Physical Science Chapters 12-13
Study Guide
The Tides
Caused mainly by differences in how much the moon pulls on different parts of the Earth.
Spring Tides: Occur when the sun, Earth, and moon are all in a line. These are the largest tides.
Neap Tides: Occur when the Sun, Earth, and moon form a right angle. During neap tides, the
differences between high and low tide are the smallest.
Atmospheres
The atmospheres of planets and moons act like an insulating blanket. The atmosphere keeps
the heat in at night and protects against the Sun’s harsh rays during the day. Planets and moons that
have no atmosphere have great swings in temperature between day and night. For example, Mercury’s
temperature changes by about 600 degrees between night and day. On the other hand, the Earth’s
temperature usually changes by about 30 degrees between day and night. This is due to the insulating
atmosphere on Earth.
Atmospheres also protect planets and moons from meteors. On Earth, most meteors burn up in
the atmosphere and never reach the surface. On the moon or on Mercury, where there is no
atmosphere, any meteor is going to strike the surface, creating a crater.
Timing of Celestial Events
1 rotation of the Earth on its axis: 24 hours
1 revolution of the Earth around the Sun: 365 days
1 revolution of the Moon around the Earth: 29.5 days
1 rotation of the Moon on its axis: 29.5 days
Effects of Celestial Events (Cause and Effect)
Earth rotates: Causes day and night on Earth
Earth revolves around the Sun: Along with the tilt of the Earth on its axis, causes the changing seasons.
Moon revolves around the Earth: The changing position of the moon causes us to see the different
phases of the moon. Different parts of the sunlit side of the moon are visible from the Earth during the
various phases.
Moon rotates on its axis: Since the rotation period is the same as the period of revolution, the same side
of the moon always faces the Earth. We never see the “Dark Side of the Moon.”
Solar eclipse: The moon is between the Earth and the Sun, casting a shadow on the Earth.
Lunar eclipse: The Earth is between the Sun and the moon, casting a shadow on the moon.
Umbra: The darkest part of the shadow cast by a planet or moon. From within the umbra, a person
would see a total eclipse.
Penumbra: The lighter part of the shadow cast by a planet or moon. From within the penumbra, a
person would see a partial eclipse.
Reasons for the Seasons
The seasons occur because of the tilt of the Earth on its axis. During the winter in the Northern
Hemisphere, the Earth is tilted away from the Sun. This means that the sunlight is less direct, so we
receive less heat from the Sun. During the summer in the Northern Hemisphere, the Earth is tilted
toward the sun. This means that sunlight is more direct, so we receive more heat from the Sun.
Summer Solstice: June 21 – the longest day of the year in the Northern Hemisphere because the Earth is
tilted directly toward the Sun.
Winter Solstice: December 22 – the shortest day of the year in the Northern Hemisphere because the
Earth is tilted directly away from the Sun.
Spring Equinox: March 20 – Day and night are of equal length. The Earth is tilted exactly sideways, with
respect to the Sun.
Fall Equinox: September 22 – Day and night are of equal length. The Earth is tilted exactly sideways,
with respect to the Sun.
Gravity and Orbits
Gravity is the force that attracts all objects to each other. The strength of gravity increases with the
mass of the objects. It also gets stronger the closer objects are to each other. For example, the force of
gravity on Earth is quite strong, but out in space it is very small.
Each object has inertia. Inertia is the tendency of an object to resists changes in speed or direction. For
example, a refrigerator is hard to move because it has a lot of mass and a lot of inertia. A large ship is
hard to turn because it has so much inertia, keeping it moving forward. The Earth is the same way – it
wants to keep moving in a straight line. However, the gravity of the Sun is so strong that it keeps
changing the direction of the Earth. These two forces, gravity and inertia, work together to keep the
Earth moving around the Sun in an orbit. If there were no gravity, the Earth would leave its orbit and
travel out into deep space. If there were no inertia, the Earth would crash into the Sun.
Phases of the Moon
Check out the diagram on pp. 480 - 481. Know what the various phases look like and the order in which
they occur. Know that each phase lasts approximately 3.75 days. Also know what events occur during
which phases:
Full Moon: Lunar eclipse and spring tides
New Moon: Solar eclipse and spring tides
First and Third Quarter: neap tides
Fun Facts
All planets and our moon revolve counterclockwise.
The moon is about a quarter of the diameter of the Earth.
The amount of matter in an object is its mass.
The amount of space that an object takes up is its volume.