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Chapter 19
The Solar System
Sun, Earth, and Moon
• The position of objects in the sky change
over time because Earth, and everything
else in the universe, is constantly moving.
– Earth’s spinning on its axis and orbiting the sun
create apparent movement in the sky
• A planet is a celestial body that orbits the
sun, is round because of its own gravity, and
has cleared the area around its orbital path.
Sun, Earth, and Moon
Sun, Earth, and Moon
• The sun is the closest
star to Earth
– Is an average star
– Has a mass about
330,000 times Earth
Sun, Earth, and Moon
• The Sun is the star at the center of the Solar System. It is
almost perfectly spherical and consists of hot plasma
interwoven with magnetic fields. It has a diameter of
about 1,392,684 km (865,374 mi), around 109 times that of
Earth, and its mass (1.989×1030 kilograms, approximately
330,000 times the mass of Earth) accounts for about
99.86% of the total mass of the Solar System. Chemically,
about three quarters of the Sun's mass consists of
hydrogen, while the rest is mostly helium. The remainder
(1.69%, which nonetheless equals 5,600 times the mass of
Earth) consists of heavier elements, including oxygen,
carbon, neon and iron, among others.
Sun, Earth, and Moon
• The Sun formed about 4.567 billion years ago
from the gravitational collapse of a region within a
large molecular cloud. Most of the matter gathered
in the center, while the rest flattened into an
orbiting disk that would become the Solar System.
The central mass became increasingly hot and
dense, eventually initiating thermonuclear fusion
in its core. It is thought that almost all stars form
by this process.
Sun, Earth, and Moon
• The sun is a major external source of heat
and light for Earth
– As the earth turns on its axis every 24 hours, we
see the sun rise and set
– Many patterns of animal behavior follow the
sun’s cycle
– Effects the weather and ocean currents
– Effects the seasons
– Promotes plant life
Sun, Earth, and Moon
• Planets and distant stars are visible in the
night sky
– Greek scholars saw and gave names to patterns
Sun, Earth, and Moon
• Family of Planets: the solar system is the
sun and all of the objects that orbit it
–
–
–
–
–
–
8 planets and their moons
Dwarf planet
Asteroids
Comets
Dust
Gas
Sun, Earth, and Moon
• Gravity holds the solar system together
– Every object in the solar system pulls on every
other object
• The force of gravity between two objects depends
upon their masses and the distance between them
– The sun exerts the largest force in the solar
system because its mass is so large
• The pull of the sun keeps Earth in its orbit
• Gravity is the force that keeps moons orbiting
around planets
Sun, Earth, and Moon
• Eight planets orbit the Sun: a planet’s
distance from the sun determines how long
the planet takes to orbit the sun
– Mercury: the closest, 88 days
– Earth: 365.25 days
– Neptune: the most distant: 165 years (60,225
days)
Sun, Earth, and Moon
Sun, Earth, and Moon
Sun, Earth, and Moon
Sun, Earth, and Moon
• A satellite is an object in orbit around a body that
has a larger mass.
– The moon is Earth’s satellite
– All of the planet’s in our solar system except Mercury
and Venus have moons
– Currently there are 135 known satellites (moons)
orbiting the planets
• Some are as small as 3km in diameter to as large as Mercury
– Can be seen because of the light they reflect
Sun, Earth, and Moon
Sun, Earth, and Moon
• The Moon:
– Orbits the Earth at a distance of about 384,000
km
– Like the Sun, the moon affects life on Earth
through its movements and gravitational
influence
Sun, Earth, and Moon
• The moon appears to have different shapes
throughout the month that are called phases
– As the Moon revolves around Earth, the
illuminated portion of the Moon that faces
Earth changes.
• When the Moon is new, the side that is dark is
facing you, so you cannot see it
• The time from one full Moon to the next is 29.5
days
Sun, Earth, and Moon
Sun, Earth, and Moon
• The Moon Phases as viewed from Earth
An easy way to remember the phases of the
moon is to think of "DOC"! On the first
night of the lunar month ("month") the
moon is not visible, this phase is called the
new moon.
Sun, Earth, and Moon
Sun, Earth, and Moon
Sun, Earth, and Moon
• The sun is shining on the back side (what
we call the "dark side" of the moon) which
can't be seen from our vantage point. We
always see the same side of the moon
because it rotates so slowly the same face is
always toward the Earth, fortunately, the
side we see is the prettiest! In 3 days a tiny
sliver of a waxing crescent can be seen, this
is the "D" in "DOC".
Sun, Earth, and Moon
Sun, Earth, and Moon
• Every night more light is added on the right side
(waxing), making more of the moon's disc visible.
One week into the lunar cycle half of the moon is
visible (but it is called the 1st Quarter). This is the
best phase to view the moon with binoculars or a
telescope because it can be seen early in the
evening and the craters are well defined by the
shadows. It is often possible to see the moon
during the afternoon in this phase. On day 12 the
moon is almost, but not quite, full. This is the
waxing gibbous phase.
Sun, Earth, and Moon
Sun, Earth, and Moon
• Two weeks into the moon's revolution
around the earth we enjoy a big, bright full
moon, the "O". Up until this point, the moon
has had more light added each night, from
here on out, there will be less light each
night. By day 16 the light begins to wane to
the right and the moon is in the waning
gibbous phase.
Sun, Earth, and Moon
Sun, Earth, and Moon
• Day 21 brings the 3rd Quarter phase when
the other half is seen. The moon rises an
hour later each night, by day 26 only a tiny
crescent is visible early in the morning, this
is the "C" phase. On Day 28 the moon has
completed its cycle. The rare times that
there are 2 full moons in one month, it is
called a "Blue Moon".
Sun, Earth, and Moon
Sun, Earth, and Moon
Sun, Earth, and Moon
• The phase of the moon appears the same all
over the world, but the orientation changes
depending on your latitude. In other words,
a full moon will be full all over the world,
but its features may appear upside down
from the opposite hemisphere. When high
in the sky, crescent moons will appear
closer to 'vertical' at higher latitudes and
'horizontal' nearer the equator.
Sun, Earth, and Moon
• Eclipses are caused by bodies casting a
shadow
– An eclipse occurs when one object moves into
the shadow cast by another object
Sun, Earth, and Moon
• A total solar eclipse requires the umbra of
the Moon's shadow to touch the surface of
the Earth. Because of the relative sizes of
the Moon and Sun and their relative
distances from Earth, the path of totality is
usually very narrow (hundreds of kilometers
across). The following figure illustrates the
path of totality produced by the umbra of
the Moon's shadow.
Sun, Earth, and Moon
Sun, Earth, and Moon
Sun, Earth, and Moon
• In case of a total solar eclipse, the Moon looks as big as the
Sun, thus covering it completely. For those few precious
moments, only the faint halo/ corona of the Sun is visible.
This happens when the Earth and the Moon are closest to
each other, since then the Moon appears big enough to
completely cover the Sun! It is a pretty rare event, not to
mention spectacular, and is only for those people who are
in the regions falling in the Moon's umbra.
Sun, Earth, and Moon
• Total Solar Eclipses occur when the umbra of the
Moon's shadow touches a region on the surface of
the Earth.
Sun, Earth, and Moon
• Partial Solar Eclipses occur when the penumbra
of the Moon's shadow passes over a region on the
Earth's surface.
Sun, Earth, and Moon
• During this eclipse, the Sun and the Moon do not come in
the same straight line. This happens because of the tilt of
the Moon's orbit to the Earth's elliptic. In this eclipse, the
Moon partly covers the Sun, so it is visible from a large
portion of the Earth. In this type of eclipse the umbra of the
Moon completely misses the Earth, so no region on Earth
will experience a total solar eclipse. Only part of the
penumbra covers a few regions of the Earth.
Sun, Earth, and Moon
Sun, Earth, and Moon
• Annular Solar Eclipses occur when a region on the
Earth's surface is in line with the umbra, but the
distances are such that the tip of the umbra does
not reach the Earth's surface.
Sun, Earth, and Moon
• This eclipse is caused when the Sun and the Moon are
positioned in one straight line, but the Moon doesn't
completely cover the Sun. During this eclipse, the Moon
appears much smaller in size as compared to the Sun. As a
result, a bright ring is visible in the sky. This happens when
the Earth is farthest away from the Moon, which is why it
appears small, and cannot hide the Sun completely. An
annular eclipse occurs more frequently than a total eclipse,
and needless to say, looks beautiful!
Sun, Earth, and Moon
Sun, Earth, and Moon
• Safety
• Permanent damage will be caused to the retina of the
eye if proper precautions are not taken while observing
the Sun. Injuries to the retina can occur without pain and
the results may not be noticeable for some hours after the
damage has been done. Viewing a partial solar eclipse,
annular solar eclipse, or the partial phase of a total solar
eclipse should not be attempted without some form of eye
protection or the use of an appropriate observing method.
• Never stare at the Sun. Even if only 1% of the Sun's
disc is visible, it is still bright enough to cause damage.
Children, especially, should be well supervised.
Sun, Earth, and Moon
• Left: Total solar eclipse, showing the solar corona and
several red prominences around the edge .
• Center: Partial solar eclipse .
• Right: Annular solar eclipse, when the moon is too far
away to completely cover the sun
Sun, Earth, and Moon
• A lunar eclipse occurs when the moon and sun are
on opposite sides of the earth. Often the entire
moon is covered by earth's shadow.
Sun, Earth, and Moon
• A sequence of three photos of the total lunar eclipse of August 16-17,
1989, taken over a time period of about half an hour. The exposure
time was shortest for the bright partial phase (left), and longest for the
dark red total phase (right). The middle photo shows how the eclipsed
portion of the moon is much darker than the small sliver that's
receiving direct sunlight.
Sun, Earth, and Moon
• The Moon affects Earth’s tides:
– Most coastal regions on Earth have two high tides and
two low tides
– Even though tides are affected by Earth’s landscape,
tides are mainly a result of the gravitational influence of
the Moon
– Moon’s gravitational pull is strongest on the side of
Earth nearest the Moon
• The ocean is pulled toward the Moon, so a slight bulge is
created. The solid Earth also moves slightly under the Moon’s
gravity, but the movement of water is more noticeable because
water is liquid
Sun, Earth, and Moon
Sun, Earth, and Moon
Sun, Earth, and Moon
• Because these external forces of gravity are so
great, not only do the oceans bulge out in the
direction towards the sun and the moon, but they
also want to bulge out in the opposite direction.
The result is an almost football like bulge of the
oceans when the gravities are aligned. That’s
because the Earth is pulled by each of these
gravities so forcefully that the planet itself
displaces water on the opposite side.
Sun, Earth, and Moon
• The earth’s inclination in relation to the sun
also effects the tides. The sun’s inclination
follows a year-long cycle, and is in highest
inclination in the summer and winter
months. During these months the "bulges"
in the ocean are offset the most from the
equator, and it is most likely to encounter
only one tide cycle per day, or diurnal tides.
Sun, Earth, and Moon
• The highest tides on planet Earth occur near
Wolfville, in Nova Scotia's Minas Basin.
The water level at high tide can be as much
as 16 meters (52 feet) higher than at low
tide.
Sun, Earth, and Moon
The Inner and Outer Planets
• The terrestrial planets are relatively small
and have solid, rocky surfaces
– Gravity keeps their orbits nearly circular
around the sun
– Metallic cores rocky surfaces with some of the
same terrain features as Earth (mountains,
canyons, and craters)
The Inner and Outer Planets
The Inner and Outer Planets
• Internal structures of the Terrestrial planets. For
Venus, the core must be similar to that of the Earth
in comparison to its size, but for Mercury, the core
is relatively large in comparison to its size, and for
Mars, the core is either relatively small, as shown
here, or made of lighter than usual materials. Note
that although Mercury's core is "large" in
comparison to the size of the planet, it is not large
compared to the Earth's core. The Earth's core is
larger than Mars, and more than twice as massive
as Mars and Mercury combined.
The Inner and Outer Planets
• Mercury has extreme temperatures
– Mercury is the smallest and closest to the Sun of the
eight planets in the Solar System
– Surface can reach as high as 720K to a low of 103K
– Rotation: One day is equivalent to nearly 59 Earth days
– Orbit: one year is approximately 88 Earth days
– No atmosphere
– No water
The Inner and Outer Planets
• Venus:
– Venus, the second planet from the sun, is named for the Roman
goddess of love and beauty
– Thick clouds cause a greenhouse effect
– Spins very slowly in a direction opposite that of most other planets
and the Sun
– Spin: One day is 243 Earth days
– Orbit: 0.6 Earth year
• Therefore, its day is longer than its year
– Atmosphere contains large amounts of sulfuric acid and carbon
dioxide (causes greenhouse effect)
– Temperature greater than 700K
– Surface Pressure is 90x the pressure on Earth
The Inner and Outer Planets
• Earth has ideal conditions for living
creatures
–
–
–
–
–
Third planet from the Sun
Rotation (spin): 1 day
Orbit: 365 days
Only planet with large amounts of liquid water
Water takes a long time to heat and cool (high
specific heat) helping moderate the temperature
of Earth.
The Inner and Outer Planets
• Earth’s atmosphere:
– 78% nitrogen, 21% oxygen, and 1% argon, carbon
dioxide, and other gases
– Moderate temperatures between day and night
– Because of the greenhouse effect, the atmosphere
absorbs energy radiated by the Sun
– Ozone: protects us from harmful ultraviolet radiation
and high-energy particles from the Sun
– Burns-up small rocks and artificial satellites as they
enter our atmosphere
– Original atmosphere was not like today (no oxygen)
The Inner and Outer Planets
• Mars: Mars is the fourth planet from the sun. Befitting the
red planet’s bloody color, the Romans named it after their
god of war.
– Polar ice caps made of frozen carbon dioxide and may contain
small amounts of water
– Terrain features indicate that water might have flowed across the
surface
– Very thin atmosphere composed mostly of carbon dioxide
– Has two small satellites (Phobos and Deimos)
– Rotation (spin): 24.7 Earth hours
– Orbit: 1.9 Earth years
– Surface temperatures range from 144K to 300K
The Inner and Outer Planets
• Mars:
– Mountain 3x larger than Everest
– Many impact craters since thin atmosphere can
not burn up objects from space
– Surface is red from iron oxide in its soil
– Frequent dust storms
The Inner and Outer Planets
• Between Mars and Jupiter lie hundreds of
smaller, rocky objects that range in diameter
from 3km to 1,000km
– These objects are called asteroids (smaller solar
bodies)
• Some wander away from this region and may cross
Earth’s orbit
The Inner and Outer Planets
Asteroid Bennu
The Inner and Outer Planets
• The Gas Giants: The outer planets are much larger
than the inner planets and have thick, gaseous
atmospheres, many satellites, and rings
– No solid surface
– All the gas giants have rings
– All have satellites: most cratered and some have thin
atmospheres
•
•
•
•
Jupiter: more than 60
Saturn: more than 40
Uranus: 27
Neptune: 13
The Inner and Outer Planets
The Inner and Outer Planets
• Jupiter: fifth planet from the Sun
– The largest planet in the Solar System
• 1,300 times the size of Earth
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–
–
–
First planet beyond the asteroid belt
Rotation (spin): 10 earth hours
Orbit: 12 Earth years
Atmosphere of swirling clouds of hydrogen, helium, methane, and
ammonia
– Enormous storms: the Great Red Spot is a huge hurricane twice the
diameter of Earth
• It has existed for hundreds of years
– The colorful bands of Jupiter are arranged in dark belts and light zones
created by strong east-west winds in the planet's upper atmosphere
traveling more than 400 mph (640 kph). The white clouds in the zones are
made of crystals of frozen ammonia, while darker clouds of other
chemicals are found in the belts.
The Inner and Outer Planets
• Saturn: Saturn is the sixth planet from the sun and
the second largest planet in the Solar System
– Considered to be still growing
– Most extensive ring system
• Narrow bands of tiny particles of dust, rock, and ice ranging in
size from a few millimeters to several meters
• Not known how they formed
– 95 times the mass of Earth
– Rotation (spin): a day is 10.7 hours
– Orbit: 29 Earth years
The Inner and Outer Planets
• Uranus: Seventh Planet in the Solar system
– Gaseous atmosphere composed of hydrogen, helium,
and methane (giving the planet a bluish color)
– Rotation (spin): 17.25 hours
– Orbit: 84 years
– Atmosphere very cold (58K)
• Wind: 200 to 700 km/h
– Most extreme tilt: 980 causing extreme seasonal
changes
– Atmosphere very cold (58K)
The Inner and Outer Planets
• Neptune: Eighth and most distant planet
from the sun
– Gaseous atmosphere composed of hydrogen,
helium, and methane (giving the planet a bluish
color)
– Rotation (spin): 16 hours
– Orbit: 165 years
– Storm systems may have winds up to 1,100
km/h
The Inner and Outer Planets
• Beyond the Gas Giants are numerous small bodies composed of ice
and rock
– One of the larger bodies is Pluto
• Was considered to be a planet up to 2006
• The definition of planet set in 2006 by the International Astronomical
Union (IAU) states that, in the Solar System, a planet is a celestial
body which:
• 1. Is in orbit around the Sun
• 2. Has sufficient mass to assume hydrostatic equilibrium (a nearly
round shape)
• 3. Has not cleared the area around its orbital path of debris.
• A non-satellite body fulfilling only the first two of these criteria is
classified as a “dwarf planet“. (PLUTO)
The Inner and Outer Planets
• Pluto: dwarf planet
– 0.002 Earth’s mass
– Orbit: 248 years
– Atmosphere: thin, gaseous and a solid, icy
surface
The Inner and Outer Planets
• Beyond Neptune lies the Kulper Belt
– Region populated by many small bodies made
of ice and rock, including Pluto
• Recently discovered another dwarf planet the size of
Pluto: Eris
– Region still being explored
Formation of the Solar System
• Early Astronomy:
• Aristotle: The first model put Earth at the of the
Solar System
• Copernicus (1543): Sun at the center and the
planets orbit in perfect circles
• Kepler (1605): orbits around the Sun are ellipses
(ovals) rather than circles
• Newton (1687): explained that gravity keeps the
planets in orbit around the Sun and satellites in
orbit around the planets
Formation of the Solar System
• Nebular Hypothesis:
– From their dating of rocks, scientist estimate
that the Solar System is approximately 4.6
billion years old
– Nebula: is a large cloud of dust and gas in space
– Explains why objects that form from a disk will
lie in the same plane, have almost circular
orbits, and orbit in the same direction
Formation of the Solar System
• Nebular Hypothesis:
– The Sun, like every star, formed from a cloud of gas and dust that
collapsed because of gravity
• In our Solar System, this process began almost 5 billion years
ago
– As the cloud collapsed, it formed into a rotating disk. In the center,
where the material became denser and hotter, a star began to form.
As the cloud continued to collapse, it spun faster and faster.
– The spinning motion of the disk caused it to flatten. Planetesimals,
or particles that became planets, began to form in the disk. Their
formation caused more changes in the disk.
Formation of the Solar System
• Nebular Hypothesis:
– As the planetesimals grew, their gravitational pull increased. The
largest planetesimals began to collect more of the gas and dust of
the nebula
– Some planetesimals collided with large ones, and the planets began
to grow larger and more stable. The warmer, inner planets were
rocky, and the colder, outer planets accumulated lightweight gases
in their atmosphere
– Because each planet swept up the material in its region, the
planetary orbits are separate from each other. Asteroids and other
small rocks are most likely leftover debris from solar system
formation
Formation of the Solar System
• Nebular Hypothesis:
– States that planets formed mostly through the process by which
small particles collide and stick together, called accretion
– Explains why the terrestrial planets are different in composition
from the gas giants
• Warm temperatures near the sun prevented light gases from remaining
in the atmospheres of the terrestrial planets
• Colder gas and dust in the outer part of the disk became the gas giants
– These planets were large enough and cold enough to hold light nebular
gases, such as hydrogen, in their atmosphere
• Satellites may have formed around gas giants through the
process of accretion or particles captured by gravitational pull
of gas giants
Formation of the Solar System
• Rocks in space:
– There are many types of small bodies in our solar
system, including satellites, comets, asteroids, and
meteoroids
• Satellites orbit planets
• Comets are probably composed of material left over from the
formation of the Solar System
• Most Asteroids can be found between Mars and Jupiter
• Most Meteoroids that strike Earth burn up in the atmosphere
– Those that do not completely burn up in the atmosphere and hit
the ground are called meteorites
Formation of the Solar System
• Comets: a small body of ice, rock, and cosmic dust that follows an
elliptical orbit and that gives off gas and dust in the form of a tail as it
passes close to the Sun
– Composed of dust and of ice made from methane, ammonia,
carbon dioxide, silicon, magnesium, iron, and water
• Because of their chemical composition they are sometimes
referred to ma dirty snowballs
– Have long tails and icy centers
• When it passes near the Sun, it gives off gases in the form of a
long tail of gases
• Tail brightens when it passes the Sun
Formation of the Solar System
• Comet formation:
– During the formation of our Solar System, some small
planetesimals did not combine with other planetesimals.
These leftovers strayed far from the Sun and developed
very long orbital periods.
• Oort Cloud and Kuiper Belt
• Most Comets are small bodies of rock and covered by ice
– Halley’s comet is one of the most famous. It travels in a
highly elliptical orbit and appears in earth’s sky every
76 years
Halley’s comet travels in a highly elliptical orbit that
takes it out into the Kuiper Belt
Formation of the Solar System
• Meteorites: three major types
– Stony: include carbon-rich specimens that
contain organic materials and water
• Greatest variety
• Have compositions like Earth and Moon
– Metallic: made of iron and nickel
– Stony-iron: are a combination of the two
Formation of the Solar System
• Meteoroids: most were
once part of asteroids
– a few came from Mars
or from our Moon
Formation of the Solar System
• Moon:
– Its composition was found to be similar to Earth’s but
not identical
• Therefore, the current theory is that the moon resulted from a
collision between Earth and another large body
– The giant impact hypothesis, sometimes called the
Big Splash, states that the Moon was formed out of the
debris left over from an indirect collision between the
Earth and an astronomical body the size of Mars,
approximately 4.5 billion years ago
Formation of the Solar System
Formation of the Solar System
Formation of the Solar System
Formation of the Solar System
• Do other Stars have planets?
– An exoplanet is a planetlike body that orbits a
star other than the Sun
• Astronomers have discovered more than 200
exoplanets by measuring the small gravitational
effects that they have on their parent Stars