(Part I) 1. Practice Quiz 2. Introduction 3. Earth Spins Around Its Axis ...


(Part I) 1. Practice Quiz 2. Introduction 3. Earth Spins Around Its Axis ...

(Part I)1. Practice Quiz2. Introduction3. Earth Spins Around Its Axis4. Earth Revolves Around the Sun5. Summary

ReviewAstronomy is the study of objects outside ofthe Earth’s atmosphereThis is a science classThe scientific method always tests andretests hypotheses and develops new theoriesif old ones failPowers of 10 are used for big and smallnumbersWe are going to study lots of interestingstuff this semester

LOTS of MotionEarth Spins Around Its Axis Once per ???Earth and Moon Revolve Around Each Other Once per ???Earth Revolves Around the Sun Once per ???Solar System is Revolving Around the Center ofthe Milky WayThe Milky Way is Moving Through SpaceWhew, do you feel dizzy?

Earth SpinsThe Earth spins around its axis once per day(24 hr)This axis runs through the Earth from theNorth Pole to the South PoleWhen viewed from above the North Pole, theEarth rotates counterclockwise.This spin causes the rising and setting of theSun (and the Moon and the stars)This effects many of our weather patternsincluding hurricanes

North Celestial Pole The Earth revolves around an axis thatruns from the north celestial pole to thesouth celestial poleCurrently, the north celestial pole pointstowards Polaris, otherwise known as theNorth StarThere is no corresponding “South Star”The Earth tilt moves some so eventually thenorth celestial pole won’t point towardsPolaris

Foucault’s PendulumA pendulum swings back and forth because ofgravity and its massWithout friction and air resistance, it will dothis foreverIn 1851, Jean-Bernard-Leon Foucaultdeveloped a test to demonstrate the Earth’srotation

Foucault’s Pendulum (cont)Imagine the pendulum is at the North PoleIt will continue to swing back and forth inthe same plane. However, the Earth willrotate underneath it.So to someone standing nearby the pendulumwill appear to rotate, completing onerevolution per dayNow consider a pendulum at the equatorwhich is swinging along the east-west lineThis pendulum will not rotateAt latitudes in the middle, the pendulum willrotate with a period more than 24 hours

Figure 2.9

The HorizonWe can only see half the sky at any givenmoment (the half above us)The other half is blocked by the EarthIf we are on the North or South Pole, we willalways see the same half of the skyIt does rotate around itselfIf we are on the equator, we will see thewhole sky once per dayIn between, we see part of the sky all daylong and part of the sky only some of theday

AST0204.swfFigure 2.4

The Earth Revolves Around the Sun The Earth revolves around the sun onceper year The distance from the Sun to the Earthchanges by about 3% over a year

What Can I See?Earth’s rotation and motion around the Sundetermine what we can see in the skyRotationDuring the day, you only see the Sun and maybe abit of the MoonAt night you see the portion of the sky above youwith stars “rising” in the east and “setting” in thewestMotion around the SunSix months from now the current sky will behidden by the Sun and we will see part which isnow behind the Sun

What Can I See? (cont.)In the northern hemisphere, the North Staris above the horizon all day longThe angle of the North Star above the horizonequals your latitudeIn the northern hemisphere, part of the skyaround the south celestial pole is never visible

The Earth is TiltedThe Earth’s axis is tilted 23.5 owith respect to its orbitaround the SunAxis always points in the samedirection, toward the northcelestial poleIt actually moves very slowly overtime, precessing like a top

Seasons Seasons are caused by the tilt of theEarth combined with motion around theSun During our summer, the north celestialpole is pointed towards the SunThe Sun is above the horizon longerWe receive more intense light During our winter, the south celestial poleis pointed towards the Sun

Intensity of Sunlight•The amount of light per square meter depends on theangle at which the light hits the surface•The amount of light determines the “heating” of theEarth•In the summer, the light is more direct and providesmore heat

Summer Solstice First day of summer, about June 22Sun appears to be 23 o north of the equatorPasses through the zenith (straight above) of places thatare 23 o N latitude at noon23 o N latitude is called the Tropic of CancerAll regions within 23 o of the North Pole see the sunfor the full day90 o -23 o = 67 o N latitude is called the Artic CircleAll regions within 23 o of the South Pole see nosunlight for the full day67 o S latitude is called the Antarctic CircleIt stays dark at the North and South Poles for 6 monthseach year

Winter Solstice and Equinoxes First day of winter, about December 22Everything is reversedSun passes through the zenith along the Tropic ofCapricorn at noon (23 o S latitude)Equinoxes Twice a year, the sun passes through the zenithalong the equator at noon (0 o latitude) Vernal Equinox, around March 21 Autumnal Equinox, around September 21 12 hours of light and 12 hours of darknesseverywhere

Real WorldEarth’s atmosphere fuzzes the edges (figurativelyand literally)Atmosphere bends light coming from the Sun,allowing us to “see over the horizon” about 18 oSun appears to rise earlier and set laterIt’s light out (twilight) in the morning when the Sun is 18 obelow the horizon and stays light in the evening until the Sunis 18 o below the horizonEffect is most noticeable at the poles – complete darknessfor only 3 months (rather than 6 months)Last week the scientific station in Antarctica starting receivingflights after the winter breakJune 22 is the longest day, but not the hottest,why?


Length of the YearIt takes the Earth 365.242199 days to gofrom one vernal equinox to the next NOT an integer number But the extra is close to ¼ = 0.25So every 4 years (leap year) we add an extraday to the calendar (Feb. 29) But this is too much (we’ve added 0.25!) So every 100 years (on the century) we don’tadd the extra day (no leap year) But this isn’t right either, so every 4 th 100years, we do include the leap yearThis is why 2000 was a leap year

SummaryThe Universe has lots of motionThe spinning of the Earth causes the risingand setting of the Sun and starsThe revolution of the Earth around the Sundetermines the yearThe tilt of the Earth determines the seasonsThe spinning, revolution and tilt determinethe part of the sky which is visibleYou want/need to understand these motionsNext time, we will look at how the Moonbehaves

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