of forces on bodies. It was familiar to all persons then, as it is to us now, that if something is at rest, it tends to remain at rest and requires some outside influence to start it in motion. Rest was thus generally regarded a... |
ath) as well as a brief explanation. Sometime in the 1590s, Galileo adopted the Copernican hypothesis of a heliocentric solar system. In Roman Catholic Italy, this was not a popular philosophy, for Church authorities still upheld the ideas of Aristotle and Ptolemy, and they had powerful political and economic reasons f... |
for astronomical observations, Galileo had to devise a stable mount and improve the optics. He increased the magnification to 30×. Galileo also needed to acquire confidence in the telescope. At that time, human eyes were believed to be the final arbiter of truth about size, shape, and color. Lenses, mirrors, and prism... |
water. These discoveries showed that the Moon might be not so dissimilar to Earth—suggesting that Earth, too, could belong to the realm of celestial bodies For more information about the life and work of Galileo, see the Galileo Project (https://openstaxcollege.org/l/30GalProj) at Rice University. After Galileo’s work... |
Mars, Jupiter, and Saturn—are more prominent than any but the brightest stars, and they can be seen even from urban locations if you know where and when to look. One way to tell planets from bright stars is that planets twinkle less. Venus, which stays close to the Sun from our perspective, appears either as an “eveni... |
on the celestial sphere 90° from the celestial poles; where the celestial sphere intersects the plane of Earth’s equator celestial poles points about which the celestial sphere appears to rotate; intersections of the celestial sphere with Earth’s polar axis celestial sphere the apparent sphere of the sky; a sphere of ... |
about a stationary Earth. We see only half of this sphere at one time, limited by the horizon; the point directly overhead is our zenith. The Sun’s annual path on the celestial sphere is the ecliptic—a line that runs through the center of the zodiac, which is the 18-degree-wide strip of the sky within which we always ... |
, discovering the nature of the Milky Way, the large-scale features of the Moon, the phases of Venus, and four moons of Jupiter. Although he was accused of heresy for his support of heliocentric cosmology, Galileo is credited with observations and brilliant writings that convinced most of his scientific contemporaries ... |
ance-in-the-stars-1527970.html. 1995 newspaper commentary attacking astrology. Copernicus and Galileo Galileo Galilei: http://www-history.mcs.st-andrews.ac.uk/Biographies/Galileo.html. A good biography with additional links. Galileo Project: http://galileo.rice.edu/. Rice University’s repository of information on Galil... |
Discuss reasons for why a person, today, may want to be acquainted with the night sky. D. Constellations commemorate great heroes, dangers, or events in the legends of the people who name them. Suppose we had to start from scratch today, naming the patterns of stars in the sky. Whom or what would you choose to commemo... |
to be a planet. Explain why. 12. Is the ecliptic the same thing as the celestial equator? Explain. 13. What is an asterism? Can you name an example? 14. Why did Pythagoras believe that Earth should be spherical? 15. How did Aristotle deduce that the Sun is farther away from Earth than the Moon? 16. What are two ways i... |
the Sun is a minor star on the outskirts of one galaxy among billions. Discuss some of the cultural and philosophical implications of each point of view. This OpenStax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 2 Observing the Sky: The Birth of Astronomy 67 29. The north celestial pole a... |
argue against the validity of astrology. 39. What did Galileo discover about the planet Jupiter that cast doubt on exclusive geocentrism? 40. What did Galileo discover about Venus that cast doubt on geocentrism? Figuring For Yourself 41. Suppose Eratosthenes had found that, in Alexandria, at noon on the first day of s... |
not know the details of Earth’s motions any better than the motions of the other planets. Their problem, as we saw in Observing the Sky: The Birth of Astronomy, was that they had to deduce the nature of all planetary motion using only their earthbound observations of the other planets’ positions in the sky. To solve t... |
Stax book is available for free at http://cnx.org/content/col11992/1.8 Chapter 3 Orbits and Gravity 71 At Hven, Brahe made a continuous record of the positions of the Sun, Moon, and planets for almost 20 years. His extensive and precise observations enabled him to note that the positions of the planets varied from thos... |
orbit. Kepler initially assumed that the orbits of planets were circles, but doing so did not allow him to find orbits that were consistent with Brahe’s observations. Working with the data for Mars, he eventually discovered that the orbit of that planet had the shape of a somewhat flattened circle, or ellipse. Next to... |
board, and then looping a string around the tacks. Each tack represents a focus of the ellipse, with one of the tacks being the Sun. Stretch the string tight using a pencil, and then move the pencil around the tacks. The length of the string remains the same, so that the sum of the distances from any point on the elli... |
cnx.org/content/col11992/1.8 Chapter 3 Orbits and Gravity 73 Kepler’s second law deals with the speed with which each planet moves along its ellipse, also known as its orbital speed. Working with Brahe’s observations of Mars, Kepler discovered that the planet speeds up as it comes closer to the Sun and slows down as it... |
spheres” as he called it. For many years he worked to discover mathematical relationships governing planetary spacing and the time each planet took to go around the Sun. In 1619, Kepler discovered a basic relationship to relate the planets’ orbits to their relative distances from the Sun. We define a planet’s orbital ... |
? Solution From Kepler’s third law, we know that (when we use units of years and AU) P2 = a3 If the object’s orbit has a semimajor axis of 4 AU (a = 50), we can cube 50 and then take the square root of the result to get P: P = a3 P = 50 × 50 × 50 = 125,000 = 353.6 years Therefore, the orbital period of the object is ab... |
= 0.38 year and a3 = 0.72 × 0.72 × 0.72 = 0.37 AU (rounding numbers sometimes causes minor discrepancies like this). The orbital period (0.38 year) approximates the semimajor axis (0.37 AU). Therefore, Venus obeys Kepler’s third law. For Earth, P2 = 1.00 × 1.00 = 1.00 year and a3 = 1.00 × 1.00 × 1.00 = 1.00 AU. The or... |
the observations and rules assembled by Galileo, Brahe, Kepler, and others. Newton was born in Lincolnshire, England, in the year after Galileo’s death (Figure 3.6). Against the advice of his mother, who wanted him to stay home and help with the family farm, he entered Trinity College at Cambridge in 1661 and eight ye... |
in opposite directions). In the original Latin, the three laws contain only 59 words, but those few words set the stage for modern science. Let us examine them more carefully. Interpretation of Newton’s Laws Newton’s first law is a restatement of one of Galileo’s discoveries, called the conservation of momentum. The l... |
that is, to start it moving, to speed it up, to slow it down, to stop it, or to change its direction. As you learned in Observing the Sky: The Birth of Astronomy, the rate of change in an object’s velocity is called acceleration. Newton showed that the acceleration of a body was proportional to the force being applied ... |
the same change of momentum by accelerating only a very small amount. Things fall toward Earth all the time, but the acceleration of our planet as a result is far too small to be measured. A more obvious example of the mutual nature of forces between objects is familiar to all who have batted a baseball. The recoil yo... |
� as indications of density (rather than weight) as, for instance, when we say that iron is heavy or that whipped cream is light. The units of density that will be used in this book are grams per cubic centimeter (g/cm3).[1] If a block of some material has a mass of 300 grams and a volume of 100 cm3, its density is 3 g... |
, if the motion of a particular object takes place at a constant velocity at a fixed distance from the spin center—then the angular momentum is also a constant. Kepler’s second law is a consequence of the conservation of angular momentum. As a planet approaches the Sun on its elliptical orbit and the distance to the sp... |
gravity might extend as far as the Moon and produce the force required to curve the Moon’s path from a straight line and keep it in its orbit. He further hypothesized that gravity is not limited to Earth, but that there is a general force of attraction between all material bodies. If so, the attractive force between t... |
but is it really universal? The gravitational theory should also predict the observed acceleration of the Moon toward Earth as it orbits Earth, as well as of any 82 Chapter 3 Orbits and Gravity object (say, an apple) dropped near Earth’s surface. The falling of an apple is something we can measure quite easily, but ca... |
Learning By what factor would a person’s weight at the surface of Earth change if Earth had its present size but only one-third its present mass? Answer: With one-third its present mass, the gravitational force at the surface would reduce by a factor of 1/3, so a person would weight only one-third as much. Gravity is ... |
), Naoko Yamzaki (JAXA), Dorothy Metcalf-Lindenburger (NASA), and Stephanie Wilson (NASA). (credit: NASA) When falling, they are in free fall and accelerate at the same rate as everything around them, including their spacecraft or a camera with which they are taking photographs of Earth. When doing so, astronauts exper... |
in which we do need to include the two mass terms—for example, when two stars or two galaxies orbit each other. Including the mass term allows us to use this formula in a new way. If we can measure the motions (distances and orbital periods) of objects acting under their mutual gravity, then the formula will permit us... |
years. The formula is a3 = M1 × P2, so a3 = 2 × 42 = 2 × 16 = 32. So a is the cube root of 32. To find this, you can just ask Google, “What is the cube root of 32?” and get the answer 3.2 AU You might like to try a simulation (https://openstaxcollege.org/l/30phetsimsunear) that lets you move the Sun, Earth, Moon, and ... |
orbits of rather low eccentricity. The most eccentric orbit is that of Mercury (0.21); the rest have eccentricities smaller than 0.1. It is fortunate that among the rest, Mars has an eccentricity greater than that of many of the other planets. Otherwise the pre-telescopic observations of Brahe would not have been suff... |
with those of the planets Mercury, Venus, Earth, Mars, and Jupiter (black circles). Shown in red are three comets: Halley, Kopff, and Encke. In blue are the four largest asteroids: Ceres, Pallas, Vesta, and Hygeia. Orbital Data for the Planets Planet Semimajor Axis (AU) Period (y) Eccentricity Mercury Venus Earth Mars... |
it is in orbit, a great deal of energy is required to lift the spacecraft off Earth and accelerate it to orbital speed. To illustrate how a satellite is launched, imagine a gun firing a bullet horizontally from the top of a high mountain, as in Figure 3.11, which has been adapted from a similar diagram by Newton. Imag... |
ictional drag is generated by the atmosphere on these satellites, eventually leading to a loss of energy and “decay” of the orbit. 90 Chapter 3 Orbits and Gravity Figure 3.12. Satellites in Earth Orbit. This figure shows the larger pieces of orbital debris that are being tracked by NASA in Earth’s orbit. (credit: NASA/... |
more than a pair of bodies revolving around each other. In fact, all the planets exert gravitational forces upon one another as well. These interplanetary attractions cause slight variations from the orbits than would be expected if the gravitational forces between planets were neglected. The motion of a body that is ... |
of a new planet in 1846. The Discovery of Neptune The discovery of the eighth planet, Neptune, was one of the high points in the development of gravitational theory. In 1781, William Herschel, a musician and amateur astronomer, accidentally discovered the seventh planet, Uranus. It happens that Uranus had been observe... |
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