The Sun-Earth-Moon System

CHAPTER 20The Sun-Earth-Moon SystemLESSON 1Earth’s MotionKey ConceptsWhat do you think? Read the two statements below and decidewhether you agree or disagree with them. Place an A in the Before columnif you agree with the statement or a D if you disagree. After you’ve readthis lesson, reread the statements to see if you have changed your mind.BeforeStatementAfter1. Earth’s movement around the Sun causes How does Earth move? Why is Earth warmer at theequator and colder at thepoles? Why do the seasons changeas Earth moves aroundthe Sun?sunrises and sunsets.2. Earth has seasons because its distance fromthe Sun changes throughout the year.Copyright Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Earth and the SunIf you look around you, it does not seem as if Earthis moving. The ground, trees, and buildings do not seem tobe moving. But Earth is always in motion. It spins andmoves around the Sun. Earth’s motion causes changes onEarth. As Earth spins, day changes to night and back to dayagain. The seasons change as Earth moves around the Sun.Summer turns to winter because Earth’s motion changeshow energy from the Sun spreads out over Earth’s surface.Identify Main IdeasHighlight each head in onecolor. Use another color tohighlight key words in theparagraphs under the headthat explain or support thehead. Use your highlightingto review the lesson.The SunThe nearest star to Earth is the Sun. The Sun is about 150million km from Earth. The Sun is much larger than Earth.The Sun’s diameter is more than 100 times greater thanEarth’s diameter. The Sun’s mass is more than 300,000 timesgreater than Earth’s mass.The Sun is a giant ball of hot gases. It emits light andenergy. Inside the Sun, the nuclei of atoms combine toproduce huge amounts of energy. This process is callednuclear fusion. Nuclear fusion produces so much energy thatthe Sun’s core temperature is more than 15,000,000 C. Evenat the Sun’s surface, the temperature is about 5,500 C. Asmall part of the Sun’s energy reaches Earth as light andthermal energy.Reading EssentialsReading Check1. Explain What is oneeffect of nuclear fusion?The Sun-Earth-Moon System345

Earth’s OrbitThe motion of one object around another object is calledrevolution. Earth makes one complete revolution around theSun every 365.24 days. The path an object follows as it movesaround another object is an orbit. Earth orbits the Sun in analmost circular path. Earth’s orbit is shown below.Earth’s motionwithout the Sun’sgravitational pullEarthVisual Check2. Describe What is theshape of Earth’s orbit aroundthe Sun? (Circle the correctanswer.)a. circleb. near circlec. a straight line3. Identify What producesEarth’s revolution around theSun?Reading Check4. Identify In whichdirection does Earth rotate?346The Sun-Earth-Moon System152 millionkm147 millionkmJanuary 3Earth is closestto the Sun.SunEarth’s OrbitThe Sun’s Gravitational PullEarth orbits the Sun because the Sun’s gravity pulls onEarth. The strength of gravity’s pull between two objectsdepends on the masses of the objects and the distancebetween them. An object with more mass has a greater pullof gravity than an object with less mass. Likewise, gravity’spull is greater on objects that are closer together.Earth’s orbit around the Sun, shown above, is like themotion of an object twirled on a string. The string pulls onthe object and moves it in a circle. If the string breaks, theobject flies off in a straight line. The Sun’s gravity is like thestring. Gravity keeps Earth revolving around the Sun in anearly circular orbit. If the pull of gravity between the Sunand Earth stopped suddenly, Earth would fly off into spacein a straight line.Earth’s RotationAs Earth revolves around the Sun, it spins. A spinningmotion is called rotation. Earth rotates on an imaginary linethat runs through its center. The line on which an object rotates isthe rotation axis.If you could look down onto Earth’s North Pole, youwould see that Earth rotates in a counterclockwise direction,from west to east. One complete rotation of Earth takes about24 hours. One rotation completes Earth’s cycle of day andnight. It is daytime on the half of Earth that faces the Sun. Itis nighttime on the half of Earth that faces away from theSun.Reading EssentialsCopyright Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Key Concept CheckJuly 4Earth is farthestfrom the Sun.Gravitationalpull of the Sun

The Sun’s Apparent Motion Each day, the Sun appears tomove across the sky from east to west. It seems as if the Sunis moving around Earth. In fact, it is Earth’s rotation thatcauses the Sun’s apparent motion.Earth rotates from west to east. This makes the Sunappear to move from east to west across the sky. The Moonand stars also seem to move from east to west across the skydue to Earth’s west-to-east rotation. Earth’s west-to-eastrotation causes apparent east-to-west motion in the sky.The Tilt of Earth’s Rotation Axis Earth’s rotation axis istilted, as shown in the figure below. The tilt of Earth’srotation axis does not change. During one-half of Earth’sorbit, the north end of the rotation axis is toward the Sun.During the other half of Earth’s orbit, the north end of therotation axis is away from the Sun.Earth’s orbitReading Check5. Describe What causesthe Sun’s apparent motionacross the sky?RotationaxisNNSSDecemberSeptemberVisual CheckSunCopyright Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.6. Identify BetweenNEarthSMarchNSwhich months is the northend of Earth’s rotation axisaway from the Sun?JuneTemperature and LatitudeAs Earth orbits the Sun, the Sun shines on the half ofEarth that faces the Sun. Sunlight carries energy. The moresunlight that reaches a part of Earth’s surface, the warmerthat part becomes. Because Earth’s surface is curved, differentparts of Earth’s surface get different amounts of the Sun’senergy.Energy Received by a Tilted SurfaceSuppose you shine a flashlight onto a flat card. The beamshines in a circle on the card. As you tilt the top of the cardaway from the beam of light, the light becomes more spreadout on the card’s surface. The energy that the light beamcarries also spreads out more over the card’s surface. An areaon the surface within the light beam receives less energywhen the surface is more tilted relative to the light beam.Reading EssentialsReading Check7. Summarize Why dosome parts of Earth’s surfaceget more energy from theSun than other parts?The Sun-Earth-Moon System347

When the surfaceis tilted, the lightbeam is spread outover a larger area.Surface is vertical.Visual Check8. Determine Is the lightenergy more spread out onthe vertical or tilted surface?Key Concept Check9. Cause and Effect Whyis Earth warmer at the equatorand colder at the poles?Surface is tilted.The dotted line showsthe area covered bythe light beam beforethe surface was tilted.The Tilt of Earth’s Curved SurfaceInstead of being flat and vertical like the card shownabove on the left, Earth’s surface is curved and tilted,somewhat like the card on the right. Earth’s surface becomesmore tilted as you move away from the equator and towardthe poles. As a result, regions of Earth near the poles receiveless energy than areas near the equator. This is why Earth iswarmer at the equator and colder at the poles.SeasonsYou might think that summer happens when Earth isclosest to the Sun. However, seasonal changes do not dependon Earth’s distance from the Sun. In fact, Earth is closest tothe Sun in January! The tilt of Earth’s rotation axis andEarth’s motion around the Sun cause the seasons to change.Spring and Summer in the Northern HemisphereWhen Earth is 147,000,000 kmfrom the Sun, how far isEarth from the Sun in miles?To calculate the distance inmiles, multiply the distancein km by the conversionfactor.0.62 miles147,000,000 km 1 km 91,100,000 miles10. Convert Units WhenEarth is 152,000,000 km fromthe Sun, how far is Earthfrom the Sun in miles?348The Sun-Earth-Moon SystemTemperatures are higher in the northern hemisphere andlower in the southern hemisphere. Daylight hours last longer inthe northern hemisphere. Nights last longer in the southernhemisphere. It is spring and summer in the northernhemisphere and fall and winter in the southern hemisphere.Fall and Winter in the Northern HemisphereDuring the other half of Earth’s orbit, the north end ofthe rotation axis is away from the Sun, as shown in the leftfigure on the next page. Then, the northern hemispherereceives less energy from the Sun than the southernhemisphere does.Temperatures are cooler in the northern hemisphere andwarmer in the southern hemisphere. It is fall and winterin the northern hemisphere. At the same time, spring andsummer occur in the southern hemisphere.Reading EssentialsCopyright Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Math SkillsDuring one-half of Earth’s orbit, the north end of therotation axis is toward the Sun. Then, the northern hemispherereceives more energy from the Sun than the southernhemisphere does. See the figure on the right on the next page.

North end of rotation axis points away from the Sun.Northern hemispherereceives less sunlight.NSouthernhemispherereceives moresunlight.SNorth end of rotation axis points toward the Sun.Southern hemispherereceives less sunlight.Solstices, Equinoxes, and the Seasonal CycleAs Earth travels around the Sun, Earth’s rotation axisalways points in the same direction in space. But the amountthat Earth’s rotation axis is toward or away from the Sunchanges. This causes the yearly cycle of the seasons.There are four days each year when the direction ofEarth’s axis is special relative to the Sun. A solstice is a daywhen Earth’s rotation axis is the most toward or away from the Sun. Anequinox is a day when Earth’s rotation axis is leaning along Earth’sorbit, neither toward or away from the Sun.Copyright Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.NNorthernhemispherereceives moresunlight.March Equinox to June Solstice The north end of therotation axis slowly points more and more toward the Sun. Asa result, the northern hemisphere slowly receives more solarenergy. Spring takes place in the northern hemisphere.SVisual Check11. Label the left figureto indicate whether it issummer or winter in thenorthern hemisphere.Make a bound book fromtwo sheets of paper toorganize information abouteach season and its solsticesand equinoxes.Solsticer EquinoxDecemberSeptembeJune Solstice to September Equinox The north end of therotation axis still points toward the Sun but does so less andless. The northern hemisphere starts to receive less solarenergy. This is summer in the northern hemisphere.September Equinox to December Solstice The north end ofthe rotation axis points more and more away from the Sun.The northern hemisphere receives less and less solar energy.Fall takes place in the northern hemisphere.December Solstice to March Equinox The north end of therotation axis still points away from the Sun but does so lessand less. As a result, the northern hemisphere starts toreceive more solar energy. This is winter in the northernhemisphere. Earth’s seasonal cycle is summarized in thetable on the next page.Reading EssentialsKey Concept Check12. Conclude How doesthe tilt of Earth’s rotation axisaffect Earth?The Sun-Earth-Moon System349