ch2 discovering the universe

7/31/2019 CH2 Discovering the Universe

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Discovering the Universe

Learning Goals

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2.1 Patterns in the Night Sky

What does the universe look like from Earth?

Why do stars rise and set?

Why do the constellations we see depend on latitude

and time of year?

2.2 The Reason for Seasons

What causes the seasons?

How does the orientation ofEarths axis change with

time?

2.3 The Moon, Our Constant Companion

Why do we see phases of the Moon?

What causes eclipses?

2.4 The Ancient Mystery of the Planets

Why was planetary motion so hard to explain?

Why did the ancient Greeks reject the real explanation

for planetary motion?


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2.1 PATTERNS IN THE NIGHT SKY

What does the universe look like from Earth?

Why do stars rise and set? Why do the constellations we see depend on

latitude and time of year?

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Stars and other celestial objects appear to lie on a great

celestial sphere surrounding Earth. We divide thecelestial sphere into constellationswith well-defined

borders. From any location on Earth, we see half thecelestial sphere at any given time as the dome of ourlocal sky, in which the horizonis the boundary betweenEarth and sky, the zenith is the point directly overhead,and the meridianruns from due south to due norththrough the zenith.

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Constellations- is a regionof thesky with well-defined borders; thefamiliar patterns of stars merely helpus locate these constellations.

The Celestial Sphere The stars ina particular constellation appear to lieclose to one another but may actuallybe at very different distances fromEarth. This illusion occurs becausewe lack depth perception when welook into space, a consequence of thefact that the stars are so far away.The ancient Greeks mistook thisillusion for reality, imagining the starsand constellations to lie on a greatcelestial sphere that surroundsEarth.

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Red lines mark official bordersof several constellations nearOrion.

Yellow lines connect

recognizable patterns of starswithin constellations.

Sirius, Procyon, and Betelgeuseform a pattern that spans severalconstellations and is called theWinter Triangle. It is easy to see

on clear winter evenings.


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The stars appear to lie on a great celestial sphere that surrounds Earth. This isan illusion created by our lack of depth perception in space, but it is useful for

mapping the sky.


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The band of light that we call the Milky Waycircles all the way around the celestialsphere, passing through more than a dozenconstellations, and bears an importantrelationship to the Milky Way Galaxy: Ittraces ourgalaxys disk of starsthe galactic

planeas it appearsfrom our location in theoutskirts of the galaxy.

The Milky Way Galaxy is shaped like a thinpancake with a bulge in the middle. We view

the universe from our location a little morethan halfway out from the center of thispancake. In all directions that we look withinthe pancake, we see the countless stars andvast interstellar clouds that make up the MilkyWay in the night sky; that is why the band oflight makes a full circle around our sky.

The Milky Way appears somewhat wider inthe direction of the constellation Sagittarius,because when we look in that direction, weare looking toward the galaxys central bulge.

We have a clear view to the distantuniverse only when we look away from thegalactic plane, along directions that haverelatively few stars and clouds to block ourview.


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The dark lanes that run down thecenter of the Milky Way contain thedensest clouds, obscuring our view ofstars behind them.

In fact, these clouds generally

prevent us from seeing more than afew thousand light-years into ourgalaxys disk.

As a result, much of our own galaxyremained hidden from view until just afew decades ago, when newtechnologies allowed us to peerthrough the clouds by observing

forms of light that are invisible to oureyes (such as radio waves and Xrays).


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Our local skythe sky as seen from

wherever you happen to be standingappears to take the shape of ahemisphere or dome. The dome shapearises from the fact that we see only halfof the celestial sphere at any particularmoment from any particular location, while

the ground blocks the other half from view.

The boundary between Earth and skydefines the horizon. The point directlyoverhead is the zenith. The meridianisan imaginary half-circle stretching from thehorizon due south, through the zenith, to

the horizon due north.

We can pinpoint the position of anyobject in the local sky by stating itsdirection along the horizon and itsaltitude above the horizon.

From any place on Earth, the local sky looks like adome (hemisphere). This diagram shows keyreference points in the local sky. It also shows howwe can describe any position in the local sky by itsaltitude and direction.


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Our lack of depth perception on the celestial

sphere makes it difficult to judge the true sizesor separations of the objects we see in the sky.However, we can describe the angularsizes orseparations of objects even without knowinghow far away they are.

The angular size of an object is the angle it

appears to span in your field of view. Forexample, the angular sizes of the Sun and theMoon are each about (a).

Note that angular size does not by itself tellus an objects true size, because angular size

also depends on distance: The farther away anobject is, the smaller its angular size.

For example, the Sun is about 400 timeslarger in diameter than the Moon, but it has thesame angular size in our sky because it is alsoabout 400 times farther away.

We measure angular sizesor angular

distances, rather than actual sizes ordistances, when we look at objects in the sky.


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The angular distance between a pair of objects in the sky is theangle that appears to separate them.

For example, the angular distance between the pointer stars at theend of the Big Dippers bowl is about 5 (b). You can use youroutstretched hand to make rough estimates of angles in the sky (c).


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For more precise astronomical measurements, we subdivide each degree into 60arcminutes (abbreviated) and subdivide each arcminute into 60 arcseconds(abbreviated ).For example, we read 3527 15 as 35 degrees, 27 arcminutes, 15 arcseconds.


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Earths rotation makes stars appear to circle

around Earth each day. A star whose completecircle lies above our horizon is said to be

circumpolar. Other stars have circles that crossthe horizon, so they rise in the east and set inthe west each day.

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Earths daily rotation explains the apparent daily motions of celestial objects in our sky.


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Key facts about the paths of various starsthrough the local sky:

Stars near the north celestial pole do notrise or set; rather, they remain above the

horizon and make daily counterclockwisecircles around the north celestial pole. Wesay that such stars are circumpolar.

Stars near the south celestial pole neverrise above the horizon at all.

All other stars have daily circles that arepartly above the horizon and partly below it.Because Earth rotates from west to east(counterclockwise as viewed from abovethe North Pole), these stars appear to risein the east and set in the west.


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The visible constellations vary with time of year

because our night sky lies in different directionsin space as we orbit the Sun. The constellations

vary with latitudebecause your latitudedetermines the orientation of your horizonrelative to the celestial sphere. The sky does notvary with longitude.

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Latitudemeasures north-southposition on Earth, and longitudemeasures east-west position.

Latitude is defined to be 0 at the

equator, increasing to 90N at theNorth Pole and 90S at the SouthPole. By international treaty,longitude is defined to be 0 alonga line passing through Greenwich,England. Stating a latitude and a

longitude pinpoints a location onEarth.

For example, Miami lies at about26N latitude and 80W longitude.


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The sky varies with latitude. Notice that the altitude of the celestial pole that is visible inyour sky is always equal to your latitude.


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The altitude of the celestial pole in your sky is

equal to your latitude.


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The night sky change throughout the year because ofEarths changing position in its orbit around the Sun. AsEarth orbits, the Sun appearsto move steadily eastwardalong the ecliptic, with the stars of different constellations inthe background at different times of year.

The constellations along the ecliptic make up what we callthe zodiac; tradition places 12 constellations along thezodiac, but the official borders include a thirteenthconstellation, Ophiuchus.

The Suns apparent location along the ecliptic determineswhich constellations we see at night.


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2.2 THE REASON FOR SEASONS

What causes the seasons?

How does the orientation of Earths axis change with

time?

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The tilt of Earths axis causes the seasons. The axis

points in the same direction throughout the year, so asEarth orbits the Sun, sunlight hits different parts of Earthmore directly at different times of year. The summer

and winter solstices are the times during the yearwhen the Northern Hemisphere gets its most and leastdirect sunlight, respectively. The spring and fallequinoxes are the two times when both hemispheres

get equally direct sunlight.

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To help us mark the changing of the seasons, we define four special moments

in the year, each of which corresponds to one of the four special positions inEarths orbit.

The summer (June) solstice, which occurs around June 21, is the momentwhen the Northern Hemisphere is tipped most directly toward the Sun (and theSouthern Hemisphere is tipped most directly away from it).

The winter (December) solstice, which occurs around December 21, is themoment when the Northern Hemisphere is tipped most directly away from theSun (and the Southern Hemisphere is tipped most directly toward it).

The spring (March) equinox, which occurs around March 21, is the moment

when the Northern Hemisphere goes from being tipped slightly away from theSun to being tipped slightly toward the Sun.

The fall (September) equinox, which occurs around September 22, is themoment when the Northern Hemisphere first starts to be tipped away from theSun.


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The equinoxes occur on the only two daysof the year on which the Sun rises preciselydue east and sets precisely due west. Theseare also the only two days when sunlight fallsequally on both hemispheres.

The summer solstice occurs on the day that

the Sun follows its longest and highest paththrough the Northern Hemisphere sky (and itsshortest and lowest path through theSouthern Hemisphere sky). It is therefore theday that the Sun rises and sets farther to thenorth than on any other day of the year, andon which the noon Sun reaches its highest

point in the Northern Hemisphere sky. Theopposite is true on the day of the wintersolstice, when the Sun rises and sets farthestto the south and the noon Sun is lower in theNorthern Hemisphere sky than on any otherday of the year.


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Earths 26,000-year cycle of precession changes theorientation of its axis in space, although the tilt remainsabout 23. Thechanging orientation of the axis does notaffect the pattern of seasons, but it changesthe identity ofthe north star and shifts the locations of the solstices andequinoxes inEarths orbit.

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Precession- a gradual wobble that changes the orientation of

Earths axis in space.


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Why does precession occur?

It is caused by gravitys effect on a tilted, rotating object that is not a

perfect sphere. A spinning top precessesbecause Earths gravity triesto pull over its lopsided, tilted spin axis. Gravity does not succeed inpulling it overat least until friction slows the rate of spinbut insteadcauses the axis to precess. The spinning Earth precesses becausegravitational tugs from the Sun and Moon try to straighten out ourplanets bulging equator, which has the same tilt as the axis. Again,gravity does not succeed in straightening out the tilt but only causesthe axis to precess.

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2.3 THE MOON, OUR CONSTANTCOMPANION

Why do we see phases of the Moon?

What causes eclipses?

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The phaseof the Moon

depends on its positionrelative to the Sun as itorbits Earth. The half ofthe Moon facing the Sunis always illuminatedwhile the other half isdark, but from Earth we

see varyingcombinations of theilluminated and darkhalves.

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The phases of the ball result fromjust two basic facts:

1. Half the ball always faces theSun (or flashlight) and thereforeis bright, while the other halffaces away from the Sun andtherefore is dark.

2. As you look at the ball atdifferent positions in its orbit

around your head, you seedifferent combinations of itsbright and dark faces.

Notice that the phases from new to fullare said to be waxing, which meansincreasing. Phases from full to neware waning, or decreasing. Also noticethat no phase is called a half moon.Instead, we see half the moons face at

first-quarter and third-quarter phases;these phases mark the times when theMoon is one-quarter or three-quarters ofthe way through its monthly cycle (takento begin at new moon).

The phases just before and after newmoon are called crescent, while thosejust before and after full moon arecalled gibbous(pronounced with a hardgas in gift).


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We see a lunar eclipse when Earths shadow falls on the Moon anda solareclipse when the Moon blocks our view of the Sun. We do notsee an eclipse at every new and full moon because the Moons orbit isslightly inclined to the ecliptic plane. Eclipses come in different types,depending on where the dark umbraland lighter penumbralshadowsfall.

A solar eclipse occurs whenthe Moon lies directly betweenthe Sun and Earth, so that theMoons shadow falls on Earth.

People living within the areacovered by the Moonsshadow will see the Sunblocked or partially blockedfrom view.

A lunar eclipse occurswhen Earth lies directlybetween the Sun and theMoon, so that Earths

shadow falls on theMoon.


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The two points in each orbit at which the Moon crosses the

surface are called the nodesof the Moons orbit.

Eclipses can occur only during these periods (called eclipseseasons) when the nodes line up with the Sun and Earth:

a) A lunar eclipse occurs when a full moon occurs at or very nearoneof the nodes.

b) A solar eclipse occurs when a new moon occurs at or very nearoneof the nodes.


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The shadow of the Moon or Earth consists of two distinctregions: a central umbra, where sunlight is completelyblocked, and a surrounding penumbra, where sunlight isonly partially blocked.


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A lunar eclipse begins at the moment when the Moons orbit firstcarries it into Earths penumbra. After that, we will see one of threetypes of lunar eclipse.

1. If the Sun, Earth, and Moon are nearly perfectly aligned, the Moonwill pass through Earths umbra and we will see a total lunar

eclipse.

2. If the alignment is somewhat less perfect, only part of the fullmoon will pass through the umbra (with the rest in the penumbra)and we will see a partial lunar eclipse.

3. If the Moon passes only through Earths penumbra, we will see apenumbral lunar eclipse.


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Penumbral eclipses are slightly more common that total lunar eclipses and

partial lunar eclipses, but they are the least visually impressive because thefull moon darkens only slightly.

Earths umbral shadow clearly darkens part of the Moons face during apartial lunar eclipse, and the curvature of this shadow demonstrates that Earthis round.

A total lunar eclipse is particularly spectacular because the Moon becomesdark and eerily red during totality, the time during which the Moon is entirelyengulfed in the umbra. Totality typically lasts about an hour. The Moonbecomes dark because it is in shadow, and red because Earths atmosphere

bends some of the red light from the Sun toward the Moon.


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If a solar eclipse occurs when the Moon is relatively close to Earth in its orbit,

the Moons umbra touches a small area of Earths surface (no more thanabout 270 kilometers in diameter). Within this area you will see a total solareclipse.

Surrounding the region of totality is a much larger area (typically about 7000kilometers in diameter) that falls within the Moons penumbral shadow. Hereyou will see a partial solar eclipse, in which only part of the Sun is blockedfrom view.

If the eclipse occurs when the Moon is relatively far from Earth, the umbramay not reach Earths surface at all. In that case, you will see an annulareclipsea ring of sunlight surrounding the moonfrom a position directlybehind the umbra; again, you will see a partial solar eclipse in the surroundingpenumbral shadow.


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The combination of the moving nodes and the 29-day cycle of lunarphases makes eclipses recur in a cycle of about 18 years 11 days.This cycle is called the saros cycle.

If a solar eclipse occurred today, the one that would occur 18 years11 days from now would not be visible from the same places on Earth

and might not be of the same type.


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Today, we can predict eclipses because we

know the precise details of the orbits of Earthand the Moon.

Table 2.1 lists upcoming lunar eclipses;notice that, as we expect, eclipses generally

come a little less than 6 months apart.

Figure 2.25 shows paths of totality for upcoming total

solar eclipses (but not for partial or annular eclipses),using color coding to show eclipses that repeat with thesaros cycle.


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2.4 THE ANCIENT MYSTERY OF THEPLANETS

Why was planetary motion so hard to explain?

Why did the ancient Greeks reject the real

explanation for planetary motion?

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Planets generally appear to moveeastward relative to the stars overthe course of the year, but forweeks or months they reverse

course during periods of apparentretrograde motion. This motionoccurs when Earth passes by (oris passed by) another planet in itsorbit, but it posed a major mystery

to ancient people who assumedEarth to be at the center of theuniverse.

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The Greeks rejected the idea that Earth goes around the Sun in

part because they could not detect stellar parallaxslightapparent shifts in stellar positions over the course of the year. Tomost Greeks, it seemed unlikely that the stars could be so far

away as to make parallax undetectable to the naked eye, eventhough that is, in fact, the case.

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They concluded that one of the

following must be true:

1. Earth orbits the Sun, but the

stars are so far away thatstellar parallax is notdetectable to the naked eye.

2. There is no stellar parallaxbecause Earth remainsstationary at the center of theuniverse.

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2The Big Picture You can enhance your enjoyment of astronomy by observing the sky. The moreyou learn about the appearance and apparent motions of objects in the sky, themore you will appreciate what you can see in the universe.

From our vantage point on Earth, it is convenient to imagine that we are at thecenter of a great celestial sphereeven though we really are on a planet orbiting

a star in a vast universe. We can then understand what we see in the local sky bythinking about how the celestial sphere appears from our latitude.

Most of the phenomena of the sky are relatively easy to observe andunderstand. The more complex phenomenaparticularly eclipses and apparentretrograde motion of the planetschallenged our ancestors for thousands of

years. The desire to understand these phenomena helped drive the developmentof science and technology.

ch2 discovering the universe

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