Item 2:  Earth’s Movements:


A few pre questions about this lesson:

  1. What is Earth’s shape?


2. What are Earth’s motions?


3. What do you know about Earth’s magnetic field?


4. What causes the seasons?


5. What is a solstice and what is an equinox?



Properties  of Earth:

Every morning, the Sun rises in the east. It moves across the sky during the day and the Sun sets in the west every evening. Is the Sun moving—or are you?

People once thought that Earth was a flat object at the center of the universe. They believed that the Sun went around Earth in a big circle each day. Now, most people know that Earth is not flat, and the Sun only looks like it is moving around Earth. Scientists have discovered that Earth spins and that Earth moves around the Sun. It is the spinning motion of Earth that makes it look like the Sun is moving across the sky.


What  is Earth’s  shape?

Basketballs, tennis balls, and Earth have something in common. They are all round, three-dimensional objects called spheres.  The distance from the center of a sphere to any point on the surface is the same.


Many people believe that Columbus made his famous voyage to prove that the Earth was round, but people have thought the Earth was round for thousands of years.  Aristotle, a Greek astronomer and philosopher who lived around 350 B.C., observed that Earth made a curved shadow on the Moon during an eclipse. His observations led him to think that Earth was a sphere.  About 2200 years ago, a Greek mathematician named Eratosthenes proved that the Earth was round by doing an experiment.  YOUTUBE NOW!!!


Today, because of pictures from artificial satellites and space probes, as well as photos taken by the astronauts, we know positively that the  Earth is round. Now we also know Earth is not perfectly round. It bulges at the equator and is somewhat flat at the poles.


Does  Earth  spin?

Earth spins like a top. The imaginary center line around which Earth spins is called Earth’s  axis. The poles are at the north and the south ends of Earth’s axis. The spinning of Earth on its axis is called  rotation.

Earth’s rotation causes day and night. As Earth rotates, your area of Earth faces toward the Sun in the morning and away from the Sun at night. Earth rotates once each day. A rotation takes about 24 hours.



What causes  changing seasons?

Flowers bloom as the days get warmer. The Sun appears higher in the sky, and daylight lasts longer. Spring seems like a fresh, new beginning. What causes these changes?


Does  Earth’s  orbit  cause  seasons?

Remember that Earth’s rotation causes day and night. Another movement of Earth is called revolution.  Revolution is Earth’s orbit, or the path of Earth, as it goes around the Sun. It takes a year for Earth to orbit the Sun.  The shape of Earth’s path around the Sun is an  ellipse(ee LIHPS)—a long, curved shape, sort of like an egg-shaped circle. The Sun is not located in the center of the ellipse but is a little toward one end. Earth is closest to the Sun around January 3, and farthest from the Sun around July 4. Although Earth’s orbit takes it nearer and farther from the Sun, the change in distance is small and does not cause seasons. If Earth’s distance from the Sun caused the seasons, January—when the Earth is nearest to the Sun—would have

the warmest days. This is not the case, however, in the northern hemisphere.


Does Earth’s tilted axis cause seasons?

Earth’s axis is tilted 23.5 degrees from a line drawn perpendicular to the plane of its orbit. It is this tilt that

causes seasons. The tilt explains why Earth receives such a different amount of solar energy from place to place during the year.


In the northern hemisphere, summer begins in June and ends in September. This is when the northern hemisphere is tilted toward the Sun. During summer, there are more hours of sunlight—or solar energy. Longer periods of sunlight are one reason that summer is warmer than winter, but this is

not the only reason.


How does Earth’s tilt affect solar radiation?

Earth’s tilt causes the Sun’s radiation to strike the hemispheres at different angles. Sunlight strikes the

hemisphere tilted toward the Sun at an angle closer to 90 degrees than the hemisphere tilted away. Thus the

hemisphere tilted toward the Sun receives more solar radiation than the hemisphere tilted away from the Sun.

Summer occurs in the hemisphere tilted toward the Sun, when its radiation strikes Earth at a high angle and for

longer periods of time. The hemisphere receiving less radiation experiences winter.


Solstices

The solstice is the day when the Sun reaches its greatest distance north or south of the equator. In the northern hemisphere, the summer solstice occurs on June 21 or 22, and the winter solstice occurs on December 21 or 22. The position of Earth in relation to the Sun at different times of the year is shown in the figure on the next page. In the southern hemisphere, the winter solstice is in June and the summer solstice is in December. Summer solstice is the longest period of daylight of the year. From the summer solstice to the winter solstice, the number of daylight hours keeps decreasing. The winter solstice is the shortest period of daylight of the year. Then the number of daylight hours begins increasing again.


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Equinoxes

An equinox occurs when the Sun is directly above Earth’s equator. The tilt of Earth’s axis means that the Sun’s position relative to the equator is constantly changing. Most of the time, the Sun is either north or south of the equator. But two times a year the Sun is directly over the equator. This results in the spring and fall equinoxes. At an equinox, the Sun strikes the equator at the highest possible angle, 90 degrees. This can be seen in the figure above.

During an equinox, neither the northern hemisphere nor the southern hemisphere is tilted toward the Sun. The

number of daylight hours and nighttime hours is nearly equal all over the world. In the northern hemisphere, the spring equinox occurs on March 20 or 21, and the fall equinox occurs on September 22 or 23. In the southern hemisphere, the spring equinox occurs in September, while the fall equinox occurs in March.


Now that you have read, answer these questions again!

  1. What is Earth’s shape?


2. What are Earth’s motions?


3. What do you know about Earth’s magnetic field?


4. What causes the seasons?


5. What is a solstice and what is an equinox?