Space & Technology
Chapter 17 - Earth's Cycles
Lesson 1: How does the Earth move?
Earth is always moving. We do not feel it because we are moving along with Earth. Also, Earth moves very smoothly. There are some clues that Earth is moving. The first clue is that the Sun and stars seem to move across the sky. This happens because the Earth is turning. The second clue is that the seasons change during the year. These changes are partly caused by the way Earth moves in space. Scientists use many tools to study how the stars and other objects seem to move.
Earth turns around an imaginary pole through its center called its axis. Earth spinning around its is called rotation. Earth takes about a day to make one rotation. Earth rotates from west to east. This makes objects like the Sun and stars seem to move from east to west in the sky.
Why Shadows Change
Shadows form when light shines on an object but does not pass through it. Earth’s rotation causes sunlight to shine on objects from different angles throughout the day. This causes the length and position of an object’s shadow to change. Earth’s rotation causes day and night. When Earth is turned towards the Sun, it is daytime. It is nighttime when Earth is turned away from the Sun.
The number of daylight hours changes during the year. The Northern Hemisphere has more daylight hours during the summer months and fewer daylight hours during the winter months.
The movement of one object around another is called a revolution. It takes Earth about one year to revolve around the Sun. The path Earth takes around the Sun is its orbit. Gravity is a force that pulls two objects towards each other. Gravity between the Earth and the Sun keeps the Earth in its orbit. Earth’s orbit is an ellipse. An ellipse looks like a circle that is stretched out.
Earth’s Tilted Axis
Earth’s axis always tilts in the same direction. The tilt affects how places on Earth receive sunlight. This is what causes Earth’s seasons. The Northern Hemisphere gets more direct sunlight when it tilts towards the Sun in the summer. Daylight lasts longer. Temperatures are higher. At the same time, it is winter in the Southern Hemisphere as it tilts away from the Sun. The days are shorter. The temperatures are lower.
Lesson 2: What patterns can you see in the sky?
The Moon does not make its own light. You can see the Moon because sunlight reflects off the Moon’s surface. The Moon orbits around Earth. Its orbit is shaped like an ellipse. It takes about 29 days to revolve around Earth. The Moon rotates once around its axis as it revolves around the Earth. This is why the same side of the Moon always faces Earth.
The Moon’s shape seems to change at different times of the month. All the Moon’s shapes are called the phases of the Moon. These phases are caused by the Moon’s revolution around Earth. Half of the Moon faces the Sun. We only see the part of the Moon that reflects the Sun. The Moon looks like a circle when its lighted half faces Earth. This is called a full Moon. Then, only parts of the lighted half of the Moon face Earth and the Moon appears to have different shapes. Then you cannot see the Moon at all. The pattern begins again.
An eclipse occurs when one object in space gets between the Sun and another object. This causes a shadow to fall on the second object.
A lunar eclipse is when the Moon passes through Earth’s shadow. A partial eclipse is when only part of the Moon is in Earth’s shadow. Then the Moon might look like something took a bite out of it. Unlike a partial eclipse, a total lunar eclipse occurs when the whole Moon is in the Earth’s shadow. The Moon does not disappear during a lunar eclipse. Earth’s atmosphere bends and scatters some sunlight, allowing some of the Sun’s rays to reach the Moon. Lunar eclipses happen several times a year.
A solar eclipse is when the Moon moves between the Sun and Earth. This causes the Moon’s shadow to fall on Earth. It can only be seen where the moon casts its shadow.
There are many, many stars in the sky. The Sun is the star nearest to Earth. The Sun gives energy and light and is a hot ball of gas. Some stars are bigger, brighter, or hotter than the Sun, and some are smaller, dimmer, and cooler. We cannot see other stars during the day because the Sun is so bright.
The patterns of stars in the sky are called constellations. Some stars in each constellation are closer to Earth than others. We cannot see the distance between these stars. This is why the stars in each constellation look like they are close together. They appear to move in the sky as Earth rotates. People who live in the Southern Hemisphere see different constellations than people in the Northern Hemisphere.