Download Look! Up in the Sky!

Look! Up in the Sky!
A quick introduction to the sky above us
and how it appears to change
Matthew A. d’Alessio
California State University, Northridge
and
Mary Lusk
Southwest Florida College
(CK-12 Open-source textbook project)
This chapter is set up to help you understand why objects in the sky appear to move. It
assumes you have some basic familiarity with the Sun, planets, and sky above you from
your K-12 education in California.
p. 2
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This work is adapted from the CK-12 FlexBook, an open-source textbook developed by a
non-profit foundation to meet science content standards in the State of California.
The chapter was originally written by Mary Lusk and distributed under the Creative
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50% of the text is unmodified from the original source and presented in this text without
quotation marks.
Original Source: http://ck12.org/flexr/chapter/1848/, accessed Feb. 17, 2010.
Original Author: Mary Lusk, Southwest Florida College
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Image Sources
Cover: See Figure 2
Fig. 1. Matthew d’Alessio, released to public domain.
Fig. 2. Brasstown Bald mountain, Georgia, in 1985. Credit: Jimmy Westlake., Released to NASA and
public domain, http://science.nasa.gov/headlines/y2004/06dec_geminids.htm
Fig. 3. http://en.wikipedia.org/wiki/File:Foucault_pendulum_at_north_pole_accurate.PNG. GNU-FDL.
Fig. 4. Modified from: http://en.wikipedia.org/wiki/File:Sidereal_day_%28prograde%29.png
Fig. 5. Matthew d’Alessio, released to public domain. Image and text inspired by
http://learn.uci.edu/oo/getOCWPage.php?course=OC0811004&lesson=002&topic=001&page=9. Data
from: http://en.wikipedia.org/wiki/Big_Dipper
Fig. 6. http://www.ck12.org/ck12/images?id=297406
Fig. 7. http://www.jpl.nasa.gov/news/news.cfm?release=2004-285
Fig. 8: http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=178
p. 3
Fig. 1. A view through a skylight at night.
An intriguing observation
One weekend, I visited family that live out in the country. They have a skylight in their
bedroom, so I stared out at the dark sky as I tried to drift off to sleep on their
uncomfortable guest bed. As my eyes adjusted to the night time, I started to notice that
my little window was filled with beautiful stars. A bright, blinking light zoomed across
my portal -- "that thing is moving, so it must be an airplane," I thought. It appeared much
brighter than the stars because it was much closer. After the plane disappeared, my eyes
were drawn to the simple shape of the constellation "the Big Dipper." I learned to
recognize it when I was a child but hardly noticed it since. I marveled as one particularly
bright star twinkled near the very edge of the skylight. My mind wandered. When I
looked up a few minutes later, that twinkling star was gone. I puzzled over its
disappearance. Surely it couldn't have been an airplane -- I watched it stand still for
several minutes, and it had not flashed like the airplane. Could it have moved? The clear
outline of the big dipper still loomed in the skylight as I drifted off to sleep. I woke up a
few hours later with a terrible kink in my neck from the lumpy bed. I grumpily stared
back up at my skylight. To my astonishment, the big dipper was nowhere to be seen. I
was sure it was there when I fell asleep. First the bright star near the edge disappeared,
and now an entire constellation! Perhaps someone had picked up the entire house and
moved it during the night?! The idea isn’t as crazy as it sounds…
The Sun rises and sets each day, meaning that it appears in one spot, moves across the
sky, and then disappears below the horizon in a different spot. The moon does the same
thing, but just at a different time. Perhaps the whole sky is moving. If you observe the
stars like I did, that's the conclusion you might come to. The moon, the stars, and even
the Sun all travel across the sky at the same speed, reappearing in approximately the same
spot once every 24 hours.
Wait! Once every 24 hours? That's the length of a day. In fact, the sky isn't really moving,
but my theory that someone picked up the house and moved is basically correct. The
house sits on the Earth, which constantly spins on its axis. The house is moving because
it’s attached to the spinning planet. How do we know?
p. 4
Earth’s rotation
Today we can go out into space
and watch Earth spin, but back in 1851, a
French scientist named Léon Foucault
figured out how to show that the Earth
rotates by staying put on the ground1. He
attached a heavy iron ball to a really long
string, pulled the ball to one side and then
released it, letting it swing back and forth
in a straight line. A ball swinging back and
forth on a string is called a pendulum. A
pendulum set in motion, will not change
its motion, so it will not change the
Fig. 3. A pendulum at the north pole always
swings in the same direction as the Earth
moves beneath it.
direction of the swinging. However, Foucault
observed that his pendulum did seem
to change direction over the course of several
hours. He knew that the pendulum itself could
not change its motion, so he concluded that
the Earth, underneath the pendulum was
moving. Placing this pendulum at the North or
South Pole, it would take 23 hours, 59 minutes
and 4 seconds for the pendulum to swing back
to where it first started – the time it takes
Earth to spin 360° around on its axis2. At the
equator, the Earth rotates at a speed of about
1,700 kilometers per hour. Thankfully, we do
not notice this movement, because it would
certainly make us dizzy.
The Sun and Moon appear to move
across the sky and the stars do the same. As
the Earth rotates, observers on Earth see the
Sun moving across the sky from east to west
1
Fig. 2. Star trails. Normally a camera
snaps a picture in a fraction of a
second. To take this picture of the
sky, the photographer left the shutter
open for several hours. The stars
appear to spin in a circle throughout
the night because the earth rotates.
The straight lines are “shooting stars”
that flew across the sky.
An excellent tutorial on how Foucalt pendulums work:
http://www.calacademy.org/products/pendulum/
2
This experiment was actually conducted in 2001. http://www.physastro.sonoma.edu/graduates/baker/southpolefoucault.html
p. 5
with the beginning of each new day. We often say that the Sun is “rising” or “setting,”
but actually it is the Earth’s rotation that gives us this perception. When we look at the
Moon or the stars at night, they also seem to rise in the east and set in the west. Earth’s
rotation is also responsible for this. A ballet dancer or a figure skater constantly sees
different things coming into and out of his vision as he spins rapidly around on his toes
even though the skating arena doesn’t move at all. The same is true for residents of Earth
looking up at the sky.
Earth’s revolution
Thus far, we’ve explained more than 23 hours and 59 minutes of Earth’s “day.” What
about the other the other 56 seconds? To understand why our day is almost exactly 24
hours, you need to know that Earth doesn’t just spin in place like an ice skater doing
pirhouettes. It also does constant laps around the Sun in a motion commonly called
Earth’s “revolution.” Both rotation and revolution have similar English meanings (to
spin), but scientists like to use the words for different types of circular motion – Earth’s
revolution on its orbit around the Sun takes much longer than its rotation on its axis. One
complete revolution takes 365.24 days, or one year.
How does revolution affect the length of a day,
which is mostly caused by rotation? Figure 4 shows
the Earth at three different times. At time 1, people
standing on top of the red triangular mountain see
the Sun by looking straight up. At time 2, Earth has
rotated half way around and those same people will
see the dark night sky when they look straight up.
After one complete rotation (time 3), the mountain
is back where it started from and the people are
looking the exact same direction when they look
straight up. While it will be daylight they don’t see
Fig. 4. Earth rotates around its axis
the Sun directly above them (it’s a little off to the
slightly more than 360° in one day.
east). Why? Because the Earth moved further along
By the time it spins 360°, it has
in its orbit. In order to rotate around to the point
moved along on its path around the
where the Sun is directly overhead, you need to time
Sun. This image exaggerates the
to rotate a bit more (time 4). In fact, it’s an extra 56
effect, which is why it has the label
seconds, during which time the Earth rotates an
“not to scale.”
extra 0.23° around its axis (Fig. 4 exaggerates this
effect, which is why it has the label “not to scale”).
One day ends up being 24 full hours. You do not need to remember these numbers, but with a
complete understanding of Earth’s rotation and revolution, you should be able to explain the
length of Earth’s “day.”3
3
A related topic is the number of hours of daylight, which is related to the reason we have
seasons on Earth. You probably know from personal experience that it gets lighter earlier and sun
sets later in the summer. To understand this, you have to know that Earth’s rotation axis is tilted
compared to its axis of revolution around the Sun. Check out this tutorial
http://www.teachersdomain.org/ext/ess05_int_seasonsgame/index.html
p. 6
How to discover planets
Even though stars rise in the East and set in the west because of Earth’s rotation,
the stars always seem to be in the same relative positions and shapes. In fact, every
human that has ever lived saw a night sky almost identical to the one you can see4. A
specific arrangement of stars that forms a recognizable shape or pattern when viewed
from Earth is called a constellation (read more about them in a later section). The
patterns of stars are so unchanging that names given to constellations by cultures
thousands of years ago are still meaningful to us today. When the Big Dipper rises, we
know that the constellation Leo will be nearby, surrounded by Cancer the crab to its west
and Virgo to its east. The entire sky rotates uniformly so that the signs of the zodiac
always appear in the same order (you’d think Leo’s mighty lion could outpace its
neighbor, Cancer, a crab. But it never happens).
The sky changes so little that people notice exceptions very easily. Shooting stars
are obvious ones. They can appear to streak half way across the visible sky in a fraction
of a second. In reality, they are bits of space dust and asteroids. Air resistance causes
them to heat up and glow as they fly through our atmosphere. From the time it starts to
glow to the time it stops, a shooting star probably travels only a few hundred miles. Why
do they look like they travel so far? Because they are so much closer than any of the other
stars. A meteor is usually just 40-60 miles above the surface of the Earth, while a regular
star is trillions of miles away. People in Los Angeles will rarely see the same shooting
stars as people in San Francisco, about 350 miles away because meteors burn up so low
to the ground (compared to regular stars).5 The rest of the stars in the sky will look
almost identical in the two cities.
A much less dramatic example of stars that move out-of-step with the rest of the
sky is a series of “wandering stars.” One of these stars can appear in front of the
constellation Leo one night and then Cancer a few nights after that. They seem to wander
across the sky, but at a much slower pace than shooting stars. While it could be that they
are moving slower than shooting stars, another option is that they are much further away
from Earth. Once humans developed telescopes, they started looking more closely at
these wandering stars and found that they were totally different than all the other stars.
Each one had a distinct round shape (like our moon does) and surface features (some had
dark spots and light patches like craters on our own moon, others had dramatic rings
around them). In fact, these wandering “stars” are not regular stars at all! They are what
we now call the planets (the word planet is derived from the word for “wanderer” in
Greek). People were able to recognize them as different from other stars long before they
could see them up close with telescopes because planets don’t rotate across the sky at the
same pace as the other constellations. Watching how fast each planet moves relative to
the rest of the stars has helped us realize that each planet revolves around the Sun at its
own pace and at a different distance away from the Sun.
4
Humans living in the Southern Hemisphere see different stars than those in the Northern
Hemisphere. Today, you can hop on an airplane to experience either hemisphere.
5
http://munnecke.com/blog/?p=332
p. 7
What are "stars"?
When you look up in the sky, you probably call every point of light a "star." When you
start looking closely at the stars, you might notice subtle differences. Some stars are
brighter and some are slightly different colors ranging from blue-ish white to orange-ish
white. But if you used a telescope to look much closer, you might see that the differences
don't stop there. There are at least 4 broad categories of "stars" that I list below in order
of distance away from Earth from closest to farthest.
Things close to Earth (satellites). Very few of the stars in the sky are in this
category that includes objects in Earth's atmosphere and orbiting the planet just outside
the atmosphere. We already discussed "shooting stars" which burn up in Earth's
atmosphere. Did you know that you can also see the communication satellites, the
International Space Station, and pieces of random space garbage orbiting Earth if you
look up in the night sky? It's rare because you have to be looking in the right place at the
right time. Because they are so close to Earth, they appear to move across the sky as you
watch them.
Things that orbit the Sun (planets). Even though these are some of the brightest
objects in the sky, they don't actually create any of their own light! Light from the Sun
bounces off the surface of planets like a mirror, sending that light into our eyes. They are
bright because they are close, compared to the rest of the stars.
Individual stars "nearby." These are the only true "stars" where a single point of
light corresponds to a single glowing ball of gas like our own Sun. They appear smaller
than our Sun because they are much further away (our Sun is actually medium-sized
compared to many of the other stars in the sky). It takes light from our Sun just 8 minutes
to get to Earth, but it takes 4 years for the light to reach us from the next closest star. All
the individual stars we see are in our own Milky Way galaxy. A galaxy is another word
for a cluster of stars that are close together. Galaxies often have spiral shapes, with
individual stars orbiting around the center of the galaxy in much the same way planets
revolve around the Sun in our solar system. Galaxies and individual stars form by a
process similar to the formation of our solar system and its individual planets – gravity
attracts clumps of material together from a really huge cloud of dust and gas. Like our
solar system, most all the stars in a galaxy rotate and revolve in the same direction that is
related to the way the huge cloud was originally rotating. (More about these last two
sentences in later sections).
Our galaxy is about 100,000 light years across, and the Sun is about 25,000 light
year's from its center. So when we say an individual star is "nearby," it's still could be
really, really far from Earth. There are perhaps 400 billion stars in our galaxy, but most of
them are too dim for us to see from Earth6. On a dark night in the countryside, your naked
eye can probably only see about 8,000 of them.7 Around cities and towns, you might only
be able to see a few hundred to a few thousand stars because the glowing light from
neighborhoods is brighter than many dim stars.
Groups of stars that are very far away (other galaxies). Some of the
"individual" lights in our sky are actually billions and billions of really bright stars
6
http://chview.nova.org/chview/chv5.htm
A good classroom activity to estimate the number of stars in the sky is here:
http://spacemath.gsfc.nasa.gov/weekly/2page7.pdf
7
p. 8
clustered together in a galaxy. Because these other galaxies are so unbelievably far away,
they look like a single dot to our naked eye. Using a powerful telescope, you can start to
see the shape and size of the galaxy and that it is made up of many individual stars. The
furthest galaxies scientists can see are more than 10 billion light years away. That's
millions of times further than any of the individual stars in our own galaxy. Fewer than
one hundred of the stars you see with your naked eye are actually galaxies, but there are
literally billions of galaxies out there that we can start to see with telescopes.
While these four categories of objects are completely different, they appear almost the
same to you as a person looking up in the sky with your naked eye.
What are constellations?
The stars that make up the handle of the Big Dipper appear related to the stars that make
up the pot to a person looking up from Earth; they are not. The problem is that the sky
looks flat, but space goes out in three dimensions. With your naked eye, you can't tell
how far away a star is, and some of the stars in the Big Dipper are much closer to us than
others. The same is true of all the constellations. A constellation is just a pattern of stars
when viewed from Earth. They appear to move together in the night sky as Earth rotates,
but an alien living around
one star in the constellation
Leo the lion's shoulder
might be able to visit Earth
a lot more easily than it
could visit the star that
makes up the lion's foot
because one star is much
closer to Earth than the
other8. Stars in a galaxy all
rotate around the same
galactic center, but stars in
a constellation can be in
completely different
Fig. 5. The stars in each constellation make look like they are
galaxies.
close to one another, but they don’t have to be. The sky
appears flat even though deep space goes off in all three
Do the stars change
dimensions.
from month to month?
Earth’s revolution around
the Sun also affects what we see during the night sky. As we do laps around the Sun, the
direction Earth faces at night changes (night time is always seen on the side of the Earth
in the direction away from the Sun). That means that people on Earth are able to see a
different set of stars from one part of the year to another. Constellations that we can see
when looking away from the Sun in the winter show up behind the Sun during the
summer. We can’t see stars when they are in the same direction as the Sun because the
Sun appears so much brighter than the stars. You can demonstrate this effect when you
8
Leo’s front paw is a star called Regulus, 77 light-years away. It’s shoulder, η-Leonis is
a whopping 2,131 light years away (http://en.wikipedia.org/wiki/List_of_stars_in_Leo).
p. 9
turn a light on during the daytime. If the Sun is shining brightly, you won’t even notice a
change when the electric light is on.
While we can see different stars at different times of year, the stars themselves don’t
change much. As we move around the Sun, you might be tempted to think that stars
might look bigger during summer compared to winter because we are closer to them. It
turns out that they don’t look closer because stars are really, really far away. One lap
around the Sun is an astonishing 584 million miles, but that’s nothing compared to the 25
trillion miles to the closest star. Scientists can measure the differences using careful
observations, but you wouldn’t notice any difference with your naked eye. So while stars
are exciting, active, and constantly changing when you look really close, our night sky is
relatively constant to an everyday observer. It only appears to change because our
perspective changes. As Earth rotates and revolves, we look in different directions at
different sections of the vast sky from hour to hour and season to season.
The origin of our Solar System: Why is everything spinning?
Earth rotates about its axis and revolves around the Sun. Look at the arrows in Figure 4
illustrating this motion as viewed from far above the North Pole. Which way does Earth
revolve, clockwise or counter-clockwise? How about its revolution direction? If you
watch the other planets in our solar system from this viewing point, all of them revolve
around the Sun counter-clockwise. All the planets
also rotate. Can you guess which direction? All
but one rotate about their own axes counterclockwise9. Even the Sun itself rotates around its
own axis in the same direction that Earth does.
This key feature is a major clue to how the solar
system formed.
A Giant Nebula
The most widely accepted explanation of how the
solar system formed is called the nebular
hypothesis. According to this hypothesis, the
solar system formed about 4.6 billion years ago
from the collapse of a giant cloud of gas and dust,
called a nebula. The nebula was made mostly of
hydrogen and helium, but there were heavier
elements as well.
9
Fig. 6. Our solar system started out
as a cloud of dust, gas, and frozen
flakes of stuff. Gravity caused it to
turn into what we see today. This
artist’s drawing shows what the early
solar system would have looked like.
Venus spins the opposite direction from all the other planets, but it spins really slowly.
From Earth on out, all of the planets take less than one Earth day to rotate 360° around
their axes. Venus takes more than 243 days
(http://www.windows.ucar.edu/tour/link=/our_solar_system/planets_table.html). That
slow speed gives us a clue about why its rotation direction is backwards. Want to find out
what scientists think? Search the Internet for “Venus rotation.” I like this site:
http://www.astronomycafe.net/qadir/q50.html
p. 10
The nebula was drawn together by gravity. As the nebula collapsed, it started to spin. As
it collapsed further, the spinning got faster, much as an ice skater spins faster when he
pulls his arms to his sides during a spin move. This effect, called “conservation of
angular momentum,” along with complex effects of gravity, pressure, and radiation,
caused the nebula to form into a disk shape, as shown in the picture below. This is why
all the planets are found in the same plane. What does “same plane” mean? If you ever
see a table-top model of the solar system, all the planets revolving around the sun in a
roughly flat region. None of them want to fly up high above the table or plunge below its
surface. They all sort of dance along the table top. In the earliest stages of the solar
system, that wasn’t the case. The nebula went off in all directions like a big ball of gas.
After spinning and spinning, gas from high above got drawn downward while the gas
from below was attracted upward. The ball flattened out until almost all the material was
in a single flat disk.
Formation of the Sun and Planets
Even though the disk was flat, gravity continued to pull on material. More and more
material accumulated in the center of the disk in a runaway process. Because the strength
that gravity pulls depends on the amount of material, the disk pulled harder and harder on
the surrounding material as it grew. Slowly, the density and pressure increased at the
center of the spinning nebula. When the pressure in the center was high enough that
nuclear fusion reactions started in the center, a star was born—the Sun.
Meanwhile, the outer parts of the disk were cooling off. Small pieces of dust in the disk
started clumping together. These clumps collided and combined with other clumps.
Larger clumps, called planetesimals, attracted smaller clumps with their gravity. I often
call them “planets-to-be.” Eventually, the planetesimals grew bigger so that they now
form the planets and moons that we find in our solar system.
The outer planets—Jupiter, Saturn, Uranus and Neptune—condensed farther from the
Sun from lighter materials such as hydrogen, helium, water, ammonia, and methane. Out
by Jupiter and beyond, where it’s very cold, these materials can form solid particles. But
in closer to the Sun, these same materials are gases. As a result, the inner planets—
Mercury, Venus, Earth, and Mars—formed from dense rock, which is solid even when
close to the Sun.
Fig. 7. An artist’s drawing of the early solar system, looking towards the newly formed Sun. Small
clumps of debris orbit the Sun and will eventually combine to become larger planets.
p. 11
Fig. 8. The Sun and planets shown with accurate relative sizes. Compare the four small
planets close to the Sun with the four huge planets further away. The distance between
planets is not accurate in this illustration.
Understanding the Solar System
Picture yourself on a spaceship leaving the massive sun and heading outward towards
each of the planets. There are dozens of online tours of the planets, so you can explore
each alien world virtually to your heart's delight. Our tour in this chapter is very brief.
Inner rocky planets versus outer gas giants
You'll pass by four small planets made of rock and metal. Each one has slightly different
surface features, but their composition, density, and even size are all remarkably similar.
Comparing the largest and smallest of these inner planets, the diameter of Earth is less
than three times bigger than Mercury. As you head out away from these small planets,
things change dramatically. The next four planets are much, much bigger. You could fit
more than 1,300 Earths inside of the planet Jupiter, but it is made mostly of hydrogen and
helium gas. While observations of their mass and size tells us that they have a rock and
metal core, anyone trying to land on the surface of these gigantic planets would sink in
because their outer layers are gas and liquid. Humans have landed probes on the rocky
surfaces of all the inner planets, but will never be able to do that for the outer gassy
planets.
Why are they different?
We can understand the difference between the inner and outer planets by
understanding how they formed from the nebular cloud. Recall that each planet was built
up as gravity drew together particles into larger and larger clumps. Whatever materials
were in the cloud would be the materials that would make up each planet as it formed.
Hydrogen and helium are by far the most common elements in the Universe, and we can
assume that this was also true about the nebular cloud that turned into our solar system.
In fact, more than 98% of the solar system mass is hydrogen and helium, which is indeed
p. 12
the same as the overall composition of the Universe.10 The composition of the Sun is
similar to the outer planets, so there is no reason to believe that some spots of the nebula
had different composition than others.
The frost line
There must be some reason that the inner planets don't have much gas. The
answer is actually pretty easy to understand: it's hot close to the Sun. Don't laugh!
Molecules move fast and freely in a gas, without feeling a strong attachment to one
another. As a planet begins to grow, its gravity attracts more and more molecules of stuff.
Gas molecules, being so free, can more easily escape the gravitational pull of a small
planet-to-be. Solids, however, tend to clump together and stick around once they are
attracted close to the growing planet-to-be. When they are small, planets grow best by
accumulating only flakes of solid, frozen material.
Inner planets are made of only rock and metal: Because it is so hot close to the
Sun, most materials are gases. Only rocks and metal are solid close to the Sun, so planets
made out of only rock and metal formed.
Outer planets are bigger: At a certain distance away from the Sun, it gets cool
enough that many common materials like water and methane gas condense into little
frozen flakes of material. Scientists like to call this special distance the frost line. When
you get further than the frost line, there are a lot more solid frozen flakes in a nebula.
Because there is more solid available to clump together, planets far from the Sun grew
bigger. They accumulated rock, metal, and solid icy materials like frozen water and
frozen methane.
Outer planets have lots of hydrogen and helium gas: If Jupiter was made of only
solid flakes, it would be about 3 times bigger than Earth. The bulk of it's mass, however,
comes from hydrogen and helium gases. Both remain gases even at the lowest
temperatures in our solar system. Since they never form solids, you might think that they
would never be captured by a planet. However, even as a gas, they still feel the pull of
gravity. Planets exert stronger and stronger gravitational pull as they get bigger, and
eventually a planet can grow large enough that it and can retain these gases. The outer
planets, especially Jupiter and Saturn, have reached that size. The inner planets were not
able to hold onto much of this gas, which is why helium balloons are such a special treat
on Earth. 11
Everything in the previous section is probably wrong
This “frost line” explanation seemed to explain everything perfectly. However, in
the last decade scientists have started discovering other solar systems around other stars.
Rocky planets like Earth are too small to detect at this point, but we have discovered
hundreds of huge planets that are made out of hydrogen and helium around hundreds of
10
http://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System
Helium balloons depend on being less dense than the air around them in order to rise
up. If there was more helium in our atmosphere, they would have the same density as the
air and would not rise.
11
p. 13
different stars. What's weird is that many of these huge gassy planets are very close to
their star. According to our theory of planet formation, they should never have been able
to accumulate gas so close to the star. The story in this chapter says that it is too hot for
these planets to form, but they do form. So while this explanation makes lots of sense,
new observations are starting to call it into question.12 Maybe there are some other minor
details that need to be considered to explain the new solar systems, or maybe the theory
we've got is dead wrong and we'll need to start over from scratch in order to explain all
the information we have now. That's what makes science exciting. Who will figure out
the answer? Maybe one of your students if you can get them energized about science and
discovery!
12
http://www.astronomynotes.com/solfluf/s11.htm
p. 14
How does gravity do it?
If gravity draws objects together, wouldn’t Earth and the other planets crash into the Sun
instead of revolving around it?
Imagine that you are a few feet of string with a rock tied to the end. If you let the rock hang
straight down, the only force the string applies is straight up towards your hand. If you try to
push the rock away from you using only the string, the string will bend. It can’t transmit a
force away from your hand. In that way, it acts like gravity that ONLY pulls objects together.
Nonetheless, you can take that string over your head and twirl the rock around in a circle.
This should help convince you that the force of gravity can keep a planet in orbit. As the
string tries to fly away, the string yanks it back. Instead of flying away, it flies around in a
circle.
The Earth revolves around the Sun because gravity keeps it in a roughly circular orbit around
the Sun. The Earth’s orbital path is not a perfect circle, but rather an ellipse, which means
that it is like a slight oval in shape (Figure 24.10). This creates areas where the Earth is
sometimes farther away from the Sun than at other times. We are closer to the Sun at
perihelion (147 million kilometers) on about January 3rd and a little further from the Sun
(152 million kilometers) at aphelion on July 4th. Students sometimes think our elliptical orbit
causes Earth’s seasons, but this is not the case. If it were, then the Northern Hemisphere
would experience summer in January!
Figure 24.10: Earth and the other planets in the solar system make regular orbits around the
Sun; the orbital path is an ellipse and is controlled by gravity. (5)
During one revolution around the Sun, the Earth travels at an average distance of about 150
million kilometers. Mercury and Venus take shorter times to orbit the Sun than the Earth,
while all the other planets take progressively longer times depending on their distance from
the Sun. Mercury only takes about 88 Earth days to make one trip around the Sun. While
Saturn, for example, takes more than 29 Earth years to make one revolution around the Sun.