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Look! Up in the Sky!A quick introduction to the sky above usand how it appears to changeMatthew A. d’AlessioCalifornia State University, NorthridgeandMary LuskSouthwest Florida College(CK-12 Open-source textbook project)This chapter is set up to help you understand why objects in the sky appear to move. It assumes you have some basic familiarity with the Sun, planets, and sky above you from your K-12 education in California.Copyright informationThis work is adapted from the CK-12 FlexBook, an open-source textbook developed by a non-profit foundation to meet science content standards in the State of California.The chapter was originally written by Mary Lusk and distributed under the Creative Commons Attribution-Share Alike 3.0 Unported copyright license (see below). About 50% of the text is unmodified from the original source and presented in this text without quotation marks. Original Source: , accessed Feb. 17, 2010.Original Author: Mary Lusk, Southwest Florida CollegeCreative Commons Attribution-Share Alike 3.0 UnportedYou are free:?to Share — to copy, distribute and transmit the work?to Remix — to adapt the workUnder the following conditions:?Attribution — You must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of the work). ?Share Alike — If you alter, transform, or build upon this work, you may distribute the resulting work only under the same, similar or a compatible license.To read the complete legal code, visit Image Sources Cover: See Figure 2Fig. 1. Matthew d’Alessio, released to public domain.Fig. 2. Brasstown Bald mountain, Georgia, in 1985. Credit: Jimmy Westlake., Released to NASA and public domain, . 3. . GNU-FDL.Fig. 4. Modified from: . 5. Matthew d’Alessio, released to public domain. Image and text inspired by . Data from: . 6. . 7. . 8: . 1. A view through a skylight at night.An intriguing observationOne weekend, I visited family that live out in the country. They have a skylight in their bedroom, so I stared out at the dark sky as I tried to drift off to sleep on their uncomfortable guest bed. As my eyes adjusted to the night time, I started to notice that my little window was filled with beautiful stars. A bright, blinking light zoomed across my portal -- "that thing is moving, so it must be an airplane," I thought. It appeared much brighter than the stars because it was much closer. After the plane disappeared, my eyes were drawn to the simple shape of the constellation "the Big Dipper." I learned to recognize it when I was a child but hardly noticed it since. I marveled as one particularly bright star twinkled near the very edge of the skylight. My mind wandered. When I looked up a few minutes later, that twinkling star was gone. I puzzled over its disappearance. Surely it couldn't have been an airplane -- I watched it stand still for several minutes, and it had not flashed like the airplane. Could it have moved? The clear outline of the big dipper still loomed in the skylight as I drifted off to sleep. I woke up a few hours later with a terrible kink in my neck from the lumpy bed. I grumpily stared back up at my skylight. To my astonishment, the big dipper was nowhere to be seen. I was sure it was there when I fell asleep. First the bright star near the edge disappeared, and now an entire constellation! Perhaps someone had picked up the entire house and moved it during the night?! The idea isn’t as crazy as it sounds…The Sun rises and sets each day, meaning that it appears in one spot, moves across the sky, and then disappears below the horizon in a different spot. The moon does the same thing, but just at a different time. Perhaps the whole sky is moving. If you observe the stars like I did, that's the conclusion you might come to. The moon, the stars, and even the Sun all travel across the sky at the same speed, reappearing in approximately the same spot once every 24 hours.?Wait! Once every 24 hours? That's the length of a day. In fact, the sky isn't really moving, but my theory that someone picked up the house and moved is basically correct. The house sits on the Earth, which constantly spins on its axis. The house is moving because it’s attached to the spinning planet. How do we know??Fig. 3. A pendulum at the north pole always swings in the same direction as the Earth moves beneath it.Earth’s rotationFig. 2. Star trails. Normally a camera snaps a picture in a fraction of a second. To take this picture of the sky, the photographer left the shutter open for several hours. The stars appear to spin in a circle throughout the night because the earth rotates. The straight lines are “shooting stars” that flew across the sky.Today we can go out into space and watch Earth spin, but back in 1851, a French scientist named Léon Foucault figured out how to show that the Earth rotates by staying put on the ground. He attached a heavy iron ball to a really long string, pulled the ball to one side and then released it, letting it swing back and forth in a straight line. A ball swinging?back and forth on a string is called a pendulum. A pendulum set in motion, will not change its motion, so it?will not change the direction of the swinging. However, Foucault observed that his pendulum did seem to?change direction over the course of several hours. He knew that the pendulum itself could not change its motion, so he concluded that the?Earth, underneath the pendulum was moving.?Placing this pendulum at the North or South Pole, it would take 23 hours, 59 minutes and 4 seconds for the pendulum to swing back to where it first started – the time it takes Earth to spin 360° around on its axis. At the equator, the Earth rotates at a speed of about 1,700?kilometers per hour. Thankfully, we do not notice this movement, because it would certainly make us dizzy.The Sun and Moon appear to move across the sky and the stars do the same. As the Earth rotates, observers on Earth see the Sun moving across the sky from east to west with the beginning of each new day. We often say that the Sun is “rising” or “setting,” but actually it is the Earth’s rotation that gives us this perception. When we look at the Moon or the stars at night, they also seem to rise in the east and set in the west. Earth’s rotation is also responsible for this. A ballet dancer or a figure skater constantly sees different things coming into and out of his vision as he spins rapidly around on his toes even though the skating arena doesn’t move at all. The same is true for residents of Earth looking up at the sky.Earth’s revolutionFig. 4. Earth rotates around its axis slightly more than 360° in one day. By the time it spins 360°, it has moved along on its path around the Sun. This image exaggerates the effect, which is why it has the label “not to scale.” Thus far, we’ve explained more than 23 hours and 59 minutes of Earth’s “day.” What about the other the other 56 seconds? To understand why our day is almost exactly 24 hours, you need to know that Earth doesn’t just spin in place like an ice skater doing pirhouettes. It also does constant laps around the Sun in a motion commonly called Earth’s “revolution.” Both rotation and revolution have similar English meanings (to spin), but scientists like to use the words for different types of circular motion – Earth’s revolution on its orbit around the Sun takes much longer than its rotation on its axis. One complete revolution takes 365.24 days, or one year. How does revolution affect the length of a day, which is mostly caused by rotation? Figure 4 shows the Earth at three different times. At time 1, people standing on top of the red triangular mountain see the Sun by looking straight up. At time 2, Earth has rotated half way around and those same people will see the dark night sky when they look straight up. After one complete rotation (time 3), the mountain is back where it started from and the people are looking the exact same direction when they look straight up. While it will be daylight they don’t see the Sun directly above them (it’s a little off to the east). Why? Because the Earth moved further along in its orbit. In order to rotate around to the point where the Sun is directly overhead, you need to time to rotate a bit more (time 4). In fact, it’s an extra 56 seconds, during which time the Earth rotates an extra 0.23° around its axis (Fig. 4 exaggerates this effect, which is why it has the label “not to scale”). One day ends up being 24 full hours. You do not need to remember these numbers, but with a complete understanding of Earth’s rotation and revolution, you should be able to explain the length of Earth’s “day.” How to discover planetsEven though stars rise in the East and set in the west because of Earth’s rotation, the stars always seem to be in the same relative positions and shapes. In fact, every human that has ever lived saw a night sky almost identical to the one you can see. A specific arrangement of stars that forms a recognizable shape or pattern when viewed from Earth is called a constellation (read more about them in a later section). The patterns of stars are so unchanging that names given to constellations by cultures thousands of years ago are still meaningful to us today. When the Big Dipper rises, we know that the constellation Leo will be nearby, surrounded by Cancer the crab to its west and Virgo to its east. The entire sky rotates uniformly so that the signs of the zodiac always appear in the same order (you’d think Leo’s mighty lion could outpace its neighbor, Cancer, a crab. But it never happens). The sky changes so little that people notice exceptions very easily. Shooting stars are obvious ones. They can appear to streak half way across the visible sky in a fraction of a second. In reality, they are bits of space dust and asteroids. Air resistance causes them to heat up and glow as they fly through our atmosphere. From the time it starts to glow to the time it stops, a shooting star probably travels only a few hundred miles. Why do they look like they travel so far? Because they are so much closer than any of the other stars. A meteor is usually just 40-60 miles above the surface of the Earth, while a regular star is trillions of miles away. People in Los Angeles will rarely see the same shooting stars as people in San Francisco, about 350 miles away because meteors burn up so low to the ground (compared to regular stars). The rest of the stars in the sky will look almost identical in the two cities.A much less dramatic example of stars that move out-of-step with the rest of the sky is a series of “wandering stars.” One of these stars can appear in front of the constellation Leo one night and then Cancer a few nights after that. They seem to wander across the sky, but at a much slower pace than shooting stars. While it could be that they are moving slower than shooting stars, another option is that they are much further away from Earth. Once humans developed telescopes, they started looking more closely at these wandering stars and found that they were totally different than all the other stars. Each one had a distinct round shape (like our moon does) and surface features (some had dark spots and light patches like craters on our own moon, others had dramatic rings around them). In fact, these wandering “stars” are not regular stars at all! They are what we now call the planets (the word planet is derived from the word for “wanderer” in Greek). People were able to recognize them as different from other stars long before they could see them up close with telescopes because planets don’t rotate across the sky at the same pace as the other constellations. Watching how fast each planet moves relative to the rest of the stars has helped us realize that each planet revolves around the Sun at its own pace and at a different distance away from the Sun. What are "stars"?When you look up in the sky, you probably call every point of light a "star." When you start looking closely at the stars, you might notice subtle differences. Some stars are brighter and some are slightly different colors ranging from blue-ish white to orange-ish white. But if you used a telescope to look much closer, you might see that the differences don't stop there. There are at least 4 broad categories of "stars" that I list below in order of distance away from Earth from closest to farthest.Things close to Earth (satellites). Very few of the stars in the sky are in this category that includes objects in Earth's atmosphere and orbiting the planet just outside the atmosphere. We already discussed "shooting stars" which burn up in Earth's atmosphere. Did you know that you can also see the communication satellites, the International Space Station, and pieces of random space garbage orbiting Earth if you look up in the night sky? It's rare because you have to be looking in the right place at the right time. Because they are so close to Earth, they appear to move across the sky as you watch them.Things that orbit the Sun (planets). Even though these are some of the brightest objects in the sky, they don't actually create any of their own light! Light from the Sun bounces off the surface of planets like a mirror, sending that light into our eyes. They are bright because they are close, compared to the rest of the stars. Individual stars "nearby." These are the only true "stars" where a single point of light corresponds to a single glowing ball of gas like our own Sun. They appear smaller than our Sun because they are much further away (our Sun is actually medium-sized compared to many of the other stars in the sky). It takes light from our Sun just 8 minutes to get to Earth, but it takes 4 years for the light to reach us from the next closest star. All the individual stars we see are in our own Milky Way galaxy. A galaxy is another word for a cluster of stars that are close together. Galaxies often have spiral shapes, with individual stars orbiting around the center of the galaxy in much the same way planets revolve around the Sun in our solar system. Galaxies and individual stars form by a process similar to the formation of our solar system and its individual planets –?gravity attracts clumps of material together from a really huge cloud of dust and gas. Like our solar system, most all the stars in a galaxy rotate and revolve in the same direction that is related to the way the huge cloud was originally rotating. (More about these last two sentences in later sections).Our galaxy is about 100,000 light years across, and the Sun is about 25,000 light year's from its center. So when we say an individual star is "nearby," it's still could be really, really far from Earth. There are perhaps 400 billion stars in our galaxy, but most of them are too dim for us to see from Earth. On a dark night in the countryside, your naked eye can probably only see about 8,000 of them. Around cities and towns, you might only be able to see a few hundred to a few thousand stars because the glowing light from neighborhoods is brighter than many dim stars. Groups of stars that are very far away (other galaxies). Some of the "individual" lights in our sky are actually billions and billions of really bright stars clustered together in a galaxy. Because these other galaxies are so unbelievably far away, they look like a single dot to our naked eye. Using a powerful telescope, you can start to see the shape and size of the galaxy and that it is made up of many individual stars. The furthest galaxies scientists can see are more than 10 billion light years away. That's millions of times further than any of the individual stars in our own galaxy. Fewer than one hundred of the stars you see with your naked eye are actually galaxies, but there are literally billions of galaxies out there that we can start to see with telescopes.While these four categories of objects are completely different, they appear almost the same to you as a person looking up in the sky with your naked eye.What are constellations?Fig. 5. The stars in each constellation make look like they are close to one another, but they don’t have to be. The sky appears flat even though deep space goes off in all three dimensions.The stars that make up the handle of the Big Dipper appear related to the stars that make up the pot to a person looking up from Earth; they are not. The problem is that the sky looks flat, but space goes out in three dimensions. With your naked eye, you can't tell how far away a star is, and some of the stars in the Big Dipper are much closer to us than others. The same is true of all the constellations. A constellation is just a pattern of stars when viewed from Earth. They appear to move together in the night sky as Earth rotates, but an alien living around one star in the constellation Leo the lion's shoulder might be able to visit Earth a lot more easily than it could visit the star that makes up the lion's foot because one star is much closer to Earth than the other. Stars in a galaxy all rotate around the same galactic center, but stars in a constellation can be in completely different galaxies. Do the stars change from month to month?Earth’s revolution around the Sun also affects what we see during the night sky. As we do laps around the Sun, the direction Earth faces at night changes (night time is always seen on the side of the Earth in the direction away from the Sun). That means that people on Earth are able to see a different set of stars from one part of the year to another. Constellations that we can see when looking away from the Sun in the winter show up behind the Sun during the summer. We can’t see stars when they are in the same direction as the Sun because the Sun appears so much brighter than the stars. You can demonstrate this effect when you turn a light on during the daytime. If the Sun is shining brightly, you won’t even notice a change when the electric light is on.While we can see different stars at different times of year, the stars themselves don’t change much. As we move around the Sun, you might be tempted to think that stars might look bigger during summer compared to winter because we are closer to them. It turns out that they don’t look closer because stars are really, really far away. One lap around the Sun is an astonishing 584 million miles, but that’s nothing compared to the 25 trillion miles to the closest star. Scientists can measure the differences using careful observations, but you wouldn’t notice any difference with your naked eye. So while stars are exciting, active, and constantly changing when you look really close, our night sky is relatively constant to an everyday observer. It only appears to change because our perspective changes. As Earth rotates and revolves, we look in different directions at different sections of the vast sky from hour to hour and season to season. The origin of our Solar System: Why is everything spinning? Fig. 6. Our solar system started out as a cloud of dust, gas, and frozen flakes of stuff. Gravity caused it to turn into what we see today. This artist’s drawing shows what the early solar system would have looked like.Earth rotates about its axis and revolves around the Sun. Look at the arrows in Figure 4 illustrating this motion as viewed from far above the North Pole. Which way does Earth revolve, clockwise or counter-clockwise? How about its revolution direction? If you watch the other planets in our solar system from this viewing point, all of them revolve around the Sun counter-clockwise. All the planets also rotate. Can you guess which direction? All but one rotate about their own axes counter-clockwise. Even the Sun itself rotates around its own axis in the same direction that Earth does. This key feature is a major clue to how the solar system formed.A Giant Nebula The most widely accepted explanation of how the solar system formed is called the nebular hypothesis. According to this hypothesis, the solar system formed about 4.6 billion years ago from the collapse of a giant cloud of gas and dust, called a nebula. The nebula was made mostly of hydrogen and helium, but there were heavier elements as well. The nebula was drawn together by gravity. As the nebula collapsed, it started to spin. As it collapsed further, the spinning got faster, much as an ice skater spins faster when he pulls his arms to his sides during a spin move. This effect, called “conservation of angular momentum,” along with complex effects of gravity, pressure, and radiation, caused the nebula to form into a disk shape, as shown in the picture below. This is why all the planets are found in the same plane. What does “same plane” mean? If you ever see a table-top model of the solar system, all the planets revolving around the sun in a roughly flat region. None of them want to fly up high above the table or plunge below its surface. They all sort of dance along the table top. In the earliest stages of the solar system, that wasn’t the case. The nebula went off in all directions like a big ball of gas. After spinning and spinning, gas from high above got drawn downward while the gas from below was attracted upward. The ball flattened out until almost all the material was in a single flat disk. Formation of the Sun and Planets Even though the disk was flat, gravity continued to pull on material. More and more material accumulated in the center of the disk in a runaway process. Because the strength that gravity pulls depends on the amount of material, the disk pulled harder and harder on the surrounding material as it grew. Slowly, the density and pressure increased at the center of the spinning nebula. When the pressure in the center was high enough that nuclear fusion reactions started in the center, a star was born—the Sun. Meanwhile, the outer parts of the disk were cooling off. Small pieces of dust in the disk started clumping together. These clumps collided and combined with other clumps. Larger clumps, called planetesimals, attracted smaller clumps with their gravity. I often call them “planets-to-be.” Eventually, the planetesimals grew bigger so that they now form the planets and moons that we find in our solar system. The outer planets—Jupiter, Saturn, Uranus and Neptune—condensed farther from the Sun from lighter materials such as hydrogen, helium, water, ammonia, and methane. Out by Jupiter and beyond, where it’s very cold, these materials can form solid particles. But in closer to the Sun, these same materials are gases. As a result, the inner planets—Mercury, Venus, Earth, and Mars—formed from dense rock, which is solid even when close to the Sun. Fig. 7. An artist’s drawing of the early solar system, looking towards the newly formed Sun. Small clumps of debris orbit the Sun and will eventually combine to become larger planets. Fig. 8. The Sun and planets shown with accurate relative sizes. Compare the four small planets close to the Sun with the four huge planets further away. The distance between planets is not accurate in this illustration.Understanding the Solar SystemPicture yourself on a spaceship leaving the massive sun and heading outward towards each of the planets. There are dozens of online tours of the planets, so you can explore each alien world virtually to your heart's delight. Our tour in this chapter is very brief.Inner rocky planets versus outer gas giantsYou'll pass by four small planets made of rock and metal. Each one has slightly different surface features, but their composition, density, and even size are all remarkably similar. Comparing the largest and smallest of these inner planets, the diameter of Earth is less than three times bigger than Mercury. As you head out away from these small planets, things change dramatically. The next four planets are much, much bigger. You could fit more than 1,300 Earths inside of the planet Jupiter, but it is made mostly of hydrogen and helium gas. While observations of their mass and size tells us that they have a rock and metal core, anyone trying to land on the surface of these gigantic planets would sink in because their outer layers are gas and liquid. Humans have landed probes on the rocky surfaces of all the inner planets, but will never be able to do that for the outer gassy planets. Why are they different?We can understand the difference between the inner and outer planets by understanding how they formed from the nebular cloud. Recall that each planet was built up as gravity drew together particles into larger and larger clumps. Whatever materials were in the cloud would be the materials that would make up each planet as it formed. Hydrogen and helium are by far the most common elements in the Universe, and we can assume that this was also true about the nebular cloud that turned into our solar system. In fact, more than 98% of the solar system mass is hydrogen and helium, which is indeed the same as the overall composition of the Universe. The composition of the Sun is similar to the outer planets, so there is no reason to believe that some spots of the nebula had different composition than others. The frost lineThere must be some reason that the inner planets don't have much gas. The answer is actually pretty easy to understand: it's hot close to the Sun. Don't laugh! Molecules move fast and freely in a gas, without feeling a strong attachment to one another. As a planet begins to grow, its gravity attracts more and more molecules of stuff. Gas molecules, being so free, can more easily escape the gravitational pull of a small planet-to-be. Solids, however, tend to clump together and stick around once they are attracted close to the growing planet-to-be. When they are small, planets grow best by accumulating only flakes of solid, frozen material. Inner planets are made of only rock and metal: Because it is so hot close to the Sun, most materials are gases. Only rocks and metal are solid close to the Sun, so planets made out of only rock and metal formed.Outer planets are bigger: At a certain distance away from the Sun, it gets cool enough that many common materials like water and methane gas condense into little frozen flakes of material. Scientists like to call this special distance the frost line. When you get further than the frost line, there are a lot more solid frozen flakes in a nebula. Because there is more solid available to clump together, planets far from the Sun grew bigger. They accumulated rock, metal, and solid icy materials like frozen water and frozen methane. Outer planets have lots of hydrogen and helium gas: If Jupiter was made of only solid flakes, it would be about 3 times bigger than Earth. The bulk of it's mass, however, comes from hydrogen and helium gases. Both remain gases even at the lowest temperatures in our solar system. Since they never form solids, you might think that they would never be captured by a planet. However, even as a gas, they still feel the pull of gravity. Planets exert stronger and stronger gravitational pull as they get bigger, and eventually a planet can grow large enough that it and can retain these gases. The outer planets, especially Jupiter and Saturn, have reached that size. The inner planets were not able to hold onto much of this gas, which is why helium balloons are such a special treat on Earth. Everything in the previous section is probably wrongThis “frost line” explanation seemed to explain everything perfectly. However, in the last decade scientists have started discovering other solar systems around other stars. Rocky planets like Earth are too small to detect at this point, but we have discovered hundreds of huge planets that are made out of hydrogen and helium around hundreds of different stars. What's weird is that many of these huge gassy planets are very close to their star. According to our theory of planet formation, they should never have been able to accumulate gas so close to the star. The story in this chapter says that it is too hot for these planets to form, but they do form. So while this explanation makes lots of sense, new observations are starting to call it into question. Maybe there are some other minor details that need to be considered to explain the new solar systems, or maybe the theory we've got is dead wrong and we'll need to start over from scratch in order to explain all the information we have now. That's what makes science exciting. Who will figure out the answer? Maybe one of your students if you can get them energized about science and discovery! How does gravity do it?If gravity draws objects together, wouldn’t Earth and the other planets crash into the Sun instead of revolving around it? Imagine that you are a few feet of string with a rock tied to the end. If you let the rock hang straight down, the only force the string applies is straight up towards your hand. If you try to push the rock away from you using only the string, the string will bend. It can’t transmit a force away from your hand. In that way, it acts like gravity that ONLY pulls objects together. Nonetheless, you can take that string over your head and twirl the rock around in a circle. This should help convince you that the force of gravity can keep a planet in orbit. As the string tries to fly away, the string yanks it back. Instead of flying away, it flies around in a circle. The Earth revolves around the Sun because gravity keeps it in a roughly circular orbit around the Sun. The Earth’s orbital path is not a perfect circle, but rather an ellipse, which means that it is like a slight oval in shape (Figure 24.10). This creates areas where the Earth is sometimes farther away from the Sun than at other times. We are closer to the Sun at perihelion (147 million kilometers) on about January 3rd and a little further from the Sun (152 million kilometers) at aphelion on July 4th. Students sometimes think our elliptical orbit causes Earth’s seasons, but this is not the case. If it were, then the Northern Hemisphere would experience summer in January!Figure 24.10: Earth and the other planets in the solar system make regular orbits around the Sun; the orbital path is an ellipse and is controlled by gravity. (5)During one revolution around the Sun, the Earth travels at an average distance of about 150 million kilometers. Mercury and Venus take shorter times to orbit the Sun than the Earth, while all the other planets take progressively longer times depending on their distance from the Sun. Mercury only takes about 88 Earth days to make one trip around the Sun. While Saturn, for example, takes more than 29 Earth years to make one revolution around the Sun. ................
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