Chapter 7: The Earth-Moon-Sun System

Section 1: Earth in Space

• Earth’s Size and Shape

  • Sphere: a round, three-dimensional object, the surface of which is the same distance from the center in all directions.
  • Aristotle made three different observations that indicated that Earth’s shape is spherical:
  • No matter where you are on Earth, objects fall straight down to the surface, as if they are falling toward the center of a sphere.
  • Earth’s shadow on the Moon during a lunar eclipse is always curved.
  • People in different parts of the world see different stars at night.
  • Gravity: the attractive force between two objects that depends on the masses of the objects and the distance between them.
  • Because Earth is a sphere, the pull of gravity on objects near Earth’s surface is always straight downward toward Earth’s center.

• Earth’s Magnetic Field

  • Earth has a magnetic field that protects us from harmful radiation from the Sun.
  • Earth’s magnetic field is concentrated at two ends of an imaginary magnetic axis running from Earth’s north magnetic pole to its south magnetic pole. This axis is tilted about 11.5° from Earth’s geographic axis of rotation.
  • The locations of Earth’s magnetic poles change slowly over time. Large-scale movements, called polar wandering, are thought to be caused by movements in Earth’s crust and upper mantle.
  • An area within Earth’s magnetic field, called the magnetosphere, deflects harmful radiation coming from the Sun, a stream of particles called the solar wind.
  • Some of these ejected particles from the Sun produce other charged particles in Earth’s outer atmosphere. These charged particles spiral along Earth’s magnetic field lines toward Earth’s magnetic poles. There they collide with atoms in the atmosphere. These collisions cause the atoms to emit light. This light is called the aurora borealis (northern lights) in the northern hemisphere and the aurora australis (southern lights) in the southern hemisphere.

• Earth Orbits the Sun

  • Earth orbits the Sun at an average distance of 149,600,000 km.
  • Ellipse: an elongated, closed curve with two foci.
  • The Sun is not located at the center of the ellipse, but at one of its two foci.
  • Earth is the only planet whose characteristics make it possible for life as we know it to survive.
  • On Earth, where the amount of atmospheric greenhouse gases is much lower, the greenhouse effect is much less. As a result, Earth’s average surface temperature is about 15°C.

Section 2: Time and Seasons

• Measuring Time on Earth

  • People can determine the approximate time of day by determining where the Sun is in the sky.
  • Earth spins and makes one complete turn in about 24 hours.
  • This spinning causes the Sun to appear to move across the sky from east to west.
  • It takes 24 hours from when the Sun is highest in the sky (noon) until it is highest in the sky again (noon the next day).
  • If Earth spins approximately 360° in 24 hours, then it spins through 15° in one hour. This led to the setting up of time zones on Earth that have the same time in minutes but vary in hours.
  • Time Zone: an area 15° wide in which the time is the same.
  • It cannot be two different days at the same spot, so a day is added to the time at the International Date Line.
  • Rotation: the spinning of Earth on its axis, an imaginary line drawn through Earth from its rotational north pole to its rotational south pole.
  • The apparent movement of the Sun from noon one day until noon the next day is called a solar day.
  • As Earth revolves in its orbit, the Sun appears to move through the skies compared to the seemingly fixed positions of the stars.
  • The time it takes for the Sun to make one complete trip through the sky in reference to the background of stars is the same amount of time it takes for Earth to complete one trip around the Sun, or one sidereal year.
  • Ecliptic: defined as the plane of Earth’s orbit around the Sun.
  • The 12 constellations (star patterns) through which we observe the Sun moving during this year is called the zodiac.

• Why Do Seasons Change?

  • Earth’s axis is tilted 23.5° from a line perpendicular to the plane of its orbit. Because of this tilt, Earth’s geographic north pole points toward the star Polaris.
  • The tilt of Earth’s rotation axis causes the angle at which the Sun’s rays strike Earth’s surface to vary through the year. The tilt also causes the number of hours of daylight to vary. It is these variations, caused by the tilt of Earth’s axis, that cause seasons to change.
  • During mid-summer in the northern hemisphere, the Sun is higher in the sky throughout most of the day compared to other times of the year.
  • The Sun is lowest in the sky during mid-winter. As a result, in the northern hemisphere the Sun’s rays strike Earth’s surface at a higher angle during the summer than during the winter. Sunlight is more intense and warms Earth’s surface more when striking the surface at higher angles than at lower angles.
  • During the summer, the Sun is above the horizon for more hours than it is when school begins in the fall. As the year progresses, the number of hours of daylight each day becomes fewer and fewer until it reaches a minimum around December 21 for the northern hemisphere.
  • Equinox: Earth’s axis is perpendicular to a line drawn from the center of Earth to the center of the Sun.
  • Solstice: occurs when Earth’s rotation axis is tilted directly toward the Sun or away from the Sun.

Section 3: Earth’s Moon

• Movement of the Moon

  • If you look at the Moon each day at the same time over a period of a few days, you will see that it moves toward the east.
  • It takes 27.3 days (a sidereal month) for the Moon to revolve once around Earth and line up with the same star again.
  • A complete lunar phase cycle takes 29.5 days, known as a synodic month.

• How does the Moon affect Earth?

  • Daily changes in the height of the ocean are one way that the Moon affects Earth. These changes are caused by the gravitational forces that the Sun and Moon exert on Earth.
  • Tide: rise and fall in sea level
  • As the crest of this large wave approaches the shore, the level of the water in the ocean rises. This rise of sea level is called high tide.
  • The attractive gravitational force exerted by the Moon on water in an ocean depends on the distance of the water from the Moon.
  • As Earth rotates and the Moon revolves, different locations on Earth’s surface pass through the high and low tides.
  • The Sun does affect Earth’s tides: it can strengthen or weaken the moon’s tidal effect.

• Moonlight

  • The Moon shines because it reflects sunlight from its surface.
  • As the Moon revolves around Earth, different portions of the side facing Earth are lighted, causing the Moon’s appearance to change.

• Moon Phases: the changing appearances of the B Moon as seen from Earth.

• A new moon occurs when the Moon is between Earth and the Sun.

• After a new moon, the moon’s phases are said to be waxing—the lighted portion that we see appears larger each night.

• The moon is in the waxing gibbous phase from the first quarter up until full moon.

• The waning crescent occurs before another new moon.

• The word month is derived from the same root word as Moon.

• Eclipses

  • Eclipses occur when Earth or the Moon temporarily blocks sunlight from reaching the other object.
  • Eclipses can occur only when the Sun, the Moon, and Earth are lined up perfectly.
  • Because the Moon’s orbit is tilted about 5° from the plane of Earth’s orbit around the Sun, eclipses happen only a few times each year.
  • Solar Eclipse: occurs when the Moon moves directly between the Sun and Earth and casts a shadow on part of Earth.
  • The darkest portion of the Moon’s shadow is called the umbra.
  • The only portion of the Sun that is visible during a total eclipse is part of its atmosphere, which appears as a pearly white glow around the edge of the eclipsing Moon.
  • Lunar Eclipse: When Earth’s shadow falls on the Moon
  • A lunar eclipse begins when the Moon moves into Earth’s penumbra
  • The Moon sometimes becomes red during an eclipse because light from the Sun is scattered and refracted by Earth’s atmosphere.
  • A partial lunar eclipse occurs when only a portion of the Moon moves into Earth’s umbra.

• The Moon’s Surface

  • Many depressions on the Moon were formed by meteorites, asteroids, and comets, which strike the surfaces of planets and their satellites.
  • Maria: the dark-colored, relatively flat regions on the Moon’s surface.
  • Regolith: the accumulation of debris on the Moon

• The Moon’s Interior

  • If cracks did form when the large depressions were produced by impacts, and lava did flow onto the lunar surface, then the interior of the Moon just below its surface must have been molten at that time.
  • Other information about the Moon’s interior comes from seismographs left on the Moon by Apollo astronauts.

• Exploring the Moon

  • Clementine compiled a detailed map of the Moon’s surface, including the South Pole-Aitken Basin.
  • This is the oldest identifiable impact feature on the Moon’s surface. It is also the largest and deepest impact basin or depression found so far anywhere in the solar system, measuring 12 km in depth and 2,500 km in diameter.
  • Data from Clementine confirmed that the crust on the side facing Earth is much thinner than on the far side.
  • Data from Lunar Prospector confirmed that the Moon has a small, iron-rich core about 600 km in diameter.

• Origin of the Moon

  • Prior to the data obtained from the Apollo space missions, there were three theories about the Moon’s origin:
  • The first was that the Moon was captured by Earth’s gravity (the capture theory). It had formed elsewhere and wandered near Earth.
  • The second theory was that the Moon condensed from the same loose material that Earth formed from during the early formation of the solar system (the binary accretion theory).
  • The third theory was that a glob of molten material was ejected from Earth while Earth was still molten (the fission theory).

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