Earth Science Chapter 21

8/12/2019 Earth Science Chapter 21

1/24


2/24


3/24


4/24


5/24


6/24


7/24


8/24

61 8 C H A P T E R 2 1 O r ig in s o f M o d e r n A s t r o n o m y

Le German astronomer Johannes Kepler (1571-1630 )helped establ ish the era of modern astronomy by deriving three lawsof planetary motion. ( P h o t o by I m a g n o / G e t ty I m a g e s )In 160 9 , after nearly a decade ofwork, Kepler proposed hisf irst two laws of p lane ta r y mot ion:

1 . The path o f each planet around the Sun, while almost cir-cular, is actually an el lipse, with the Sun at one focus(Figure 21.11).2. Each planet revolves s o that an imaginary line connectingit to the Sun sweeps over equal areas in equal intervals oftime 1 . = ; F 1 ) . This law ofequal areas geometricallyexpresses the variations in orbital speeds of the planets.

Figure 21.12 i l lustrates the second law. Note that in order fo ra planet to sweep equal areas in the same amount o f t ime, itmusttravel more rapidlywhen it is nearer the Su n and more slowlywhen it is farther from the Sun.Kepler was devou t and believed that t he Crea to r made anorderly universe and that th is orderwould be reflected in the posi-t ions an d mot ions of the planets . The uniformity he tried to f indeluded him fo r nearly a decade. Then in 1619, Kepler publishedhis third law in The Harmony of the Worlds:3. The orbital periods of t he p lanet s and their distances to

the Su n ar e proportional.

ll

i

i//\/ -_/'/if \_nTl _, , ___ _ _ _ _ _ _ _ ~ f IiFocus Focus

(3

. >

Y _ _ _ ___;_________=-_=:; _ _ _ _ _ _ .. . _ .1-w--~ ~Focus Focus

; I : ' . l~ f l . - - ; -. Drawing ellipses with various eccentr ic i t ies. Usingtwo straight p in s f or f o ci an d a loop of string, trace ou t a curve whilekeep ing the str ing t aut , and yo u will have drawn an el lipse. Thefarther t he p ins (the foci) are moved apart , the more flat tened (moreeccentric) is the result ing ellipse.

In its simplest f orm , t he o r bi ta l pe ri od is measured in Earthyears, an d the p lanet s d is tance to the Sun is expressed in termsof Earths mean distance t o the Sun. The latter yardstick is calledthe astronomical unit (AU) an d is equal t o about 15 0 millionki lometers (9 3millionmiles). Using these units, Keplers third lawstates that the planet's orbital period squared is equal to i ts meanso la r d i st ance cubed . Consequen t ly , t he so la r d is tances of theplanets c an b e calculated when their periods of revolution areknown. Fo r examp le , Mars has an o rb ita l pe r iod of 1.88 years,which squared equals 3.54. The cube r oot o f 3.54 is 1.52, an d that

Kepler's law of equal areas. A l in e connecting a planet(Earth) to the Sun sweeps ou t an area in such a manner that equalareas are swept ou t in equal t imes. Thus, Earth revolves slowerwhen it is farther f rom t he Sun (aphelion) an d fas te r when it isc losest (per ihel ion). The eccen tr i c it y o f Earth's orbit is greatlyexaggerated i n t hi s diagram.

-QF a s t e r J e m o9

January Sun JulyT

,1~n-


9/24

The Birth of Modern Ast ronomy 619is the average distance from Mars t o the Sun, in astronomical units( T a b l e 2 1 . 1 ) .Keplers laws assert that t he p lanet s revo lve a round the Sun,and therefore suppor t the Copern ican theory. Kepler, however,did not determine the forces that ac t to produce the planetarymotion he had so ably described. That task would remain forGalileo Galilei and Si r Isaac Newton.

Galileo GalileiGalileoGalilei ( 15 64 -1642 ) wa s the greatest Italian scientist of theRenaissance 1 . Y ~ :. ' ). He wa s a contemporaryofKepler and,like Kepler, strongly supported the Copernican theory of a Sun-centeredsolar system. Gal i leo s greatest contributions to sciencewere his descriptions of the behavior of moving objects, whichhe derived from experimentation. The method of using experi-ments to determine natural laws had essentially been lost s incethe time of the early Greeks.

All astronomical d iscover ies before Galileo's time were madewithout the aid of a telescope. ln 1609 , Galileo heard that a Dutchlens maker had dev ised a system of lenses that magnified objects.Apparentlywithout eve r see ing a telescope, Galileo constructedh is own, which magnified d is tant objects three t imes the size seenby th e unaided eye. He immediatelymade o the rs , t he bes thavinga magn i fi ca ti on o f about 3 0 . 1 ? 1 5 1 . 1 . ti).With the telescope, Gal i leo was able to view the universe in ane w way . He made many important discoveries that supportedthe Copern ican v iew o f the universe, including the following:

1 . The discovery of ]upiters four largest satellites, ormoons 1%).his find dispelled the ol d ideat ha t Ear th wa s the sole center ofmotion in the un iverse;for here , plainly visible, wa s another center ofmotion-Iupiter. It also countered the frequently used argumentthat the Moon wou ld be left behind if Earth revolvedaround the Sun.

2 . The discovery that th e planets ar e circular disks ratherthan just po in ts o f light, a s was previously thought. Thisindicated that the p lanets must be Earth-like a s opposedto star- like.

TABLE 2 1 . 1 Pe ri od o f Revolut ion an d Solar Distanceso f PlanetsSolar Distance Period EllipticityPlanet (AU) (years) 0 = circle

Mercury 0.39 0.24 0.205Venus 0.72 0.62 0.007Eth iii1.00fi* H 1.00 i g FL017Mars 1.52 1.8 8 0.094Jupiter 5.20 11.8 6 0.049Saturn 9.54 29.46 if 0.057Uranus 19 .1 8 84.01 0.046eptune 30.06 164.80 0 . 0 1 1

*AU = astronomical unit

;l~. I tal ian scientist Galileo Galilei (1564-1642) used ane w invention, the telescope, to observe the Sun, Moon , and planetsin more deta il than ever before. (Nimatallah/Art R e s o u r c e , N . Y . )it 1 ; - I . One of Gal i leo's telescopes. Although Galileo d id n otinvent the telescope, he built severalthe l a rges t o f which ha d amagnification Of 30. (Photo by Giann i Tor to li /Photo Researchers, Inc.)


10/24


11/24


12/24


13/24


14/24


15/24

The Motions of Earth 625North celestial pole

Celestialsphere Star

._

Celestialequator

South celestial pole1 1 1 % iiiQ - i i z l it; % t i Astronomical coordinate system on the celestial sphere.

C O N C E P T c n r z c r c 2 1 .3QHow do modern ast ronomers use constel lat ions?Q Howmany constel lat ions ar e currently recognized?Q How ar e the brightest stars in a constellation denoted?QBriefly descr ibe the equatorial system.

f . i i . . ? . ' l . Star t ra i ls i n t he reg io n o f Polaris (north celestial pole) on a t ime exposure. ( P h o t oby Doug las Kirkland/CORBIS)

i i i 1 : 5 - i i l i i i i i Locat ing t he No r th Star (Polaris) from the pointer starsin t he B ig Dipper, which is part of the conste l la tion Ursa Major. TheBig D ippe r is shown soon after sunset in December ( lower f igure),April (upper figure), an d August (left).

The Mot ions of EarthThe tw o primarymotions of Earth are rotation an d revolution.A lesser motion is axial precession. Rotation is the turning, orsp inn ing , o f a body on its axis. Revolution is the motion of abody, such a s a planet or moon, along a path around some pointin space. For example, Earth revolves a r ound t he Sun, and t heMoon revolves around Earth. Earth a lso has another ver y s lowmotion known a s axial precession, which is the gradual changein the orientation of Earths axis over a period of 26,000 years.RotationThe main consequences of Earths rotation are day an d night.Earths rotation has become a s tandard method of measur ingtime because it is s o dependable and easy to use. Yo u maybe sur-pr ised to learn that Earths rotation i s measured in two ways, mak-in g tw o k in d s o f days. Most familiar is themean so la r day, the t ime interval f rom on enoon t o t he next, which averages about 24hours. Noon is when the Sun has reached

its highest point in the sky.The sidereal (sicler: s t a r , at = pertainingto) day, on the other hand, is the t ime it takesfo r Earth to make one complete rotation (360degrees) with respectto a star other t han ou rSun. The siderealday is measured bythe t imerequiredfor a star to reappear a t the ident icalposit ion in the sky. The s ide rea l day has aperiodo f 23 hours, 5 6 minutes, an d four sec-onds (measured in solar time), which isalmost 4minutes shorter than the mean solarday. This difference results because the direc-tion to distant stars changes only infinitesi-mally, whereas the direction to the Su nchanges by a lmost I degree each day. Thisdifference is shown in . - i j - : i _ . . ' T - i i


16/24

626 C H A P T E R 2 1 O r ig in s o f M o d e rn A s t r o n o m y

The difference between a so la r day an d a sidereal day. Locat ions X an d Y are directly opposite each other . Ittakes Earth 23 hour s and 56 minutes t o make one rotat ion with respect t o t he stars (s idereal day). However , no ti ce thatafter Earth has rotated once with respec t to the stars, poin t Y is no t ye t returned t o t he noon posit ion with respect to theSun. Earth has t o ro ta te ano the r 4 minut es t o complete the solar day.

Why do we use the mean so la r day to measure t ime rather thanthe s iderea l day? Consider the fact that in s iderea l t ime, noonoccurs 4 minutes ear li e r each day. Therefore , a f ter a span of 6months, noon would occur at midnight. However, observato-ries use clocks that keep s idereal t ime because the stars appear tomove through the sky in sidereal t ime. Simply, if a star is sighteddirectly south of an observatory a t 9 : 00 P . M . (siderealt ime) itwill appear in the same direction at t ha t t ime K

Precess ionA third an d very slow movement of Earth is called axial preces-sion. Although Earths axis mainta ins approximately the sameangle of tilt, the direction in which the axis points continuallychanges ( i + i g n ; i .- e : % ' i . ; 2 s ' n .) . As a result, the axis traces a circle on the

_ rr rtottittt Earths orb ita l mot ion causes t he apparen t position of the Sun toevery (sidereal) daY- shift about 1 degree each da y on the celestial sphere.RevolutionEarth revolves around the Sun in an ell iptical orbit a t a naverage speed of 107,000 kilometers (66,000 miles) pe rhour. Its average distance from the Sun is 1 5 0 mill ion kilo-meters (9 3 million miles), but because its orbit is ane ll ipse, Eart h' s distance from the Sun var ie s. Atperihelion(peri = near, hel ios= sun) it is 147 million kilo-meters (91.5 millionmiles) distant, which occurs aboutJanuary 3 each year. At aphelion (apo = away, helios =sun) Earth is 1 5 2 million kilometers (94.5 millionmiles)distant, which occurs about July 4 .Because of Earths orbital movemen t the Sunappears to be displaced among the conste lla tions at adistance equal t o abou t twice its width, or I degree eachday. The apparent annual path of the Sun agains t thebackdrop of the celestial sphere is called the ecliptic

; ? 1 ) i . . ; i 4 ) . The p la ne ts a nd the Moon travel innearly the same plane a s Earth . Hence, their paths onthe celest ial sphere also l ie nea r the ecliptic.The imaginary plane that connects points along theecliptic is cal led the plane of the ecliptic. As measuredfrom t hi s imag inary plane, Earths axis is tilted about2 3 % degrees (Figure 21.24). This angle is very impor-tant to Earths inhabitants because the inclinationofEarths ax is causes the year ly cycle of seasons, a topicdiscussed in detail in Chapter 1 6 .

Celestialsphere/North celest ia l pole


17/24


18/24

Ec li pse s o f t he Sun an d Moon 629


19/24

Fm

B. Sithases of the Moon. A . T he ou te r f igu re s show the phases as seen from Earth. B. Compare these photographs with

the diagram. (Pho tos UC Regen ts /L ick Obse rva to ry )

Eclipses of the Sun and MoonAlong with understanding the Moons phases, t he ea r ly Greeksalso real ized that eclipses are simply shadow effects. When theMoon moves in a line directly between Earth an d the Sun, whichca n occur only during the new-Moon phase, it casts a darkshadow on Earth, producing a solar eclipse (eclipsis = failure toappear) ? i ' l ' l l . I i - l l i l i - ) . Conversely, the Moon is eclipsed (lunareclipse) when it moves withinEarths shadow, a situation that ispossible only during the full-Moon phase I 3 . l l . T l _ , T - i . f E I i ) .

Why does a so lar ec l ipse not occur with every new-Moonphase an d a lunar eclipse with every full Moon? They would, ifthe orbit of the Moon l a y exac tl y a long the p lane of Earths orbit.However , t he Moon s orbit is inclined about 5 degrees to th eplane of the ecliptic. Thus, during mos t new -Moon phases, theshadow of the Moon passes either above or below Earth; andduring most full-Moon phases, the shadow of Earth misses theMoon. An eclipse ca n only take place when a new - o r full-Moonphase occurs while the Moons orbit crosses the plane of theecliptic.


20/24


21/24


22/24


23/24


24/24

Earth Science Chapter 21

Documents

Transcript of Earth Science Chapter 21