Antecedentes

Analyze and interpret data of observable​patterns to show that Earth rotates on its axis and revolves around the Sun. Emphasize patterns that provide evidence of Earth’s rotation and orbits around the Sun. Examples of patterns could include day and night, daily changes in length and direction of shadows, and seasonal appearance of some stars in the night sky. Earth’s seasons and its connection to the tilt of Earth’s axis will be taught in Grades 6 through 8. (ESS1.B)

There are observable patterns of the Earth, Sun and Moon that we will learn about in this chapter. Using what we can see with our own eyes, these patterns will give us data to analyze and help us explain what we see around us, such as day and night, length and direction of shadows, and star patterns of constellations at different times of the year.

The Earth moves

We know that the Earth moves in two different ways. The Earth orbits the Sun and the Earth also spins on its own axis. But what does this mean? Imagine an orange with a pencil stuck through it. Look at the following picture. If you hold the pencil in your fingers, you can spin the orange around. The pencil is the axis of the orange.

The Earth does not really have a pencil through it, but it does spin around. We can imagine a big pencil through the middle of the Earth.

The Earth is like the orange and the pencil is like the axis. The curved arrows show which way the Earth spins.

We are on the Earth. Let us imagine we are at the point where you see the red "X" on the orange:

● The Sun shines on the Earth and so we, at X, see the Sun. We call that daytime.

● But the Earth never stops spinning. So we, at X, move around into the shaded part of the Earth. Then we cannot see the Sun any longer and it is

● The Earth spins right around in 24 hours, so it will take 24 hours for us to come around to the same position you see in the picture.

● We call the 24 hours a day. When we say "a day" we really mean a day and night; together they last 24 hours.

If we are at position X, we move past the Sun. But to us, it looks as if the Sun is moving. The Sun seems to move from the East to the West. The Sun seems to come up (rise) in the East, move across the sky during the day and go down (set) in the West. But the Sun does not actually move.

The Earth moves in an orbit around the Sun

The Earth moves around the Sun. While the Earth orbits the Sun, it is also spinning on its own axis. It spins round 365 times while it completes one orbit of the Sun. That means 365 days pass and we call that a year.

The Earth is a planet. There are 7 other planets also moving around the Sun. You can see one of the other planets on most evenings, or early in the morning. This planet is called Venus. It is not a star, although sometimes it is called the morning or evening star.

Venus also moves around the Sun but its orbit is a smaller circle than Earth's orbit. The orbits of the outer planets are actually much bigger than what is shown in this image. But, if we tried to draw the orbits to scale, they definitely would not fit on this page!

The Rotating Earth

So how do we know that the Earth rotates on its axis? Before this century, all evidence collected about Earth's motion came from Earth-bound observations. From Earth-bound observation it is easy to see why people used to believe that the Earth was stationary. They believed that it was the Sun and the stars that orbited the Earth. We have learned that it is the Earth that rotates which causes the cycles we see in the sky.

An experiment designed by Léon Foucault, a French scientist, in the year 1851 demonstrates movement of the Earth. He hung a heavy iron weight from a long wire. He pulled the weight to one side and then released it. The weight swung back and forth in a straight line. If Earth did not rotate, the pendulum would never change direction as it moved back and forth. But it did not continue to swing in the same direction. It was thought to change because the Earth was rotating beneath it. Figure below shows how this might look.

Earth’s Rotation

Imagine a line passing through the center of the Earth. The line extends through both the North Pole and the South Pole. This imaginary line is called an axis. Earth spins around its axis, just as a top spins around its spindle. This movement is called Earth’s rotation.

Day-Night Cycle

all points on Earth, the Sun appears to move across the sky from east to west.

Of course, the Sun is not moving from east to west at all. The Earth is actually

rotating on its axis. The Moon and stars also seem to rise in the east and set in

the west. We often say that the Sun is “rising” or “setting.” Actually, it is the

Earth’s rotation that makes it appear that way. The Moon and the stars at night

also seem to rise in the east and set in the west. Earth’s rotation is also

responsible for this too. As Earth turns, the Moon and stars change position in

the sky. All the motion in the sky that you notice is due to the Earth’s rotation. The daylight and darkness cycles are determined based on how far north or

south they are. At the equator, there is an even amount of daylight and

darkness. No matter what day of the year it is, the equator always experiences

12 hours of daylight every 24 hours. At the poles, the amount of daylight varies

greatly. In their summer months. The poles receive 24 hours of continuous

daylight for 6 months. During their winter months they have 24 hours of

darkness. This too lasts for 6 months. Earth’s Revolution Earth’s revolution around the Sun takes 365.24 days. That is equal to one year.

Why does the Earth stay in orbit around the Sun? It is the Sun's gravity ​(​Figure below​) that holds the Earth in place. If not for gravity, the Earth would continue

to go in a straight line. Earth's orbit is not a circle. It is somewhat elliptical. So as we travel around the

Sun, sometimes we are a little farther away from the Sun. Sometimes we are closer to the Sun.

eight minutes for light to get from the Sun to Earth. The ​distance between the Earth and the Sun is about 93 million miles. That’s about 150 million kilometers. Earth revolves around the Sun at 27 kilometers per second. It is going really fast. Even at that speed it still takes a long time to go around the Sun. It takes 365.24 days to be exact. ​Mercury and​Venus are closer to the Sun. They take a shorter time to make one orbit. Mercury takes only about 88 Earth days to make one trip around the Sun. All of the other planets take longer. The exact amount depends on the planet's distance from the Sun. ​Saturn takes more than 29 Earth years to make its journey around the Sun. How old would you be if you were on Saturn?

Putting It Together

Why does the Sun appear to move across the sky?

How long does it take the Earth to move around the Sun?