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The Sun-Earth-Moon System

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1. How the Earth Moves

2.1. Earth's Rotation
The spinning of the Earth on its axis is known as rotation. As viewed from space, the Earth rotates in a counterclockwise direction called prograde rotation. Venus and Uranus spin in a clockwise direction called retrograde rotation.
2.2. Rotation Causes Day and Night
As the Earth rotates east, the Sun appears to move toward the west. We experience a period of daylight when the Earth is facing the Sun and a period of darkness when the Earth is facing away from the Sun.
2.3. Earth's Period of Rotation
The Earth rotates at 1,670 kilometers per hour. The amount of time it takes for the Earth to rotate once is known as its period of rotation, or what we call a day. Earth's period of rotation is 24 hours.
2.4. Earth's Revolution
As the Earth spins on its axis, it also revolves around the Sun. It moves on an elliptical orbit path at a speed of 107,000 kilometers per hour. The amount of time it takes for the Earth to revolve one time around the Sun is its period of revolution, or what we call a year. Earth's period of revolution is 365.25 days.

2. Seasons on Earth

3.1. Earth's Tilt Causes Seasons
As the Earth revolves around the Sun, the planet tilts 23.5 degrees on its axis of rotation. The degree to which various points on the globe are pointing toward or away from the Sun determines the seasons. The Northern and Southern hemispheres experience opposite seasons throughout the year.
3.2. Latitude and the Sun's Rays
Latitude is a key factor affecting the climate in a particular location on Earth. Because the Earth is a globe, the Sun's rays hit more directly at the equator than near the poles. This means the Sun's energy is more concentrated near the equator and more spread out near the poles. Average temperatures become cooler moving from the equator to higher latitudes.
3.3. Solstice
In the Northern hemisphere, the summer solstice occurs on June 21 when the Sun's rays are directly hitting the Tropic of Cancer, 23.5 degrees north of the equator. Six months later, the winter solstice occurs on December 21 when the most direct sunlight hits the Tropic of Capricorn in the Southern hemisphere.
3.4. Equinox
March 20 and September 22 are the equinox dates when the Sun's rays are directly hitting the Earth's equator. Neither hemisphere is tilted toward or away from the Sun. All locations on Earth experience approximately equal hours of day and night on these days.
3.5. Seasons in Tropical Regions
Throughout the year, the tropical regions near the Earth's equator receive fairly direct sunlight. These regions experience little variation in daylight hours and temperature. There is little or no change in seasons in the tropics.
3.6. Seasons in Temperate Regions
Regions located in the temperate zone latitudes experience four distinct seasons during the Earth's yearly cycle. Summer temperatures are warm because the Earth is tilted toward the Sun and there are more daylight hours. Winter temperatures are cold because the Earth is tilted away from the Sun and there are fewer daylight hours.
3.7. Seasons in Polar Regions
The Sun's rays hit the Earth's polar regions at very slanted angles. These regions have year-round cold temperatures and do not experience four distinct seasons. Instead, these areas have six months of daylight when the Earth's pole is tilted toward the Sun, followed by six months of darkness when the pole is tilted away from the Sun.

3. Pause and Interact

4.1. Review
Use the whiteboard tools to complete the activity.

4. Gravity and Motion

5.1. Strength of Gravity
The law of gravity states that every object in the universe attracts every other object. The strength of gravity increases as mass increases. The strength of gravity decreases as distance increases. Both the Earth and Moon exert gravitational pulls on each other.
5.2. Gravity of the Earth and Moon
The Earth has a greater gravitational pull on objects because it has a larger mass than the Moon. Earth's gravitational pull is about six times greater than the Moon's gravitational pull. On the Moon, you would weigh about one-sixth of your weight on Earth.
5.3. Earth and Moon Have Inertia
Inertia is the tendency of an object to resist change once it is in motion. An object will stay in motion at a constant speed and direction, unless it is acted on by another force. The Moon and the Earth are in constant motion as they rotate on their axes, and therefore they have inertia.
5.4. Gravity, Inertia and Orbit
The combination of gravity and inertia keeps the Moon moving in orbit around the Earth. If there were no gravitational pull on the Moon, it would move in a straight line away from the Earth into space. If the Moon did not have inertia, then it would be pulled by Earth's gravity and crash into Earth. The same forces keep the Earth in orbit around the Sun.

5. Earth's Moon

6.1. Theory of Moon's Origin
Scientists theorize that the Moon was formed billions of years ago when a planet-sized object collided with the newly forming Earth. A large piece of Earth's mantle and debris material was blasted into space. Over millions of years the molten material accumulated and crystallized to form the lunar crust.
6.2. Synchronous Rotation
The Moon is Earth's only natural satellite, making one complete revolution every 27.3 days. The Moon's rotation and revolution periods are the same, which is called synchronous rotation. This means that the same side of the Moon faces the Earth at all times. Humans did not see the dark or far side of the Moon until the space flights of the 1960s and 1970s.
6.3. Moon Composition and Surface
The Moon is composed of a solid ball of basalt covered by rocks and dust. Since the Moon's formation billions of years ago, the surface has been struck by asteroids, meteoroids and comets. These impacts have created dust, rocks, boulders and basins. There is no tectonic activity on the Moon, so the rock composition has remained unchanged since its formation.
6.4. Moon Facts
The Moon is the fifth largest known moon in the solar system. Its diameter is 3,474 km. The Moon's volume is about 1/50th that of the Earth. Its gravitational pull is about 1/6th that of the Earth.

6. Pause and Interact

7.1. Review
Use the whiteboard tools to complete the activity.

7. Phases, Eclipses and Tides

8.1. Cause of phases, eclipses and tides
The Moon is in motion as it rotates on its axis and revolves around the Earth. The changing relative positions of the Sun, Earth and Moon cause moon phases, eclipses and tides.
8.2. What are moon phases?
The changing appearances of the Moon as it makes one complete revolution around the Earth are known as moon phases. The positions of the Sun, Earth and Moon determine the portion of the Moon that is lit by sunlight. Sometimes the entire face of the Moon is lit, and at other times only part of the Moon is lit and the rest is in shadow.
8.3. Moon Phases
A new moon occurs when the sunlit portion of the Moon is facing away from the Earth. The Moon is waxing when the sunlit portion is getting larger. A full moon is seen when the side of the Moon facing the Earth is entirely sunlit. The Moon is waning when the lit portion is getting smaller. Crescent and gibbous are shapes of the Moon that we see during a month.
8.4. What is an Eclipse?
When one celestial body comes between the Sun and another celestial body, a shadow is cast on the second celestial body. This phenomenon is called an eclipse. When the Moon's shadow hits the Earth, a solar eclipse occurs. When the Earth's shadow hits the Moon, a lunar eclipse occurs.
8.5. Solar Eclipse
A solar eclipse occurs when the Moon passes directly between the Earth and the Sun, and sunlight is blocked from hitting the Earth. A total solar eclipse will only occur during a new moon. The sky turns dark and the temperature drops during the eclipse. The Sun's outer atmosphere called the corona also becomes visible.
8.6. The Moon's Umbra and Penumbra
During a solar eclipse, the Moon will cast the darkest part of its shadow, called it's umbra, on a small portion of the Earth. Only these locations will experience a total solar eclipse. Other locations within the Moon's penumbra, a wider shadow, will experience a partial solar eclipse.
8.7. Lunar Eclipse
A lunar eclipse occurs when the Earth comes between the Sun and the Moon, and sunlight is blocked from hitting the Moon. A total lunar eclipse only occurs when the Moon is full. The entire Moon falls within the Earth's dark shadow, or umbra. The completely darkened Moon can be seen at all locations on Earth where the Moon is visible.
8.8. Moon's Orbit is Tilted
One might expect that lunar and solar eclipses would occur every month as the Moon orbits the Earth. This does not happen because the Moon's orbit is actually tilted by a little more than five degrees relative to the Earth's orbit around the Sun. It is rare that the Moon is in the Earth's shadow during a full moon or between the Earth and Sun during a new moon.
8.9. What Causes Ocean Tides?
Changing water levels of the oceans are known as tides. Both the Sun and Moon influence tides. The gravitational pull of the Moon creates bulges in the Earth that impact the timing and height of tides. When ocean water is pulled by the Moon's gravitational force to create high tides in one area, water leaves other areas and creates low tides.
8.10. Spring Tides
When the Sun, Earth and Moon line up with each other, the gravitational pull on the oceans is at its maximum. This alignment causes unusually high tides called spring tides. These tides occur twice a month during the new moon and the full moon.
8.11. Neap Tides
When the Sun and Moon are at 90 degrees to one another, the total gravitational pull on the oceans is at its minimum, and the high tide is called a neap tide. These tides have the least difference between consecutive high and low tides. Neap tides occur twice a month during the first and third quarter moon phases.

8. Pause and Interact

9.1. Review
Use the whiteboard tools to complete the activity.

9. Missions to the Moon

10.1. NASA and the First Man on the Moon
The National Aeronautics and Space Administration (NASA) was established in 1958. Neil Armstrong was the first human to step on the Moon when the Apollo 11 landed on the Moon's surface in 1969.
10.2. Apollo Missions
The Apollo missions from 1969 through 1972 were a series of space flights that landed many Americans on the Moon. During these missions, the Moon's surface was studied using a Lunar Roving Vehicle. Hundreds of kilograms of rock and soil samples were collected for further study.
10.3. Current Moon Exploration
NASA anticipates sending humans back to the Moon in the near future. Current lunar research focuses on determining the feasibility of a permanent moon base to be used for further space exploration and research. The Lunar Reconnaissance Orbiter, launched in 2009, is collecting highly detailed information about the lunar environment.

10. Vocabulary Review

11.1. Vocabulary Matching Review
The National Aeronautics and Space Administration (NASA) was established in 1958. Neil Armstrong was the first human to step on the Moon when the Apollo 11 landed on the Moon's surface in 1969.

11. Virtual Investigation

12.1. Moon Phases, Eclipses and Tides
The changing relative positions of the Sun, Earth and Moon cause the moon phases, eclipses and tides. In this virtual investigation you will move the Moon to its correct orbit position relative to the Earth and Sun as you review the different moon phases, eclipses and tides. You will also identify images that match each of these phenomena.