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Planetary Motion
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Planetary Motion
A Revolution in Astronomy
• Each planet spins on its axis. The spinning of a
body, such as a planet, on its axis is called
rotation.
• The orbit is the path that a body follows as it
travels around another body in space.
• A revolution is one complete trip along an orbit.
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Planetary Motion
A Revolution in Astronomy
• When Earth completes one FULL rotation around
it’s axis, what have we experienced? How long does
this take?
•What Earth completes one FULL revolution around
the sun, what have we experienced? How long does
this take?
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Earth’s Rotation and Revolution
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Planetary Motion
A Revolution in Astronomy, continued
• Johannes Kepler made careful observations of
the planets that led to important discoveries
about planetary motion in the early 1600s.
• Kepler’s First Law of Motion: Kepler
discovered that the planets move around the sun
in elliptical orbits.
What type of model is this???
Heliocentric model
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Planetary Motion
A Revolution in Astronomy, continued
• Kepler’s Second Law of Motion: Kepler noted
that the planets seemed to move faster when they
are close to the sun and slower when they are
farther away.
• Kepler’s Third Law of Motion: Kepler observed
that planets more distant from the sun, such as
Saturn, take longer to orbit the sun.
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Planetary Motion
Newton to the Rescue!
• Kepler did not understand what causes the planets
farther from the sun to move slower than the closer
planets.
• Sir Isaac Newton’s description of gravity provides
an answer.
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Planetary Motion
Newton to the Rescue! continued
• The Law of Universal Gravitation: Newton’s law
of universal gravitation states that the force of gravity
depends on the product of the masses of the objects
divided by the square of the distance between the
objects.
• According to this law, if two objects are moved
farther apart, there will be less gravitational
attraction between them.
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Our Solar System
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Our Solar System
Our Solar System
Our solar system includes the sun, the planets,
and many smaller objects.
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Our Solar System
Measuring and Interplanetary
Distances
Scientists use the astronomical unit to measure
distances in space. One astronomical unit is the
average distance between the sun and Earth, or
approximately 150,000,000 km (or 93,205,678.8 miles)
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Our Solar System
The Inner and Outer Solar Systems
• The Inner Planets: The planets closest to the
sun include Mercury, Venus, Earth, and Mars.
• The Outer Planets: The outer planets include
Jupiter, Saturn, Uranus, and Neptune.
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• The Inner Planets:
The planets closest
to the sun include
Mercury, Venus,
Earth, and Mars.
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• The Outer
Planets: The
outer planets
include Jupiter,
Saturn, Uranus,
and Neptune.
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The Inner Planets
Mercury: Closest to the Sun
Mercury is a very hot, small planet. It only takes
Mercury 88 days to revolve around the sun.
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The Inner Planets
Venus: Earth’s Twin?
• The Atmosphere of Venus: Of all the inner
planets, Venus has the densest atmosphere.
• Mapping Venus’s Surface: The Magellan
spacecraft mapped the surface of Venus by using
radar waves.
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The Inner Planets
Earth: An Oasis in Space
• Water on Earth: Earth is warm enough to keep
most of its water from freezing and cool enough to
keep its water from boiling away. Liquid water is
important to life on Earth.
• The Earth from Space:
Satellites are used to
study the Earth from
space in order to better
understand global
systems.
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The Inner Planets
Mars: Our Intriguing Neighbor
• The Atmosphere of Mars: Mars has a thin
atmosphere with low air pressure.
• Water on Mars: Liquid water cannot exist on
Mars’s surface today, but most likely it was there
in the past.
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The Inner Planets
Mars: Our Intriguing Neighbor,
continued
• Where Is the Water Now?: Mars has two polar
icecaps made of frozen water and carbon
dioxide. Many scientists think that there is more
frozen water beneath the Martian soil
• Martian Volcanoes: Mars has two large
volcanic systems, one of which includes the
largest mountain in the solar system.
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The Inner Planets
Mars: Our Intriguing Neighbor,
continued
• Missions to Mars: Several recent missions to
Mars were launched to gain a better
understanding of the planet.
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The Outer Planets
Jupiter: A Giant Among Giants
• Jumbo Sized: Jupiter is the largest planet in our
solar system.
• NASA Missions to Jupiter: NASA has sent five
missions to Jupiter to study Jupiter’s atmosphere
and moons.
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The Outer Planets
Saturn: Still Forming
• The Rings of Jupiter: Saturn’s rings are the
largest of all of the gas giants’ rings.
• NASA Exploration of Saturn: Launched in 1997,
the Cassini spacecraft is designed to study
Saturn’s rings, moon, and atmosphere.
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The Outer Planets
Uranus: A Small Giant
• Uranus’s Atmosphere: The atmosphere of
Uranus is mainly hydrogen and methane, which
makes the planet appear to be blue-green in
color.
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The Outer Planets
Uranus: A Small Giant, continued
• A Tilted Planet: Unlike most other planets,
Uranus is tipped over on its axis. So its axis of
rotation is tilted by almost 90° and lies almost in
the plane of its orbit.
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The Outer Planets
Neptune: The Blue World
• Discovery of Neptune: Neptune was not
discovered until 1846.
• The Atmosphere of Neptune: The composition
of Neptune’s atmosphere is similar to that of
Uranus’s atmosphere, but Neptune has belts of
clouds that are much more visible.
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The Outer Planets
Pluto: A Dwarf Planet
• A Small World: Pluto is less than half the size
of Mercury.
• Reclassified: Since its
discovery in 1930, Pluto
has been called the ninth
planet. However, in 2006,
it was reclassified as a
dwarf planet.
• A dwarf planet is any object that orbits the sun,
is round because of its own gravity, but has
not cleared its orbital path.
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• "Hasn't cleared its orbital path" means that in Pluto's
orbital space there are other objects still there.
• Planets have cleared their orbital path of debris and
small objects, but Pluto orbits within the Kuiper Belt, a
region filled with millions of small objects.
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