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30 7. 6. 5. 4. skyalsoappeartomove.Theyseemchangelocationsnightly. Just asthesunappearstomoveinapathacross thesky, starsinthe Changes in Seenin theNight 3. are different inwinter, ,, andfall. change throughout theyear. Sotheconstellationsvisibleinearly movement ofthestarsacross theskyeverynight,theirrisingtimes not alwaysinthesamepositionnightsky. Inadditiontothe . ButifyoulookfortheBigDipper, youwillseethatitis UrsaMajoralsohasaninnerpattern knownasthe A group ofstarsthatformapattern iscalleda The studentwhorepresents Earthseesdifferent starsfrom different positionsinthe Intheactivity, ifthe studentplayingEarthturnstofacethesun,whattimeofdayisit Whydothestarsappeartomoveacross theskyeverynight? Unit 1 circle. Howdoesthismodel suggestevidencethatEarthcircles thesunonceayear? for the If thepersonnowturnshalfwayaround andlooksatthestars,whattimeofdayisit same people? for people who live on that side of ? Act Discuss • Earth movestoawinterpositionand finallyaspringposition.Notehowthestars • Earth movestoapositionone-fourth ofthewayaround thesun.Notewhichstars • Earth standsfacingawayfrom thesun. Earth looksstraight ahead,totheleft, and • Earth willmovearound thesuninacounterclockwise direction. Thesunandstars • Several studentsformalarge circle. Theyrepresent thestarsinnightsky. One C. B. A. seen inthenightskychange. Earth cansee.Thisisthefallnightsky. to theright.NotewhichstarsEarthcansee.Thisisnightskyinsummer. do notmove. represents theNorthPole ofEarth. and standsbetweenthestarssun.TheheadofstudentwhoisEarth student represents thesunandstandsincenter. Onestudentrepresents Earth BecauseEarthrotates onceevery24hours. Because Earthtiltsonitsaxis. BecauseEarthorbitsthesunonceayear. Withyourclass, investigatewhystarpatterns changeyearly. Patterns inthe SolarSystem Whenyoulookatthenightsky, whatdoyousee? Analyzing anEarth- Model to Explain the Stars Seen in the Sky 1 constellation . The like aladleusedtoscoopwater. patterns inthenorthernsky. Itlooks The BigDipperisoneofthemostfamiliar

© Houghton Mifflin Harcourt • Image Credits: ©Gerard Lodriguss/Science Source Seasonal Star Patterns (the Winged Horse) Earth’s place in its affects which stars can be seen during different . The location of stars in the changes as Earth moves along its orbit. During the , you will be able to see all the stars visible from one specific place on Earth.

On a given , you will only be able to see the (the Swan) stars that are in the opposite direction of . (the Great Dog) The stars seen from the may be different from the stars seen from the Southern Hemisphere. If you are on the North or Pole, (the Hero)

the stars will appear to rotate around a point directly not to scale above your head because of Earth’s . The sky Earth’s place in its orbit affects which stars are seen at seen from the is completely different than different of the year. For example, Cygnus is seen in sky seen from the . As you from night sky of the Northern Hemisphere in summer. the North Pole toward the South Pole, the sky will change. The sky seen by someone in Florida may have some of the same stars in the sky seen by someone in Brazil, but the whole pattern of stars seen is not the same.

Some Stars Are Seen in Different Seasons

Castor Pollux

Leo

Orion

Hydra

These are some of the visible in the night As Earth continues its orbit around the sun, sky of the Northern Hemisphere during winter. During changes to spring. Earth faces a different direction the day, the sun blocks summer constellations from sight. during the night and new constellations become visible.

Vega Great Square Borealis Pegasus

Markab

Aquila

In fall, Earth is three-fourths of the way through its orbit Earth’s summer position brings new constellations into around the sun and new constellations are visible. Soon, view. the winter constellations are opposite the sun winter constellations will start to be seen once more. and cannot be seen. © Houghton Mifflin Harcourt 8. Why do you see different stars in spring’s night sky than in fall’s night sky? Explain.

Lesson 2 Seasons 31 32 10. determine yourlatitudeonEarth. a measure oftheangleelevationNorthStar, youcan At theequator, theNorthStarisseenathorizon.Bytaking night. AttheNorthPole, the North Starisseendirectly overhead. a guidebywhichsailorscouldnavigatethenorthernseas at points toit. fixed inplacethroughout the yearbecauseEarth’s northaxis In theNorthernHemisphere, theNorthStar’s positionremains Using theStars TheBigDipperappearsto 9. seen intheskyallyear. Your locationonEarthdetermineswhichstarsstay visible allyear. depending onhowfarnorthorsouthoftheequatoryouare, there are somestars thatcanbe Earth’s placeinitsorbitdetermineswhichstarscanbeseenduringdifferent seasons,but tointheNorthernHemisphere. These photos,taken ataboutthesametimeofnight,showpositionBigDipperfrom Circle thecorrect words toexplainwhatyouobserveinthephotosabove. Some Stars Are Seen All Year All Seen Stars Are Some Unit 1 CouldapersonattheSouthPole seetheNorthStar?Explain. The BigDipperseemstomoveinacircular pattern asthe Because itdoesnotshift position, theNorthStarbecame Patterns inthe SolarSystem keep /change keep winter spring itspositionfrom seasontoseason. seasons / around theNorthStar. the slow, continuousmovementofotherstars is photographed at regular intervalstoshow this imageoverthecourseofonenight.Thesky -lapse wasusedtocapture change. summer fall

© Houghton Mifflin Harcourt • Image Credits: (tl) ©Gerard Lodriguss/Science Source; (tr) ©Atzori Riccardo/Shutterstock; (cl) ©steffiiiii/iStock/Getty Images Plus/Getty Images; (cr) ©Cultura RM/Preserved Photography/Alamy; (b) ©snike/Fotolia © Houghton Mifflin Harcourt • Image Credits: (t) ©Irina Volkova/Fotolia; (b) ©Jana Horti/ Fotolia season. meltsand is During ayear, manyplacesonEarthexperiencefourseasons. closer totheequatorhavemilderweatherinwinterthanareas closertothepoleshave. The farthernorthyougo,thegreater thedifferences intheseasons.For example,areas hot. Springandfalltemperatures are warmerthanwinter’s butcolderthansummer’s. follow apredictable cyclethroughout theyear. Wintermaybecold.Summer . conditionsanddailytemperatures atanylocationonEarth A Seasons of theYear 11. then winterbeginsagain.

season generally daylight hoursincreases. gradually rise. The sunmoveshigheracross theskyandnumber of In theNorthernHemisphere, springbeginsInMarch. associated withwintermonths.Thesunsets earlyintheday. season—winter. Freezing temperatures andsnowfallare In December, theNorthern Hemisphere beginsitscoldest The Four Seasons oftheYearThe Four Seasons Whatseasonalchangesdoyouseewhere youlive? Fall isadivisionoftheyearthatassociated withparticularweatherpatterns and Relating Seasons toEnergy from the Sun

andmaybringsnowice.Astemperatures warm, (also called spring begins.Nextcomes autumn ) follows,temperatures getcooler, and summer EXPLORATION 2 winter spring , thewarmest part oftheday. Northern Hemisphere. Thesunisintheskyforagreater Summer isthewarmestseason,beginninginJune the skymoveslower. Temperatures gradually cool. number ofdaylighthoursdecreases asthesun’s path across Fall beginsinSeptember intheNorthernHemisphere. The Winter on thewayandwinterisending. Early bloomsannouncethatspringis Lesson 2 Seasons summer fall 33 Changes across the Seasons Because of Earth’s rotation, we see the sun appear to move across the sky. For people living on the , the path of the sun in the sky does not change very much throughout the year. The sun rises on the eastern , goes nearly overhead, and then sets on the western horizon. The sun reaches its highest point in the sky at about . lasts about 12 hours all year long, and the weather is nearly always warm. But as you move north or south away from the equator, the sun’s path in the sky changes during the year. For example, in the Northern Hemisphere, the sun rises north of in the summer. In winter, the sun rises south of east. follow the same pattern. Summer sunsets in the Northern Hemisphere are north of , while winter sunsets are south of west. The sun’s path changes as seasons change.

The Path of the Sun in Summer and Winter in the Northern Hemisphere

12. Write summer or winter to label each path of the sun.

sun’s path in ———————————

sun’s path in ———————————

E

S N

Explore W ONLINE! not to scale

Daylight Hours In winter, it may be dark when you wake up for school and dark again soon after school ends. The sun does not go very high in the sky so its path across the sky is shorter. Because the sun is not up in the sky very long, there is less daylight time during this time of year. There is less time for the to up Earth during the daytime and more time for Earth to cool during the longer nighttime hours. The shortest amount of daylight time is on December 21 or 22 in the Northern Hemisphere. In summer, the sun goes higher in the sky. Days are longer because it takes more time for the sun to complete its longer path across the sky. It is warmer in summer than in winter because the sun is up for a longer of time. There are more hours of daylight to warm Earth during daytime, and fewer nighttime hours for Earth to © Houghton Mifflin Harcourt cool. The day with the greatest amount of daylight time is June 20, 21, or 22 in the Northern Hemisphere. 13. How is the number of daylight hours of sunlight related to the seasons?

34 Unit 1 Patterns in the Solar Hands-On Lab Model Sunlight Distribution

Do the Math You will explore what happens when light is spread out compared to when it is not spread out. MATERIALS • flashlight • graph paper Procedure • metric ruler STEP 1 Work with a partner. One partner shines a flashlight straight down on graph • pencil paper from a height of 15 cm. Using a protractor, the second partner makes • protractor sure the light strikes the paper at a 90° . The second partner then traces around the lit area on the paper with a pencil. Label it 90°. STEP 2 Switch holding the flashlight between partners. Keep the flashlight at the same height as it was in STEP 1 (15 cm). Using the protractor, one partner guides the other to change the position of the flashlight so that the angle of the light striking the paper is 60°. That partner traces the lit area and labels it 60°. STEP 3 Next, using the protractor, change the angle so that the light striking the paper is 30°. Trace the lit area and label it 30°. STEP 4 Calculate the total area of each lit area using this method: a. Count and record the number of full squares in each area. Example: 4 full squares b. Count the number of partial squares and divide the number by 2. Example: 12 partial squares ÷ 2 = 6 c. Add the number of full squares to the number calculated for partial squares to find the total area. Example: 4 + 6 = 10

Angle of light 4a. Full squares 4b. Partial squares 4c. Total area

90°

60°

30°

Analysis © Houghton Mifflin Harcourt STEP 5 Compare the total areas. What do you think the data mean?

Lesson 2 Seasons 35 36 14. strikes Earthaffect thechangesintemperature thatoccuracross theseasons. in winter. Thesechangesinthesun’s pathacross theskyandangleatwhichsunlight sunlight strikes EarthinfluencesEarth’s temperatures, makingithotinsummerandcold at alesser angle,makingthesun’s rays strikingEarthless intense.Theangleatwhich at alesser angle.Solarenergy passes through more whenitstrikes Earth more intense.Inwinter, whenthesunislowerinsky, thesun’s reaches Earth a spread-out area. Insummer, whenthesunishigherinsky, thesolarenergy is smaller area willresult inwarmertemperatures thanthesameamountofenergy striking meter ismore intense—thelightisless spread out.Theamountofenergy strikinga Earth. Asshownintheactivity, whenthesunisoverhead,energy oneachsquare more hoursofdaylight,sothere ismore timeforEarthtoabsorbthesolarenergy. across thesky, whichmeansthesunis intheskyforalongerperiodoftime.There are height ofthesuninmiddleday. Inthesummer, thesunhasalongerpath and winter:changesinthelengthofsun’s pathacross theskyandchangesin There are tworeasons whywereceive different amountsofthesun’s energy insummer Energy from theSun Unit 1

light ismore spread out?Relatethisideatothesun’s energy andEarth. a givenarea. Whathappenswhenlightisnotspread out?Whathappenswhenthe Discuss Theheightofthesuninskydeterminesangleatwhichsunlightstrikes Patterns inthe SolarSystem Together with apartner, talkabouttheamountofsolarenergy thatfallson strikes atanangle. light whenthesun only apartofthe overhead, butgets the sunisdirectly of thelightwhen ground getsall same amountof larger area. The over alarger and sunlight isspread the energy from lower inthesky, As thesunmoves

© Houghton Mifflin Harcourt 15. Language SmArts On a separate sheet of paper, write a short essay to compare what you observed in your investigation to what you saw in the and what you read about the of sunlight striking Earth.

EVIDENCE NOTEBOOK 16. How do the length of days and the path of the sun across the sky help to explain why winter is cold with shorter days? Record your evidence.

Patterns of Sunlight and Light comes from the sun. The rays of sunlight move in straight lines. Because the sun is so far away, the rays that strike Earth are very nearly parallel. Because of Earth’s spherical shape, these parallel rays strike Earth most directly at the equator. As you move from the equator to the poles, the rays strike at lesser angles. The diagram shows Earth lit by the sun on a day in the spring or fall. The diagram shows that the sun appears overhead as viewed from the equator. So, people who live on or near the equator feel the intense energy of the sun. People who live at the North Pole would see the sun on this same day as very low in the sky. This helps to illustrate why it is nearly always warm near the equator and cold at the poles. polar

equator

Due to the shape of Earth, sunlight strikes different on Earth’s at different not to scale angles. polar region 17. Think about a person at the equator, a person at the North Pole, and a person somewhere in between. Each person points toward the sun. Describe where in the sky they are pointing. © Houghton Mifflin Harcourt

Lesson 2 Seasons 37 18. Which of these describes the differences in sunlight striking Earth at different latitudes? Circle all that apply. A. The intensity of the sun’s energy received at the equator is greater than the intensity of the energy received at the poles. B. Sunlight strikes at a greater angle at the equator, which spreads out the sunlight. C. As you move away from the equator, the rays of sunlight striking Earth are no longer parallel. D. Sunlight passes through less atmosphere at the equator, so more sunlight gets through, which makes locations around the equator hotter.

Analyze How Earth’s Shape Affects Patterns of Sunlight 19. If Earth were instead of curved, how would that affect temperatures from pole to pole? Explain how the range of temperatures at noon at different latitudes on a cube-shaped would compare to temperatures on our .

20. Draw To the right of the cube, draw a model of the way the sun’s rays would strike a cube-shaped planet.

polar region

equator © Houghton Mifflin Harcourt

polar region

38 Unit 1 Patterns in the © Houghton Mifflin Harcourt one year. Earth-Sun Models Earth-Sun One completeorbitaround thesuniscalleda how muchsunlightreaches different areas ofEarthastheplanetmovesaround thesun. Earth orbitsthesuninapredictable pattern. Thepattern ofEarth’s seasons dependson 21. axis, animaginarylinepassing through Earthfrom poletopole. Earth hasanearlycircular orbitaround thesun.Earth alsorotates around itsnorth-south 22. where Earth isinitsorbitaround thesun. axis ispointedinthesamedirection nomatter remains thesamethroughout Earth’s orbit.Earth’s perpendicular totheplane of itsorbit.Thistilt around thesun.Earth’s axisistilted23.5° from is notperpendiculartothe ofEarth’s orbit Unlike theplanetinfirstmodel,Earth’s axis Examinetheimage.Whatdoesthismodel Doesthisfirstmodelexplaintheseasonsthat C. B. A. show? Circle allthatapply. to supportyouranswer. happen onEarth?Giveatleastoneexample There are coldertemperatures atthe conditionsontheplanet Everyplaceontheplanetgets12 hoursof during theyear. equator, but notemperature changes poles andwarmertemperatures atthe the sun. change dependingondistancefrom light and12 hoursofdarkeachday. Analyzing Model anEarth-Sun to Explain Seasons EXPLORATION 3 revolution This modelshows Earthwithitsaxistilted23.5°. fictional Earthshowsanaxisthatisnottilted. Earth hasanearlycircular orbitaround thesun.Thismodelofa not toscale not toscale . Onecompleterevolution takes Lesson 2 Seasons 39 Hands-On Lab Model Patterns of Sunlight throughout Earth’s Revolution

You will model the tilt of Earth. You will show the way different areas of receive more or less sunlight throughout the year. • construction paper • foam ball, 1” Procedure • light source STEP 1 Use to make a base for your foam ball . • marker • metric ruler STEP 2 With the marker, mark both poles and draw an equator on the • modeling clay, sphere. Push the toothpick carefully through the sphere from non-drying pole to pole. • protractor STEP 3 Insert the toothpick into the base. Using the protractor, • toothpick set the tilt of the axis at 23.5° from vertical. STEP 4 Cut the construction paper so that it is a square and then it exactly in half. Draw a line along the fold. Use the protractor to mark 90° on both sides of the line and connect those marks. Label the four connected folds beginning with Spring and moving counterclockwise to label Summer, Fall, and Winter. STEP 5 Place the light source on the center where the lines cross. This is your sun. STEP 6 Set the sphere directly on Summer. The North Pole (top of the toothpick) should tilt toward the light. Observe where the sphere is light and dark. Record your data by drawing and shading to show your Summer sphere in the below. STEP 7 Keep the angle and direction that the sphere is pointing the same. Move the sphere to Fall. Observe where the light falls. Record your data in the table for Fall. STEP 8 Repeat STEP 7, moving the sphere to Winter and then Spring.

Summer Fall Winter Spring © Houghton Mifflin Harcourt

40 Unit 1 Patterns in the Solar System © Houghton Mifflin Harcourt Analysis 23. in . those intheNorthernHemisphere. InDecember, itiswinterinCanadaandsummer points awayfrom thesun.SeasonsinSouthernHemisphere are opposite from seasons isbecauseofacombinationEarth’s tiltandEarth’s revolution. changes becauseEarthisalwaystiltedinthesamedirection. So,thereason wehave change. AsEarthorbitsthesun,area ofEarththatispointed more toward thesun Earth’s tiltdidnotchange,theamountofsunlightreaching aspecificarea ofEarthdid these ideasalonedonotcompletelyexplainwhyitiswarmerinsummerthanwinter. and whytheheightofsunappearslowerinskyasyougetclosertopoles.But The sphericalshapeofEarthexplainswhyitgetscolderasyougetclosertothepoles, The Tilt of Earth STEP 10 STEP 9 DotheNorthandSouth Poles alwaysstayinthesamepositionrelative tothesun? Explain yourreasoning. When theNorthernHemisphere pointstoward thesun,SouthernHemisphere What didyoulearnasyourmodelEarthmovedaround themodelsun?Because Hemisphere? Why? sun, whatseasonwouldyouexpecttoexperienceintheSouthern Look atthesphere. WhentheNorthernHemisphere tiltsawayfrom the observed toexplain. Northern Hemisphere havelowertemperatures inwinter?Usewhatyou What didyouobserveaboutsunlightonEarthinwinter?Whydoesthe Lesson 2 Seasons 41 42 of daylightthanthoseareas pointedaway. each dayeverywhere. Because ofthetilt,areas pointedtoward thesunhavemore hours is theresult ofEarth’s tilt.IfEarthhadnotilt,daysandnightswouldlastabout12hours The numberofhoursdaylightincreases asspringchangestosummer. Thischange of Earth’sThe Effect Tilt onDaylight Hours 24. not toscale not toscale Southern HemisphereSouthern winter HemisphereNorthern summer late June 25. Hours ofDaylight by Latitude Unit 1 Earth’s Sun the Revolution around • 24 • 15 • 12 • 9 • 0 Itissummerinthehemisphere tilted are winter inthehemisphere tilted Completethelabelsto showhowmuchdaylightanarea ateachlatitude would havewithEarthinthisposition. Patterns inthe SolarSystem the same/reversed the summer June May July April S August March away from /toward intheNorthernandSouthern Hemispheres. spring

away from /toward

fall equator N February September ——— ——— ——— ——— ——— January 15 October December November

hours ofdaylight hours ofdaylight hours ofdaylight hours ofdaylight hours ofdaylight thesun.Theseasons Southern HemisphereSouthern summer HemisphereNorthern winter late December thesun.Itis tilted 23.5° Earth’s axis winter seasons. causes Earth’s the sun.Thistilt points awayfrom and sometimes toward thesun sometimes points of Earth’s axis The northerntip

© Houghton Mifflin Harcourt © Houghton Mifflin Harcourt • Image Credits: ©Planet Observer/Universal Images Group Editorial/Getty Images 26. , there are equalhoursofdayandnightatalllocationsonEarth. tilt directly toward orawayfrom thesun.Theword marked bytheequinoxes.Earth’s axisdoesnot December solsticeisthelongestdayofyear. Hemisphere. IntheSouthern Hemisphere, the fewest numberofdaylighthoursintheNorthern The Decembersolsticeisthedaywith tilted awayfrom thesun,around December 21. winter ,occurswhenEarth’s northaxisis Hemisphere. as theshortestdayofyearinSouthern the NorthernHemisphere isatthesametime Hemisphere. Thislongestdayoftheyearin greatest numberofdaylighthoursintheNorthern The Junesolsticeisthedayofyearwith toward thesun,betweenJune 20andJune 22. solstice, occurswhenEarth’s northaxisistilted the sun.TheJunesolstice,alsocalledsummer Earth’s axisistilteddirectly toward orawayfrom andEquinoxesThe Solstices 27. those areas around theequator. revolves around thesun.Areas are warmerwhensunlightisnotasspread out,suchasin North Pole thanattheequator, thetemperature isnotaswarm. strikes atalesser angle,thelightspreads out.So,althoughdaylight lastslongeratthe energy isreceived inagivenarea attheNorthPole thanattheequator. Whensunlight shines ontheNorthPole atalesser angleinsteadofstriking from directly overhead,less mid-September. Thesundoesnotcompletely setduringthattime.Butsincethesun means thatthesunrisesabovehorizoninmid-March andcontinuestoshineuntil some partsofEarthreceive more solarenergy thanothers.AttheNorthPole, Earth’s tilt Earth’s tiltaffects thetemperatures atdifferent locationsonEarth.BecauseofEarth’s tilt, theEnergy Received from theSun Affects The Tilt of Earth tilt thatdetermines theseasons. closest tothe sun around January 3and farthest from thesunaround July 4. ItisEarth’s So, Earth’s distancefrom thesunisnotwhat determinestheseasons.Infact,Earthis The WhentheSouthPole istilted move from theJunesolstice totheDecembersolsticeinNorthernHemisphere. the area Hemisphere experienceswinter. Theamountofthesun’s energy thatstrikes increase /decrease summer. Thedaylight hoursare directly toward orawayfrom thesun.Thedaysget The daysthatbeginspringandfallare The Decembersolstice,alsocalledthe The angleatwhichsunlightstrikes aparticularlocationonEarthchangesas markthetwodaysofyearwhen solstices / solstices increases /decreases . toward /awayfrom longer /shorter longer markthedatesonwhichEarth’s axisistilted as compared tothesun’s energy inthe equinox darkness attheSouthPole. Pole, 12hoursofdaylightattheequator, and24hoursof On theJunesolstice,there are 24hoursofdaylightattheNorth shorter / longer means“equal night.” Onan , andthearea temperatures thesun,Southern as you Lesson 2 Seasons 43