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Neil F. Comins Discovering the Essential Universe Fifth Edition CHAPTER 1 Discovering the Night Sky In this chapter you will discover… • how astronomers organize the night sky to help them locate objects in it. • that Earth’s spin on its axis causes day and night. • how the tilt of Earth’s axis of rotation and Earth’s motion around the Sun combine to create the seasons. • that the Moon’s orbit around Earth creates the phases of the Moon. • what causes both lunar and solar eclipses. • the scales of the universe. WHAT DO YOU THINK? 1. What do astronomers define as constellations? 2. Is the North Star—Polaris—the brightest star in the night sky? 3. What causes the seasons? 4. How many zodiac constellations are there? 5. Does the Moon have a dark side that we never see from Earth? 6. Is the Moon ever visible during the daytime? 7. What causes lunar and solar eclipses? Navigating the Night Sky • Constellations • Seasonal Asterisms – Star Guides In order to more easily locate objects in the sky, we divide the sky into regions named after familiar patterns of stars called constellations. The Night Sky Without and With Light Pollution Ancient constellations were imaginary pictures outlined by familiar patterns of stars. Modern astronomers divide the sky into 88 official constellations or regions of space, many of which contain the ancient star patterns. 1 Common Guides to Finding Constellations and Stars Using the “Big Dipper” as a guide Common Guides to Finding Constellations and Stars Common Guides to Finding Constellations and Stars The “Winter Triangle” Insight into Science Compare What You Believe with What Nature and Science Show You If you think Polaris is the brightest star in the night sky, how did you learn that? There is likely to be a lot of information you learned from a variety of sources that you will have to unlearn while learning astronomy. While this process is often uncomfortable, it is essential if you want to understand how science works and how scientists view the world. Be careful: It is human nature to change new information to fit our current beliefs rather than change what we believe. Fight that urge if science can show you that your beliefs are incorrect! The “Summer Triangle” The Celestial Sphere • Projection of Earth’s equator, poles • Earth Latitude = Celestial Declination • Earth Longitude = Celestial Right Ascension Astronomers describe the universe as an imaginary sphere surrounding the Earth on which all objects in the sky can be located, called the CELESTIAL SPHERE. As viewed from Earth, the celestial sphere appears to rotate around two axis points, the north and south celestial poles, which are located directly above the Earth’s poles. Between these is the celestial equator, which divides the celestial sphere into northern and southern hemispheres. We define the position of an object on the celestial sphere using two coordinates, right ascension and declination. 2 Angles and Arc Angles Cyclic motions of the Sun and stars in our sky are due to motions of Earth. 1. ROTATION = the spin of Earth on its axis. It takes one day for Earth to complete one rotation. 2. REVOLUTION = the movement of Earth in orbit around the sun. It takes one year for Earth to complete one revolution. 3. PRECESSION = the slow conical (top-like) motion of Earth’s axis of rotation. It takes 26,000 years for Earth to complete one cycle of precession. • • • • • The Big Dipper 90 – horizon to top of sky (zenith) 5 – angle between Pointer Stars of Big Dipper 1 = 60 arcminutes = 60’ 1’ = 60 arcseconds = 60” The Moon is ½ or 30’ or 1800” in diameter Star Motions • • • • • Circumpolar Equatorial In Between Rotation of Earth Revolution of Earth around Sun The angular distance between the two “pointer stars” at the front of the Big Dipper is about 5°. Circumpolar Star Trails The apparent westward motion of the Sun, Moon, and stars across our sky each day is caused by Earth’s rotation. At middle latitudes, we see the Sun, Moon, and many of the stars first come into view moving upward, rising at some point along the eastern horizon. Then, they appear to arc across the sky. Finally, they disappear beneath the western horizon. We generalize this motion to make statements such as, “The Sun rises in the east and sets in the west.” The stars near the poles of the celestial sphere (shown here) move in trails that circle the pole and never set. They are called circumpolar. 3 Motion of Stars at the Poles Because Earth rotates around its poles, stars seen from these locations appear to move in huge, horizontal circles. This is the same effect you would get by standing up in a room and spinning around; everything would appear to move in circles around you. At the North Pole, stars move left to right, while at the South Pole, they move right to left. Rising and Setting of Stars at Middle Northern Latitudes Unlike the motion of the stars at the poles, the stars at all other latitudes do change angle above the ground throughout the night. This time-lapse photograph shows stars setting. The latitude determines the angle with respect to the horizon at which the stars rise and set. Earth also revolves around the Sun, which changes our view of the stars. Rising and Setting of Stars at the Equator Standing on the equator, you are perpendicular to the axis around which Earth rotates. As seen from there, the stars rise straight up on the eastern horizon and set straight down on the western horizon. We can see how different stars appear at different times of day by looking at the position of the Sun against the backdrop of stars. The side of Earth facing the Sun is experiencing “day,” while the side of Earth turned away from the Sun is experiencing “night.” SEPTEMBER MARCH Insight into Science Define Your Terms From our perspective, the Sun appears to move through the stars along a special path called the ecliptic. From an outside view, we see Earth revolve around the Sun. We define the plane of Earth’s orbit as the ecliptic plane. As with interpersonal communication, using the correct words is very important in science. Usually, scientific words each have a specific meaning, such as “rotation” denoting spin and “revolution” meaning one object orbiting another. Be especially careful to understand the context in which words with more than one meaning, such as “ecliptic” and “constellation,” are used. 4 Seasons • The reason for the seasons is the Earth’s tilt • In different seasons we receive different intensities of sunlight and heat • In the summer, sunlight is more direct and less spread out • In the winter, sunlight is less direct and more spread out The seasons we experience are due to the Sun’s motion across the celestial sphere. Seasons are caused because Earth’s axis is tilted with respect to its orbital plane; as Earth revolves around the Sun, different parts of Earth receive more direct sunlight (summer), whereas other parts of Earth receive sunlight that is more spread out (winter). The Sun’s Daily Path and the Energy It Deposits Here The point of the Sun’s path farthest north on the celestial sphere is called the summer solstice (JUN 21), whereas the point of the ecliptic farthest south is called the winter solstice (DEC 21). The two points on the ecliptic where the Sun crosses the celestial equator are called equinoxes. During the vernal equinox (MAR 21), the Sun is moving north, while during the autumnal equinox (SEPT 21), the Sun is moving south. Remember that the seasonal names of the equinoxes and the solstices refer to seasons in the NORTHERN hemisphere. The seasons occurring in the SOUTHERN hemisphere are exactly opposite. Insight into Science (a) On the winter solstice―first day of winter―the Sun rises farthest south of east, it is lowest in the noontime sky, stays up the shortest time, and its light and heat are least intense (most spread out) of any day of the year in the northern hemisphere. (b) On the vernal equinox―first day of spring―the Sun rises precisely in the east and sets precisely in the west. Its light and heat have been growing more intense, as shown by the brighter oval of light than in (a). (c) On the summer solstice―first day of summer― the Sun rises farthest north of east of any day in the year, is highest in the sky at noontime, stays up the longest time, and its light and heat are most intense of any day in the northern hemisphere. (d) On the autumnal equinox, the same astronomical conditions exist as on the vernal equinox. The Midnight Sun Expect the Unexpected The process of science requires that we question the obvious, that is, what we think we know. Many phenomena in the universe defy commonsense explanations. The fact that the changing distance from Earth to the Sun has a minimal effect on the seasons is an excellent example. This time-lapse photograph was taken on July 19, 1985, at 69° north latitude in northeastern Alaska. At that latitude, the Sun is above the horizon continuously from mid-May until the end of July. 5 Precession This precession causes the position of the North Celestial Pole to slowly change over time. Today, the North Celestial Pole is near the star Polaris, which we call the “North Star.” However, in 3000 BC, Thuban was close to the North Celestial Pole and in 14,000 AD, Vega will be in this location. Gravitational forces of the Sun and the Moon pulling on Earth as it rotates cause Earth to undergo a top-like motion called precession. Over a period of 26,000 years, Earth’s rotation axis slowly moves in a circular motion. Phases of the Moon • Caused by the changing amount of illumination seen from the Earth as the Moon revolves around it. Another familiar cycle is the lunar cycle. When the Moon orbits Earth, the amount of the side facing Earth that is lit changes, creating the Moon’s phases. This phase cycle is called the synodic period and is 29½ days long. 6 A synodic month is the time it takes for the Moon to orbit Earth with respect to the Sun and is 29½ days long. One common misconception is that the Moon is only visible at night. However, the time of day in which the Moon is in our sky varies depending on its phase. This picture clearly displays the Moon, visible during the day. A sidereal month is the time it takes for the Moon to orbit Earth with respect to the stars and is 27.3 days long. The two times are different because Earth moves in its orbit around the Sun as the Moon moves in its orbit around Earth. Eclipses • Solar Eclipse – the Moon shadows the Earth During a new or full moon phase, when the Moon, Sun, and Earth are aligned, the Moon may enter the shadow of Earth, or the shadow of the Moon may reach Earth, creating eclipses. However, these eclipses do not occur during every full or new moon because the Moon’s orbit is tilted by 5 with respect to the EarthSun (ecliptic) plane. – New Moon • Lunar Eclipse – the Earth shadows the Moon – Full Moon PENUMBRAL = the Moon appears dimmed. PARTIAL = part of the Moon enters the umbra of Earth’s shadow and is darkened. TOTAL = all of the Moon enters Earth’s shadow and becomes a reddish color, only lit from light bending around Earth’s atmosphere. During a total lunar eclipse, the Moon moves in and out of the umbra of Earth’s shadow. 7 Unlike lunar eclipses, solar eclipses occur at specific places on Earth, indicated by the arrow. Eclipse Paths for Total and Annular Eclipses 2001–2020 A Total Eclipse of the Sun This map shows the eclipse paths for the 14 total solar and 13 annular eclipses that occur between 2001 and 2020. In each eclipse, the Moon’s shadow travels along the eclipse path in a generally eastward direction across Earth’s surface. Sometimes eclipses occur when the Moon is too far away from Earth to completely cover the Sun in our sky. When this occurs, the Moon appears in the center and a thin ring, or “annulus,” of light surrounds it. These are called annular eclipses. 8 What Have Astronomers Discovered in Our Universe? THE SCALES OF THE UNIVERSE The range of objects we study are from the extremely small subatomic particles, to objects that are gigantic, such as a galaxy or the size of the known universe itself. Each division up the line indicates an increase in size by 100,000. Navigating the Night Sky Summary of Key Ideas Earthly Cycles • The stars appear to revolve around Earth once each night. In fact, it is actually Earth’s rotation that causes this apparent motion. • Earth revolves around the Sun in 365¼ days, defining a year. • The celestial sphere is an imaginary hollow sphere centered on Earth. A map of the sky, analogous to a map of the entire Earth, is impressed on it. The poles and equator of the celestial sphere are determined by extending the axis of rotation and the equatorial plane of Earth out onto the celestial sphere. • The surface of the celestial sphere is divided into 88 unequal areas called constellations. The Seasons • Earth’s axis of rotation is tilted at an angle of 23½ from a line perpendicular to the plane of Earth’s orbit around the Sun (the plane of the ecliptic). This tilt, combined with Earth’s revolution around the Sun, causes the seasons. • Equinoxes and solstices are significant points along Earth’s orbit that are determined by the relationship between the Sun’s path on the celestial sphere (the ecliptic) and the celestial equator. 9 The Seasons The Phases of the Moon • The length of the day is based upon Earth’s rotation rate and the average motion of Earth around the Sun. These effects combine to produce the 24-h day upon which our clocks are based. • Earth’s axis of rotation slowly changes direction relative to the stars over thousands of years, a phenomenon called precession. • The phases of the Moon are caused by the changing relative positions of Earth, the Moon, and the Sun. The Moon completes one cycle of phases in a synodic month, which averages 29½ days. • The Moon completes one orbit around Earth with respect to the stars in a sidereal month, which averages 27.3 days. Eclipses Eclipses • The shadow of an object has two parts: the umbra, where direct light from the source is completely blocked; and the penumbra, where the light source is only partially obscured. • A lunar eclipse occurs when the Moon moves through Earth’s shadow. During a lunar eclipse, the Sun, Earth, and the Moon are in alignment, with Earth between the Sun and the Moon, and the Moon in the plane of the ecliptic. • A solar eclipse occurs when a strip of Earth passes through the Moon’s shadow. During a solar eclipse, the Sun, Earth, and the Moon are in alignment, with the Moon between Earth and the Sun, and the Moon in the plane of the ecliptic. • Depending on the relative positions of the Sun, the Moon, and Earth, lunar eclipses may be penumbral, partial, or total; solar eclipses may be annular, partial, or total. Scales of the Universe WHAT DID YOU THINK? • Astronomy examines objects that range in size from the parts of an atom (~10–15 m) to the size of the observable universe (~1026 m). • Scientific notation is a convenient shorthand for writing very large and very small numbers. • What do astronomers define as constellations? Astronomers sometimes use the common definition of a constellation as a pattern of stars. Formally, however, a constellation is an entire area of the celestial sphere and all the stars and other objects in it. Viewed from Earth, the entire sky is covered by 88 different-sized constellations. If there is any room for confusion, astronomers refer to the patterns as asterisms. 10 WHAT DID YOU THINK? WHAT DID YOU THINK? • Is the North Star—Polaris—the brightest star in the night sky? No. Polaris is a star of medium brightness compared other stars visible to the naked eye. with WHAT DID YOU THINK? • How many zodiac constellations are there? There are 13 zodiac constellations, the least-known one being Ophiuchus. WHAT DID YOU THINK? • Is the Moon ever visible during the daytime? The Moon is visible at some time during daylight hours almost every day of the year. Different phases are visible during different times of the day. • What causes the seasons? The tilt of Earth’s rotation axis with respect to the ecliptic causes the seasons. They are not caused by the changing distance from Earth to the Sun that results from the shape of Earth’s orbit. WHAT DID YOU THINK? • Does the Moon have a dark side that we never see from Earth? Half of the Moon is always dark. Whenever we see less than a full Moon, we are seeing part of the Moon’s dark side. So, the dark side of the Moon is not the same as the far side of the Moon, which we never see from Earth. WHAT DID YOU THINK? • What causes lunar and solar eclipses? When the Moon is crossing the ecliptic in the full or new phase, the shadows of Earth or the Moon, respectively, then fall on the Moon or Earth. These shadows on the respective surfaces are eclipses. 11