OverDrive, Inc.Scientific American's Ask the Experts.jpg

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Answers to the Most Puzzling and Mind-Blowing

Science Questions

by the editors of

Scientific Americans

ASK THE EXPERTS

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An e -book e xce rpt from

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Contents

1 Celestial BodiesASTRONOMY

It Came From Outer Spaceas te ro ids , me teo rs , and come ts

How crowded is the asteroid belt? 1

What causes a meteor shower? 4

Is it possible that a meteorite could strike

a commercial airliner and cause it to explode? 5

Why are impact craters always round? 6

Heavenly Bodies p lane t s and moons

What defines a true planet, and why might

Pluto not qualify? 8

Why do the moon and the sun look so much larger

near the horizon? 11

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What is a blue moon? 13

Why are planets round? 14

How do scientists measure the weight of a planet? 15

How fast is the earth moving? 17

Why and how do planets rotate? 19

Star Light, Star Brights ta r s

What exactly is the North Star? 21

How long do stars usually live? 23

Why do stars twinkle? 25

Far, Far Away . . .t he un i ve rse

How do we know our location within

the Milky Way galaxy? 26

Why is the night sky dark? 28

Does the fact that the universe is continually

expanding mean that it lacks a physical edge? 31

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2 Its Alive!B IOLOGY

The Grass Is Always Greener p lan t s

What causes the leaves on trees to change

color in the fall? 35

How does the Venus flytrap digest flies? 37

How do trees carry water from the soil around

their roots to the leaves at the top? 39

Creepy Crawlers i nsec t s

How is bug blood different from our own? 41

What kind of illnesses do insects get? 43

How do flies and other insects walk up walls? 44

Why is spider silk so strong? 45

If a used needle can transmit HIV, why cant a mosquito? 46

Under the Seaocean l i f e

How do squid and octopuses change color? 47

Why do some fish normally live in freshwater and others

in saltwater? How can some fish adapt to both? 50

How can sea mammals drink saltwater? 53

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How do deep-diving sea creatures withstand

huge pressure changes? 56

How do whales and dolphins sleep without drowning? 57

Thats a Horse of a Different Coloran ima l k ingdom

Do hippopotamuses actually have pink sweat? 61

Why do cats purr? 62

Why do dogs get blue, not red, eyes in flash photos? 64

How do frogs survive winter? Why dont they

freeze to death? 66

Do unbred animals lack the individual distinctiveness

of humans? 69

Talkin About Evolutionevo lu t i on

Is there any evolutionary advantage to gigantism? 71

What is the point in preserving endangered

species that have no practical use to humans? 74

What do we know about the evolution of sleep? 76

When Dinosaurs Ruled the Earthd inosau rs

What are the odds of a dead dinosaur becoming

fossilized? 78

What kind of evidence could be found in the

fossil record (or anywhere else) that would prove

whether some dinosaurs were warm-blooded? 80

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How close are we to being able to clone a dinosaur? 84

Did any dinosaurs have poisonous saliva,

as in Jurassic Park? 85

If T. rex fell, how did it get up, given its tiny

arms and low center of gravity? 86

3 Being Human

Its All in the Geneshuman evo lu t i on

Is the human race still evolving? Isnt culture

a more powerful force? 89

Can the human race be devolving? 92

Why are we getting taller as a species? 94

Why do men have nipples? 96

Oh, Behave!human behav io r

How did the smile become a friendly

gesture in humans? 99

Why are more people right-handed?

Do other primates show a similar tendency

to favor one hand over the other? 100

How long can humans stay awake? 103

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Do humans have some kind of homing

instinct like certain birds do? 105

Why do we yawn when we are tired? And why

does it seem to be contagious? 107

You Havent Aged a Bitgrowing o lde r

Why does hair turn gray? 109

Do people lose their senses of smell and taste as

they age? 111

Anatomy 101t he human body

What is the function of the human appendix? 113

What makes the sound when we crack our knuckles? 115

Why does your stomach growl when you are hungry? 118

How can you live without one of your kidneys? 120

Why do fingers wrinkle in the bath? 122

If the cells of our skin are replaced regularly,

why do scars and tattoos persist indefinitely? 124

Why does fat deposit on the hips and thighs of

women and around the stomachs of men? 125

The Dr. Is Inhea l th and med ic ine

Why do hangovers occur? 128

Why does reading in a moving car cause

motion sickness? 130

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Why do we get the flu more often in the winter

than in other seasons? 132

What happens when you get a sunburn? 134

There are many kinds of cancer, so why is there

no heart cancer? 136

Is there any proof that Alzheimers disease is related

to exposure to aluminumfor instance, by using

aluminum frying pans? 138

How long can the average person survive without water? 139

4 As a Matter of FactCHEMISTRY

Elementary, My Dear Watson . . .t he e lemen ts

Why doesnt stainless steel rust? 141

If nothing sticks to Teflon, how does it stick to pans? 143

What determines whether a substance is transparent? 144

If You Cant Stand the Heat, Get Out of the Kitchen!eve ryday chemis t r y

Why do my eyes tear when I peel an onion? 145

Why do spicy (or hot) foods cause

the same physical reactions as heat? 148

Why does bruised fruit turn brown? 150

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How is caffeine removed to produce decaffeinated coffee? 151

What is the difference between artificial and natural flavors? 153

How can an artificial sweetener contain no calories? 155

Do vitamins in pills differ from those in food? 157

Where Theres Smoke, Theres a Fire more chemis t r y

How does a flame behave in zero gravity? 159

How does fingerprint powder work? 161

5 Theres No PlaceLike Home

EARTH SC IENCE

Everybody Talks About It . . .wea the r

Why do clouds float when they have so much water

in them? 163

What causes thunder? 165

Why are snowflakes symmetrical? 167

Why are some rainbows bigger than others? 168

What is the meaning of the phrase, It is too cold

to snow? Doesnt it have to be cold for it to snow? 170

Why do hurricanes hit the East Coast of the

United States but never the West Coast? 171

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Up Abovet he a tmosphe re

If chlorofluorocarbons are heavier than air,

how do they reach the ozone layer? 173

What determines the shape of a mushroom

cloud after a nuclear explosion? 174

The Upper Crustear th s su r face and be low

How do volcanoes affect world climate? 175

Where do geysers get their water from? 179

How do scientists measure the temperature

of the earths core? 182

What causes the regular, wavelike shapes that

form in the sand on beaches? 184

What is quicksand? 186

Lets Get Wet oceans

How did the oceans form? 188

Why does the ocean appear blue? Is it because

it reflects the sky? 190

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6 Count on Me MATHEMAT ICS

AND COMPUTERS

Much Ado About Nothingze ro

What is the origin of zero? 191

Give Em an Inchmeasuremen t

On average, how many degrees apart is any

one person in the world from another? 193

Where does the measurement of the meter come from? 195

How does a laser measure the speed of a car? 197

Does Not Computecompu te rs

Why do computers crash? 199

How do Internet search engines work? 201

How do rewritable CDs work? 203

When did the term computer virus arise? 205

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7 Lets Get PhysicalPHYSICS

Let There Be Lightl i gh t

How do surfaces, such as pavement, become

heated from the sun? 207

What is the physical process by which a

mirror reflects light rays? 208

How does sunscreen protect the skin? 210

Why are sunsets orange? 212

Im Very Particularpar t i c les

If we cannot see electrons and protons, or

smaller particles such as quarks, how can we be

sure they exist? 213

Is glass really a liquid? 215

Now Hear Thissound

How can the extremity of a whip travel

faster than the speed of sound to produce the

characteristic crack? 216

What causes the noise emitted from

high-voltage power lines? 217

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What are booming sands and what

causes the sounds they make? 219

What happens when an aircraft breaks the

sound barrier? 221

In Theoryt heo re t i ca l phys ics

Is it theoretically possible to travel through time? 223

Is dark matter theory or fact? 227

Would you fall all the way through a theoretical

hole in the earth? 230

What is antimatter? 232

Does the speed of light ever change? 234

You Wont Believe Your Eyest he phys ics o f see ing

Why do beautiful bands of color appear in

the tiny oil slicks that form on puddles? 236

Why is it that when you look at the spinning

propeller of a plane or fan, at a certain speed,

the blades seem to move backward? 237

Why do jets leave a white trail in the sky? 239

Shake It Upeve ryday phys ics

Does hot water freeze faster than cold water? 241

How does a microwave oven cook foods? 243

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Why does shaking a can of coffee cause the

larger grains to move to the surface? 244

Why does a shaken soda fizz more than an

unshaken one? 246

Bottom of the 9th, Bases Loadedt he phys ics o f baseba l l

What makes a knuckleball appear to flutter? 248

Why does a ball go farther when hit with an

aluminum bat? 250

Index

About the Authors

Credits

Cover

Copyright

About the Publisher

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1

CelestialBodies

A S T R O N O M Y

It Came From Outer Spaceasteroids, meteors, and comets

How crowded is the asteroid belt?

A N S W E R E D B Y :

Tom Gehrel, University of Arizona, Tuscon, Arizona.

A veteran asteroid hunter, he and his colleagues find

roughly 20,000 objects a yearmany of them

uncatalogued asteroidsusing the Spacewatch

Telescope on Kitt Peak.

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2 Celestial Bodies

Some scientists were seriously concerned about the pos-sible high density of objects in the asteroid belt,which lies between the orbits of Mars and Jupiter, whenthe first robotic spacecraft were scheduled to be sentthrough it. The first crossing of the asteroid belt tookplace in the early 1970s, when the Pioneer 10 and Pioneer11 spacecraft journeyed to Jupiter and beyond. The dan-ger does not lie in the risk of hitting a large object. In fact,such a risk is minuscule because there is a tremendousamount of space between Mars and Jupiter and becausethe objects there are very small in relation. Even thoughthere are perhaps a million asteroids larger than one kilo-meter in diameter, the chance of a spacecraft not gettingthrough the asteroid belt is negligible.

Even if there were 100,000 sizable asteroids (morethan a few kilometers in size) in the asteroid beltand thereal number is quite likely about 10 times lessthe aver-age separation between them would be about five millionkilometers. That is more than 10 times the distancebetween the earth and the moon. If you were standing onone of those asteroids and looked up, you would not see asky full of asteroids; your neighbors would appear so smalland dim that you would be quite lucky to even see one, letalone hundreds.

In some ways, the asteroid belt is actually emptier thanwe might like. In the early 1990s, the National Aeronau-tics and Space Administration wanted the Galileo space-

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It Came From Outer Space 3

craft to encounter an asteroid while it was passing throughthe asteroid belt on its way to Jupiter. But it took someeffort to find an object that was located even roughlyalong Galileos path. Special targeting was required toreach this object, but the result was the first close-up viewof an asteroid, the one called Gaspra.

The number of objects in the asteroid belt increasessteeply with decreasing size, but even at micrometer sizesthe Pioneer spacecraft were hit only a few times duringtheir passage. That is not to say that asteroids cannot poseany danger, however. It is worth noting that for a largeplanet like Earth, over a long period of time, there is anappreciable chance of being hit. This hazard comes fromthe fragments of mutual collisions in the asteroid belt;after their break-up, some of these fragments move towardthe earth under the gravitational action of Jupiter.

An asteroid about 12 kilometers in diameter crashedinto the earth 65 million years ago, killing nearly 90 per-cent of the animals, including the dinosaurs. Such majorimpacts are very rare events, but for smaller objects thelikelihood of impact increases; the chance of the earthbeing struck by an object approximately one kilometer insize is about one in 5,000 in a human lifetime. An objectone kilometer across would still be large enough to cause aglobal disaster because of the enormous energy it wouldrelease upon impact: at least a million times the energy ofthe bomb dropped on Hiroshima in 1945.

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M eteor showers occur when the earth in its orbit aroundthe sun passes through debris left over from the disin-tegration of comets. Although the earths orbit around thesun is almost circular, most comets travel in orbits that arehighly elongated ellipses. As a result, some comets haveorbits that intersect or partially overlap the earths path.

Because a comets nucleus is made up of a combina-tion of icy materials and loosely consolidated dirt, whena comet is heated by passing close to the sun, it more orless slowly disintegrates, producing the visible tail. Therocky debris, consisting of mostly sand-size particles, con-tinues in an elongated orbit around the sun close to thatof its parent comet. When the earth intersects this orbit inits annual trip, it can run into this debris, which burns upon entry into the earths atmosphere, producing a visibleshower of meteors.

Meteor showers associated with particular cometorbits occur at about the same time each year, because it is

What causes a meteor shower?

A N S W E R E D B Y :

Gregory A. Lyzenga, Professor of Physics,

Harvey Mudd College, Claremont, California.

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at those points in the earths orbit that the collisions occur.However, because some parts of the comets path arericher in debris than others, the strength of a meteorshower may vary from one year to the next. Typically ameteor shower will be strongest when the earth crosses thecomets path shortly after the parent comet has passed.

I t is certainly possible, although the probability is low.We can make a very rough estimate by comparing thearea of airliners with the area of cars in the United States.A typical car has an area on the order of 10 square meters,and there are roughly 100 million cars in the UnitedStates, for a total area of about 1,000 square kilometers.The typical airliner has a cross-sectional area of severalhundred square meters, but the number of planes is muchsmaller than the number of cars, perhaps a few thousand.

Is it possible that a meteorite could strike acommercial airliner and cause it to explode?

A N S W E R E D B Y :

David Morrison, NASA Ames Research Center,

Maffett Field, California.

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The total area of airliners is therefore no more than 10square kilometers, or a factor of at least 100 less than thatof cars. Three cars are known to have been struck bymeteorites in the United States during the past century, soit would appear that the odds are against any airplaneshaving been hit, but it is not impossible that one mighthave been.

If an airplane were hit, it would be more likely tooccur on the ground than in the air, because airplanesspend more time overall on the ground.

W hen geologists and astronomers first recognized thatcraters were produced by impacts, they surmisedthat much of the impacting body might be found stillburied beneath the surface of the crater floor. Much later,however, scientists realized that at typical solar systemvelocitiesseveral to tens of kilometers per secondany

Why are impact craters always round?

A N S W E R E D B Y :

Gregory A. Lyzenga, Professor of Physics, Harvey Mudd

College, Claremont, California.

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impacting body must be completely vaporized when it hitsan object.

At the moment an asteroid collides with a planet,there is an explosive release of the asteroids huge kineticenergy. The energy is very abruptly deposited at whatamounts to a single point in the planets crust. This sud-den, focused release resembles more than anything else thedetonation of an extremely powerful bomb. As in the caseof a bomb explosion, the shape of the resulting crater isround: Ejecta are thrown equally in all directions regard-less of the direction from which the bomb may havearrived.

This behavior may seem at odds with our daily expe-rience of throwing rocks into a sandbox or mud, becausein those cases the shape and size of the crater is domi-nated by the physical dimensions of the impactor. In thecase of astronomical impacts, though, the physical shapeand direction of approach of the meteorite is insignifi-cant compared with the tremendous kinetic energy that itcarries.

An exception to this rule occurs only if the impactoccurs at an extremely shallow, grazing angle. If the angleof impact is quite close to horizontal, the bottom, middle,and top parts of the impacting asteroid will strike the sur-face at separate points spread out along a line. In thiscase, instead of the energy being deposited at a point, itwill be released in an elongated zoneas if our bombhad the shape of a long rod. This requires an impact at an

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angle of no more than a few degrees from horizontal. Forthis reason, the vast majority of impacts produce roundor nearly round craters, just as is observed.

Heavenly Bodiesplanets and moons

A nything in the solar system that is larger than a fewmeters in size and that does not produce stellar quanti-ties of heat and light is properly considered a planet of somesort (major planet, minor planet, small planet, tiny planet),unless it orbits another body besides the sunin which caseit is usually called a satellite of the larger body. Though notusually called planets, even comets could be thought of assmall, icy planets. For this reason, Pluto is surely a planet.

What defines a true planet, and why mightPluto not qualify?

A N S W E R E D B Y :

Daniel W. E. Green, Harvard-Smithsonian Center for

Astrophysics, Cambridge, Massachusetts.

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There are different kinds of planets in the solar system,and we cannot adequately classify all of them, because wedo not have enough information. A large group of planets1,000 kilometers across and smaller orbit the sun in a largebelt between the orbits of Mars and Jupiter; these smallplanets are usually referred to as asteroids or minor planets.Most asteroids have orbits that keep them always betweenMars and Jupiter (that is, their orbits are not terribly elon-gated). Many of them, however, have orbits that take themacross the orbits of major planets, including that of Earth.Comets are well known to have extremely elongated, orhighly elliptical, orbits. Some comets, such as Halleyscomet, have orbits that cross the orbits of many of theeight major planets.

Eight major planets, you might ask? Well, in reality, ithas come time to stop talking generally about the numberof planets in the solar system, because such figures can bemisleading. It is probably better to say simply that thesolar system is a system of objects that is dominated by astar (the sun), around which are found many orbitingbodies of sizes ranging from particles of dust and gas tothe giant gaseous planet Jupiter. There are four notablelarge gaseous planets (Jupiter, Saturn, Uranus, and Nep-tune), and inside the main asteroid belt are some smaller,rocky bodies (the most sizable of which are Mercury,Venus, Earth, and Mars). Orbiting the large gaseous plan-ets are dozens of satellites, seven of which, along with ourown moon, are larger than Pluto.

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It is only since 1978 that we have known the real sizeand approximate mass of Pluto; both of these are far smallerthan astronomers had thought soon after Plutos discovery in1930. For numerous reasons, Pluto was called the ninthplanet at the time; with little information to support orrefute that assertion, this classification became locked inastronomy textbooks for decades. But all along, Plutoappeared different from the eight known larger planets: Forinstance, its orbit is much more elliptical and more highlyinclined with respect to the ecliptic than are the orbits of thelarger planets, and its orbit brings it inside that of Neptune,so that Neptune is currently the outermost major planet.Pluto is so small that calling it a major planet is misleadingin the context of what we now know about the solar system.It is more accurately described as a planetesimal or aminor planet. There is even evidence that Pluto may ineffect be a giant comet. But much more work and observa-tion is needed before drawing clear conclusions.

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T he so-called moon illusion is one of the oldestknown psychological phenomena; records of it goback to ancient China and Egypt. It may be the mostancient scientific puzzle that is still unexplained.

People trained in the physical sciences often think thatthe illusion is real, that the moon actually looks large whenit is near the horizon because of the refraction of light bythe atmosphere. In fact, there is a very small refractiveeffect, but it is not the cause of the illusion.

There are a couple of ways you can prove to yourselfthat the light reaching the eye from the moon remains thesame as the moon changes position in the sky. Forinstance, if you photograph the moon at various heightsabove the horizon, you will see that the images of themoon are all the same size. My students frequently sendme photos of a giant harvest moon in which the moonlooks like a small spot in the sky. (The same thing happens

Why do the moon and the sun look so muchlarger near the horizon?

A N S W E R E D B Y :

Maurice Hershenson, Professor of Psychology,

Brandeis University, Waltham, Massachusetts.

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in photos of seemingly spectacular sunsetsthe illusionworks for the sun as well.) Another way to break the holdof the illusion is to cup your hand into a fist and lookthrough it at the large horizon moon. It will immedi-ately shrink in size. Clearly, this is a psychological effect.

My own view is that the moon illusion is linked to themechanism that produces everyday sizedistance percep-tion, a genetically determined brain process that allows usto translate the planar images that fall on the retina into aview of rigid objects moving in space. I believe the moonillusion results from what happens when the mechanismoperates in an unusual situation. In normal perception,when rigid objects move in depth (distance), the angularsize of the light image stimulating our eyes grows orshrinks. The brain automatically translates this changingstimulation back into the perception of rigid objectswhose position in depth is changing.

When the moon is near the horizon, the ground andhorizon make the moon appear relatively close. Becausethe moon is changing its apparent position in depth whilethe light stimulus remains constant, the brains sizedistance mechanism changes its perceived size and makesthe moon appear very large.

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T he definition has varied over the years. A blue moononce meant something virtually impossible, as in theexpression When pigs fly! This was apparently the usageas early as the sixteenth century. Then, in 1883, theexplosion of the volcano Krakatau in Indonesia releasedenough dust to turn sunsets green worldwide and themoon blue. Forest fires, severe drought, and volcaniceruptions can still do this. So a blue moon became syn-onymous with something rarehence the phrase oncein a blue moon.

The more recent connection of a blue moon with thecalendar apparently comes from the 1937 Maine FarmersAlmanac. The almanac relies on the tropical year, whichruns from winter solstice to winter solstice. In it, the sea-sons are not identical in length and the earths orbit is ellip-tical. Further, the synodic, or lunar, month is about 29.5days, which doesnt fit evenly into a 365.24-day tropicalyear or into seasons roughly three months in length.

What is a blue moon?

A N S W E R E D B Y :

George F. Spagna, Jr., Chair, Department of Physics,

Randolph-Macon College, Ashland, Virginia.

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Most tropical years have 12 full moons, but occasion-ally there are 13, so one of the seasons will have 4. Thealmanac called that fourth full moon in a season a bluemoon. (The full moons closest to the equinoxes and sol-stices already have traditional names.) J. Hugh Pruett,writing in 1946 in Sky and Telescope, misinterpreted thealmanac to mean the second full moon in a given month.That version was repeated in a 1980 broadcast ofNational Public Radios Star Date, and the definitionstuck. So when someone today talks about a blue moon,he or she is referring to the second full moon in a month.

P lanets are round because of their gravitational fields.A planet behaves like a fluid, and over long periodsof time it succumbs to the gravitational pull from its cen-ter of gravity. The only way to get all the mass as close to

Why are planets round?

A N S W E R E D B Y :

Derek Sears, Professor of Cosmochemistry,

University of Arkansas, Fayetteville, Arkansas,

and Editor of Meteoritics and Planetary Science.

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the planets center of gravity as possible is to form asphere. The technical name for this process is isostaticadjustment.

With much smaller bodies, such as the 20-kilometerasteroids we have seen in recent spacecraft images, thegravitational pull is too weak to overcome the asteroidsmechanical strength. As a result, these bodies do notform spheres, but they maintain irregular, fragmentaryshapes.

T he weight (or the mass) of a planet is determined byits gravitational effect on other bodies. Newtons Lawof Gravitation states that every bit of matter in the uni-verse attracts every other with a gravitational force that isproportional to its mass. For objects of the size weencounter in everyday life, this force is so minuscule that

How do scientists measure the weight of a planet?

A N S W E R E D B Y :

Gregory A. Lyzenga, Professor of Physics,

Harvey Mudd College, Claremont, California.

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we dont notice it. However, for objects the size of planetsor stars, it is of great importance.

In order to use gravity to find the mass of a planet, wemust somehow measure the strength of its tug onanother object. If the planet in question has a moon (anatural satellite), then nature has already done the workfor us. By observing the time it takes for the satellite toorbit its primary planet, we can utilize Newtons equationsto infer what the mass of the planet must be.

For planets without observable natural satellites, wemust be more clever. Although Mercury and Venus, forexample, do not have moons, they do exert a small pullon each other and on the other planets in the solar system.As a result, the planets follow paths that are subtly differ-ent than they would be without this disturbing effect.Although the mathematics is a bit more difficult, and theuncertainties are greater, astronomers can use these smalldeviations to determine how massive the moonless plan-ets are.

What about those objects such as asteroids, whosemasses are so small that they do not measurably disturbthe orbits of the other planets? Until recent years, themasses of such objects were simply estimates, based uponthe apparent diameters and assumptions about the possiblemineral makeup of those bodies.

Now, however, several asteroids have been (or soonwill be) visited by spacecraft. Just like a natural moon, aspacecraft flying by an asteroid has its path bent by an

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amount controlled by the mass of the asteroid. Thisbending is measured by careful tracking and Dopplerradio measurement from the earth.

Q uestions about how fast the earthor anything, forthat matteris moving are incomplete unless they alsoask, Compared to what? Without a frame of reference,questions about motion cannot be completely answered.

Consider the movement of the earths surface withrespect to the planets center. The earth rotates onceevery 23 hours, 56 minutes, and 4.09053 seconds, andits circumference is roughly 40,075 kilometers. Thus,the surface of the earth at the equator moves at a speedof 460 meters per secondor roughly 1,000 miles perhour.

As schoolchildren, we learn that the earth is movingabout our sun in a very nearly circular orbit. It covers thisroute at a speed of nearly 30 kilometers per second, or

How fast is the earth moving?

A N S W E R E D B Y :

Rhett Herman, Professor of Physics, Radford

University, Radford, Virginia.

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67,000 miles per hour. In addition, our solar systemEarth and allwhirls around the center of our galaxy atsome 220 kilometers per second, or 490,000 miles perhour. As we consider increasingly large size scales, thespeeds involved become absolutely huge!

The galaxies in our neighborhood are also rushing at aspeed of nearly 1,000 kilometers per second toward astructure called the Great Attractor, a region of spaceroughly 150 million light-years (one light-year is about sixtrillion miles) away from us. This Great Attractor, havinga mass 100 quadrillion times greater than our sun and aspan of 500 million light-years, is made of both the visiblematter that we can see and the so-called dark matter thatwe cannot see.

Each of the motions described above were given rela-tive to some structure. Our motion about our sun wasdescribed relative to our sun, while the motion of our localgroup of galaxies was described as toward the GreatAttractor. The question arises: Is there some universalframe of reference relative to which we can define themotions of all things? The answer may have been providedby the Cosmic Background Explorer (COBE) satellite.

In 1989, the COBE satellite was placed in orbit aboutthe earth (again, the earth is the frame of reference!) to mea-sure the long-diluted radiation echo of the birth of our uni-verse. This radiation, which remains from the immenselyhot and dense primordial fireball that was our early uni-verse, is known as the cosmic microwave background radia-

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tion (CBR). The CBR presently pervades all of space. It isthe equivalent of the entire universe glowing with heat.

One of COBEs discoveries was that the earth wasmoving with respect to this CBR with a well-definedspeed and direction. Because the CBR permeates all space,we can finally answer the original question fully, using theCBR as the frame of reference.

The earth is moving with respect to the CBR at aspeed of 390 kilometers per second. We can also specifythe direction relative to the CBR. It is more fun, though,to look up into the night sky and find the constellationknown as Leo (the Lion). The earth is moving toward Leoat the dizzying speed of 390 kilometers per second. It isfortunate that we wont hit anything out there during anyof our lifetimes!

S tars and planets form in the collapse of huge cloudsof interstellar gas and dust. The material in these

Why and how do planets rotate?

A N S W E R E D B Y :

George Spagna, Chair, Department of Physics,

Randolph-Macon College, Ashland, Virginia.

?

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clouds is in constant motion, and the clouds themselvesare in motion, orbiting in the aggregate gravity of thegalaxy. As a result of this movement, the cloud will mostlikely have some slight rotation as seen from a point nearits center. This rotation can be described as angularmomentum.

As an interstellar cloud collapses, it fragments intosmaller pieces, each collapsing independently and eachcarrying part of the original angular momentum. Therotating clouds flatten into protostellar disks, out of whichindividual stars and their planets form. By a mechanismnot fully understood, but believed to be associated withthe strong magnetic fields associated with a young star,most of the angular momentum is transferred into theremnant disks. Planets form from material in this disk,through accretion of smaller particles.

In our solar system, the giant gas planets (Jupiter, Sat-urn, Uranus, and Neptune) spin more rapidly on theiraxes than the inner planets do and possess most of the sys-tems angular momentum. The sun itself rotates slowly,only once a month. The planets all revolve around the sunin the same direction and in virtually the same plane. Theyalso all rotate in the same general direction, with theexceptions of Venus and Uranus. These differences arebelieved to stem from collisions that occurred late in theplanets formation.

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Star Light, Star Brightstars

T he North Star, or Polaris, is the brightest star in theconstellation Ursa Minor, the Little Bear (also knownas the Little Dipper). As viewed by observers in the North-ern Hemisphere, Polaris occupies a special place. It liesroughly one-half degree from the North Celestial Pole(NCP), the point in the night sky directly in line with theprojection of the earths axis. As the earth rotates on its axis(once every 24 hours), the stars in the northern sky appearto revolve around the NCP, so this particular star appearsto remain stationary hour after hour and night after night.

Because the earth is spherical, the position of Polarisrelative to the horizon depends on the location of anobserver. For example, when viewed from the equator

What exactly is the North Star?

A N S W E R E D B Y :

Rich Schuler, Adjunct Instructor and Outreach

Coordinator, Department of Physics and Astronomy,

University of Missouri-St. Louis.

?

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(0 degrees latitude), Polaris lies on the northern horizon.As the observer moves northward from the equatorsay,to Houston, Texas (30 degrees latitude)Polaris islocated 30 degrees above the northern horizon. This trendcontinues until the traveler reaches the geographic (notmagnetic) North Pole. At this point, Polaris is 90 degreesabove the northern horizon and appears directly overhead.

A traveler on land or sea need only measure the anglebetween the northern horizon and Polaris to determine hisor her latitude. Thus, Polaris is a handy tool for finding thenorthern extent of ones position, or latitude, and wastherefore heavily utilized by travelers in the pastespe-cially sailors.

There is currently no known star in the SouthernHemisphere that coincides with the South Celestial Pole.Also, Polaris is not an absolute guide to measuring latitudeon the earth for Northern Hemisphere observers. This isbecause the axis of the earth precesses in a conical motion.The location of the North (and South) Celestial Pole isdefined by projecting the axis of the earth onto the celes-tial sphere; consequently, as the axis changes position, so,too, does the North Star. As a result, 5,000 years ago theearths axis pointed toward the star Draco, and the starThuban was the North Star. Similarly, in 12,000 years thestar Vega (in the constellation Lyra) will be the NorthStar.

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T he length of a stars life depends on how fast it uses upits nuclear fuel. Our sun, in many ways an averagesort of star, has been around for nearly five billion yearsand has enough fuel to keep going for another five billionyears. Almost all stars shine as a result of the nuclearfusion of hydrogen into helium. This takes place withintheir hot, dense cores where temperatures are as high as 20million degrees. The rate of energy generation for a star isvery sensitive to both temperature and the gravitationalcompression from its outer layers. These parameters arehigher for heavier stars, and the rate of energy genera-tionand in turn the observed luminositygoes roughlyas the cube of the stellar mass. Heavier stars thus burntheir fuel much faster than less massive ones do and aredisproportionately brighter. Some will exhaust their avail-able hydrogen within a few million years. On the otherhand, the least massive stars that we know are so parsimo-nious in their fuel consumption that they can live to ages

How long do stars usually live?

A N S W E R E D B Y :

John Graham, Astronomer, Carnegie Institution of

Washington, Washington, D.C.

?Star Light, Star Bright 23

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older than that of the universe itselfabout 15 billionyears. But because they have such low energy output, theyare very faint.

When we look up at the stars at night, almost all ofthe ones we can see are intrinsically more massive andbrighter than our sun. Most longer-lasting stars that arefainter than the sun are just too dim to view without atelescope. At the end of a stars life, when the supply ofavailable hydrogen is nearly exhausted, it swells up andbrightens. Many stars that are visible to the naked eye arein this stage of their life cycles because this bias bringsthem preferentially to our attention. They are, on aver-age, a few hundred million years old and slowly comingto the end of their lives. A massive star such as the redBetelgeuse in Orion, in contrast, approaches its demisemuch more quickly. It has been spending its fuel soextravagantly that it cannot be older than about 10 mil-lion years. Within a million years, it is expected to gointo complete collapse before probably exploding as asupernova.

Stars are still being born at the present time fromdense clouds of dust and gas, but they remain deeplyembedded in their placental material and cannot be seenin visible light. The enveloping dust is transparent toinfrared radiation, however, so scientists using moderndetecting devices can easily locate and study them. In sodoing, we hope to learn how planetary systems like ourown come together.

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H ave you ever noticed how a coin at the bottom of aswimming pool seems to wobble? This occursbecause the water in the pool bends the path of lightreflected from the coin. Similarly, stars twinkle becausetheir light has to pass through several miles of Earths at-mosphere before it reaches the eye of an observer. It is asif we are looking at the universe from the bottom of aswimming pool. Our atmosphere is turbulent, withstreams and eddies forming, churning, and dispersing allthe time. These disturbances act like lenses and prismsthat shift a stars light from side to side by minute amountsseveral times a second. For large objects such as the moon,these deviations average out. (Through a telescope withhigh magnification, however, the objects appear to shim-mer.) Stars, in contrast, are so far away that they effec-tively act as point sources, and the light we see flickers inintensity as the incoming beams bend rapidly from side toside. Planets such as Mars, Venus, and Jupiter, which

Why do stars twinkle?

A N S W E R E D B Y :

John Graham, Astronomer, Carnegie Institution

of Washington, Washington, D.C.

?Star Light, Star Bright 25

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appear to us as bright stars, are much closer to Earth andlook like measurable disks through a telescope. Again, thetwinkling from adjacent areas of the disk averages out,and we see little variation in the total light emanating fromthe planet.

Far, Far Away . . .the universe

F inding ones location in a cloud of a hundred billionstarswhen one cant travel beyond ones ownplanetis like trying to map out the shape of a forestwhile tied to one of the trees. One gets a rough idea of theshape of the Milky Way galaxy by just looking aroundaragged, hazy band of light circles the sky. It is about 15

How do we know our location withinthe Milky Way galaxy?

A N S W E R E D B Y :

Laurence A. Marschall, Department of Physics,

Gettysburg College, Gettysburg, Pennsylvania.

?

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degrees wide, and stars are concentrated fairly evenly alongthe strip. That observation indicates that our Milky Waygalaxy is a flattened disk of stars, with us located some-where near the plane of the disk. Were it not a flatteneddisk, it would look different. For instance, if it were asphere of stars, we would see its glow all over the sky, notjust in a narrow band. And if we were above or below thedisk plane by a substantial amount, we would not see itsplit the sky in halfthe glow of the Milky Way would bebrighter on one side of the sky than on the other.

The position of the sun in the Milky Way can be fur-ther pinned down by measuring the distance to all thestars we can see. In the late eighteenth century, theastronomer William Herschel tried to do this, concludingthat the earth was in the center of a grindstone-shapedcloud of stars. But Herschel was not aware of the presenceof small particles of interstellar dust, which obscure thelight from the most distant stars in the Milky Way. Weappeared to be in the center of the cloud because we couldsee no further in all directions. To a person tied to a tree ina foggy forest, it looks like the forest stretches equally awayin all directions, wherever one is.

A major breakthrough in moving the earth from the cen-ter of the galaxy to a point about three-fifths away from theedge came in the early decades of the twentieth century,when the astronomer Harlow Shapley measured the distanceto the large clusters of stars called globular clusters. He foundthat they were distributed in a spherical distribution about

Far, Far Away . . . 27

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100,000 light-years in diameter, centered on a location in theconstellation Sagittarius. Shapley concluded (and otherastronomers have since verified) that the center of the distri-bution of globular clusters is the center of the Milky Way aswell, so our galaxy looks like a flat disk of stars embedded ina spherical cloud, or halo, of globular clusters.

In the past 75 years, astronomers have refined this pic-ture, using a variety of techniques of radio, optical,infrared, and even x-ray astronomy, to fill in the details: thelocation of spiral arms, clouds of gas and dust, concentra-tions of molecules, and so on. The essential modern pictureis that our solar system is located on the inner edge of a spi-ral arm, about 25,000 light-years from the center of thegalaxy, which is in the direction of the constellation ofSagittarius.

Why is the night sky dark?

A N S W E R E D B Y :

Karen B. Kwitter, Ebenezer Fitch

Professor of Astronomy, Williams College,

Williamstown, Massachusetts.

?

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W e see stars all around, so why doesnt their com-bined light add up to make our night skyandsurrounding space, for that matterbright? The Germanphysicist Heinrich Wilhelm Olbers put the same puzzlethis way in 1823: If the universe is infinite in size, andstars (or galaxies) are distributed throughout this infiniteuniverse, then we are certain to eventually see a star in anydirection we look. As a result, the night sky should beaglow. Why isnt it?

In fact, the answer is far more profound than itappears. There have been many attempts at explaining thispuzzle, dubbed Olbers Paradox, over the years. One ver-sion implicated dust between stars and perhaps betweengalaxies. The idea was that the dust would block the lightfrom faraway objects, making the sky dark. In reality, how-ever, the light falling on the dust would eventually heat itup so that it would glow as brightly as the original sourcesof the light.

Another answer proposed that the tremendous redshift of distant galaxiesthe lengthening of the wave-length of light they emit due to the expansion of the uni-versewould move light out of the visible range into theinvisible infrared. But if this explanation were true,shorter-wavelength ultraviolet light would also be shiftedinto the visible rangewhich doesnt happen.

The best resolution to Olbers Paradox at present hastwo parts. First, even if our universe is infinitely large, it isnot infinitely old. This point is critical because light travels

Far, Far Away . . . 29

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at the finite (though very fast!) speed of about 300,000kilometers per second. We can see something only afterthe light it emits has had time to reach us. In our everydayexperience that time delay is minuscule: even seated in thebalcony of the concert hall, you will see the conductor ofthe symphony raise her baton less than a millionth of asecond after she actually does.

When distances increase, though, so does the timedelay. For instance, astronauts on the moon experience a1.5-second time delay in their communications with Mis-sion Control due to the time it takes the radio signals(which are a form of light) to travel round-trip betweenearth and the moon. Most astronomers agree that the uni-verse is between 10 and 15 billion years old. And thatmeans that the maximum distance from which we canreceive light is between 10 and 15 billion light-years away.So even if there are more distant galaxies, their light willnot yet have had time to reach us.

The second part of the answer lies in the fact that starsand galaxies are not infinitely long-lived. Eventually, theywill dim. We will see this effect sooner in nearby galaxies,thanks to the shorter light-travel time. The sum of theseeffects is that at no time are all of the conditions for creat-ing a bright sky fulfilled. We can never see light from starsor galaxies at all distances at once; either the light from themost distant objects hasnt reached us yet, or if it has, thenso much time would have had to pass that nearby objectswould be burned out and dark.

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A tricky part of this question is the wording: The uni-verse, by definition, is all there is! To speak of theuniverse expanding into something would mean that therewas something bigger, which we ought to have called theuniverse in the first place.

Perhaps the easiest way to see what is meant by anexpanding universe is to imagine what life would be likefor two-dimensional ants living on the surface of anexpanding spherical balloon. They can crawl around, butbeing unable to fly, or to penetrate the balloons surface,they live in what is essentially a two-dimensional world.For the ants, provided nothing disturbs them from out-side, the universe is the surface of the balloonthats allthere is! Being confined to the surface of the balloon,there is no way for the ants to discover anything at allabout what we would term up and down.

The area of this two-dimensional universe is finite,

Does the fact that the universe is continuallyexpanding mean that it lacks a physical edge?

A N S W E R E D B Y :

Stephen Reucroft and John Swain,

Professors of Physics, Northeastern University,

Boston, Massachusetts.

?Far, Far Away . . . 31

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but nowhere will the ants find a boundary or an edge.Here you have to ignore the rubber neck of the balloonand the person blowing it up; think of a balloon sealedsmoothly into a spherical shape hovering in a tank inwhich the air pressure could be lowered to make the bal-loon expand.

Now as the balloon expands, the ants see one anothergetting farther and farther apart. Each sees the same thing:All its neighbors are moving away. The ants live in whatfor them is an expanding universe with no physical edge.If an ant walks quickly enough, it could conceivably get allthe way around the balloon and return to its starting pointwithout encountering an edge.

You may object at this point and claim that we see theballoon expanding into the surrounding space. But we haveaccess to an extra dimension in which to movethe onethat would correspond to up and down for the ants, werethey able to move in those directions. As far as the ants areconcerned, they can learn everything they want to knowabout their world by making measurements on the bal-loons surface, with no reference to the surrounding space.

Experience in physics has taught us that when we finda concept that is spuriousin the sense that it leads to nopredictable effectswe do better just to assume its notthere. In other words, the ants would do well not to talkabout their space expanding into something that theycant measure. Nothing is lost and there is a substantialgain in simplicity.

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To make an analogy between the ants situation andour own, you have to imagine space expanding in all direc-tions. Everybody in the universe sees everything rushingaway from everything else; but the universe need have nophysical edges, and there is no need to describe it asexpanding into anything; it can just expand.

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