Earth’s Orbit & Rotation

Before Video

• Because the Earth rotates on its axis, the sun appears to move across the sky. Long shadows point away from the sun as it rises in the east. As it gets higher in the sky, the shadows get smaller. After it passes overhead, the shadows begin to grow again in the opposite direction.

• Sunlight shines on one side of Earth, but the other side faces away from the sun and is dark. Earth rotates on its axis once every 24 hours, so the part of the Earth that is in the light (day) rotates into the darkness (night).

• In one year, the Earth orbits the sun one time. That takes 365.25 days. It also spins on its axis once per day, which takes 24 hours. It keeps moving like this all year long.

• As the Earth orbits the sun, the night side of the earth faces a different part of the galaxy. Certain stars are visible from Earth at certain times each year. The cycle repeats each year.

After Video

• The length of a shadow is determined by where the sun appears to be in the sky. If the sun is close to the horizon, a shadow will be long. If the sun is directly overhead, the shadow will be very short.

• Earth rotates once on its axis once every 24 hours. During that time, all locations on Earth experience day and night because they will be facing toward the sun for part of the day and away from the sun for another part (night).

• Earth rotates (spins) 365.25 times in a year. It rotates once every twenty four hours, while also orbiting the sun once every 365.25 days. These two motions are happening at the same time.

• Since it takes Earth 365.25 days to orbit the sun, but our normal calendar is 365 days, we add an extra day to our calendar every four years. (4 years x 0.25 day = 1 day added every 4 years.)

• An invention called a Foucault pendulum provides evidence that the Earth is rotating on its axis. The swinging pendulum would always trace a straight line as it swings back and forth if it were sitting on an unmoving surface. However, as time goes by the path of the pendulum changes, providing evidence that the surface it is sitting on (the earth) is rotating.

• A geocentric view of the universe is the idea that everything orbits around the Earth. Since the sun appears to move across the sky, it is easy to understand why humans believed this for a long time. A concept called stellar parallax provided evidence that we orbit the sun.

• Earth and all the other planets in our solar system are both rotating and orbiting the sun. Sometimes planets are on the same side of the sun and sometimes on opposite sides. The view of planets from earth changes all the time.

• Each day the sun appears to rise, move across the sky and set. However, this doesn’t happen at the same time in every location on Earth. To adjust our clocks so that everyone has daytime during the same hours, it is necessary to use time zones. This way, anywhere you are on earth, the sun will be as high in the sky as it can be at noon.

• Due to the vast scale of objects in space, it is not always practical to use scale models. For example, if the sun was modeled to be the size of a baseball, Earth would need to be smaller than a grain of rice (2 mm)! You would barely be able to see it. Also, if you modeled the distance to scale, the baseball would be about 100 feet away from the tiny grain of rice. While accurate, it is hard to study something so tiny and far apart.