Earth-Sun-Moon System

In this lesson, students investigate the Earth-sun-moon system. They explore the phenomenon of gravity in keeping the moon in orbit around Earth and the Earth-moon system in orbit around the sun, and then evaluate how these motions result in patterns over time, including seasons, moon phases, and solar and lunar eclipses.

• Gravity is an attractive force between all matter. On Earth, we experience the downward pull of Earth’s gravity because Earth is so massive.
• Earth’s gravitational field extends beyond Earth’s atmosphere and into space. Interactions between the force of gravity and inertia cause objects to move in orbit.
• At the same time as the moon and the space station orbit Earth, Earth itself is in motion, orbiting the sun. All three objects form a system because they interact with one another.
• Earth orbits the sun in a regular, repeated way, which results in patterns in the length of a year and in seasons.
• Earth rotates on its axis, which causes day and night.
• The moon’s predictable patterns of movement as it travels across the night sky happen because of the moon’s position in space relative to Earth and the sun.
• When the sun, moon, and Earth are lined up in just the right way, an eclipse will occur. An eclipse occurs when one object in the solar system moves into the shadow of another object. There are two kinds of eclipses on Earth: an eclipse of the sun and an eclipse of the moon.

• What causes the moon’s orbital motion?
• How is Earth’s orbit around the sun related to the length of a year?
• What is the primary cause of seasons?
• Why are seasons different at the equator from how they are at the North Pole?
• How does Earth’s motion cause day and night?
• How is the moon cycle an example of interactions among Earth, the sun, and the moon?
• Why is the same side of the moon always facing Earth, even though the moon also rotates on its axis, like Earth does?
• What causes the moon to sometimes appear full and bright, and sometimes just a sliver in the sky?
• What has to happen for an eclipse to occur?
• How would you compare a solar eclipse with a lunar eclipse?
• Why don’t eclipses happen every month during the new moon and full moon?

Misconception: Seasons happen because Earth is closer to the sun in the summer and farther away in the winter.

Fact: The seasons are actually a result of Earth’s tilt. The part of Earth that is tilted toward the sun experiences summer, while the part tilted away experiences winter. As Earth orbits the sun, the tilt always points in the same direction, which explains the seasons.

Misconception: Gravity only exists on Earth.   

Fact: All matter, both on Earth and in space, has gravity. The sun has the strongest gravity in our solar system, which is why planets orbit it.

Eclipse : occurs when one object in the solar system moves into the shadow of another object

Force : a push or pull that acts on an object, changing its speed, direction, or shape

Gravity : the attractive force between all matter

Moon Cycle : the changing appearance of the moon (as seen from Earth) as it orbits Earth

Orbit : to travel in a circle around an object

Season : a period of time characterized by specific weather patterns and by the length of day and night

Experimenting in Space

On March 1, 2016, an astronaut named Scott Kelly returned to Earth. He had just spent 340 days in space, orbiting Earth in the International Space Station. The space station is a moving science laboratory that orbits 386 kilometers (240 miles) above Earth. To orbit means to travel in a circle around an object. Astronauts from around the world live together on the space station for months at a time. The space station orbits Earth once every 90 minutes. From space, Scott saw more than 10,000 sunrises and sunsets. He took more than one thousand photographs.

Scientists on the space station are busy conducting a variety of different experiments. One question that Scott Kelly was helping to investigate was how life in space affects human health. Scientists are trying to answer this question with an eye toward human exploration of the planet Mars. Mars is so much farther away from Earth than the space station that a trip to Mars will require a much longer time in space.

Scott Kelly had an unusual control in his experiments. He has an identical twin brother, Mark, who stayed on Earth while Scott was in space. Because Mark and Scott are identical twins, they have an almost-identical genetic makeup. This allows scientists to compare the two brothers to try to determine the long-term effects of a low-gravity environment on the human body. Scientists are particularly interested in the effects of the low-gravity environment of space.

Gravity is a force of attraction between all matter. Because it is an attractive force, it pulls objects toward one another. Here on Earth, you always experience the pull of Earth’s gravity. It pulls all objects on or near its surface down toward the planet’s center.

The effects of gravity are different on the International Space Station from how they are on Earth. There is much less gravity on the space station than on Earth. It is because of this low-gravity that astronauts float in space, and why they can lift objects that weigh hundreds of pounds.

Low gravity also affects the muscles of astronauts. On Earth, we have to use our muscles just to stand up against the force of gravity, which is constantly pulling on us. This requires our bodies to maintain enough muscle to support our own weight.

In the low-gravity environment of space, astronauts lose muscle since it is not required to support their weight. To counter this, astronauts have to exercise regularly.

One experiment that Scott Kelly was part of on the space station was investigating how a low-gravity environment affects a person’s muscle mass over an extended period of time. As soon as Scott returned to Earth, he and Mark began a series of tests by different scientists who are looking for differences between the brothers. There isn’t a quick answer to this question. Scientists plan to continue to study the Kelly brothers over time to answer the question of whether there are any long-term effects that don’t show up right away.

The low gravity of the space station occurs because the space station is actually in free fall around Earth. Because of Earth’s gravitational pull, the space station is constantly falling toward Earth's surface, in the same way that a pen you throw up in the air falls back to the ground.

This happens because of two interacting factors: gravity and inertia. Gravity is a result of an object’s mass. All matter has gravity. Dust has gravity. Rocks have gravity. Even people have gravity. Objects don't orbit us, however, because we don’t have enough mass. The more massive an object is, the more its gravity will pull on other objects and cause them to move.

Because gravity is an attractive force, objects don’t need to come into contact with one another to exert a force on each other. Instead, objects have gravitational fields. A gravitational field is the area around one object where another object will feel the gravitational force of the first object. Earth is so much more massive than the space station that its gravitational field extends beyond the space station, pulling it toward Earth’s center.

Gravity exists because matter changes space. Picture Earth as a large bowling ball sitting on a blanket. If a marble (the space station) is placed near the depression the bowling ball (Earth) creates, the marble will move toward it.

This gravitational field causes patterns in movement. Remember that a pattern is something that happens in a regular and repeated way. Every time you release a pen in the air, the pen will fall back to Earth’s surface because the pen is within Earth’s gravitational field.

Students develop and use models to describe the phenomena of how the Earth sun moon system causes the cyclic patterns of lunar phases and eclipses. Students use the data from the activity to figure out the cause and effect relationship between gravity and the resulting patterns in the motion of Earth and the moon relative to each other and the sun.