How does the Earth Orbit the Sun?

Look at the image above. Notice that the earth’s orbit is not in the shape of a circle. The earth’s orbit is slightly oval-shaped. This shape is called an ellipse.

What is the difference between a circle and an ellipse?

In a circle, the distance from the center to any point along the edge is the same. In an ellipse, the distance from the centre to the edge changes as you move along the edge.

Since the Earth’s orbit is an ellipse, the sun is not exactly in the centre – it is slightly off-centre. For these reasons, the earth is not the same distance from the sun at all times.

About January 2, the earth is closest to the sun. It is about 147 million km away. The point in the orbit that is closest to the sun is called the perihelion. About July 3, the earth is farthest from the sun. It is about 152 million km away. The point in the orbit that is farthest from the sun is called the aphelion.

All of the planets – not just the earth – orbit the sun in elliptical paths. The closer a planet is to the sun, the faster it moves; the farther away from the sun, the slower it moves. The speed of the planet’s orbit is called the orbital velocity.

Fill-in-the-blanks (terms may be used more than once):

aphelion centre distance ellipse farthest January 2July 3 Mercury nearest Neptune perihelion

The Earth’s orbit is in the shape of an _______________. The sun is not found in the _________ of

the ellipse so the earth is not the same ______________ from the sun along the entire orbit. The

earth is closest to the sun on ______________. This point of the orbit is called the

__________________. The earth is furthest from the sun on _______________. This point of the

orbit is called the ____________________.

The planet with the slowest orbital velocity is __________ because it is __________ from the sun.

The planet with the fastest orbital velocity is _____________ because it is ______________ from the

sun.

The Earth moves fastest when it is closest from the sun. This occurs on _______________. The Earth

moves slowest when it is furthest from the sun. This occurs on ______________.

Gravity

You throw a ball into the air. It moves upward for a short time, and then starts falling back to the ground. Why does it fall back? A force called gravity pulls it back toward the earth. Gravity is pulling force that attacts objects toward each other.

Gravity pulls the earth and anything on the earth towards one another. As you stand on the earth, you are pulled downward, toward the centre of the earth. Every object in the universe pulls on every other object in the universe, from the tiniest atom to the largest star. However, all objects do not have the same gravitational pull. The strength of a gravitational pull depends on two variables:

The amount of matter, or mass, of the object: The more massive the object, the stronger the gravitational pull. The pull of an atom is tiny. The pull of a star is huge.

Distance between the objects: The closer two object ar to each other, the more they pull on each other. The pull of Earth on you is very large since you are so close to Earth. The pull of Jupiter on you is smaller since Jupiter is so far away.

Gravity holds the planets around the sun and moons around the planets. If there was no gravity the planets would just fly off into space away from the sun.

Fill in the Blanks

downward distance gravity mass moon pulling sun

The force that pulls objects towards one another is called ______________. Gravity is a

_____________ force. Gravity pulls things on the earth __________________. Gravity depends upon

two things: the _________ of the objects, and the ____________ between the objects. Gravity keeps

the ___________ moving around the Earth. It also keeps the planets moving around the _________.

TRUE OR FALSE:

_____ 1. The planets move in circular orbits around the sun.

_____ 2. The sun is the very centre of the Earth’s orbit.

_____ 3. The earth is the same distance from the sun at all times.

_____ 4. The earth’s orbital velocity changes as it moves around the sun.

_____ 5. Aphelion is the closest point of an orbit.

_____ 6. Aphelion is the furthest point of an orbit.

_____ 7. The earth is at aphelion in July.

_____ 8. The earth is closest to the sun in July.

_____ 9. The closer a planet is to the sun, the faster it moves.

_____ 10. The orbital velocity of Saturn is faster than the orbital velocity of Earth.

What causes the change of seasons?

Most people live in places that have four seasons – spring, summer, autumn and winter. The four seasons are marked by differences in temperature and in the lengths of day and night. Summer, for example, is much warmer than winter, and summer days are longer than winter days.

What causes seasons? Seasons are caused by the earth’s revolution around the sun and the unchanging tilt of the earth’s axis. Part of the year, the axis leans towards the sun. Part of the year, the axis leans away from the sun.

For example, June 21st is the first day of summer in the Northern hemisphere. On that day, the Northern hemisphere leans towards the sun the most. The sun’s rays strike head-on or nearly head-on over a large part of this hemisphere. These are ‘direct’ rays, and are very strong; they do not spread over a large area. Direct rays heat up the earth the most.

December 21st is the first day of winter in the Northern hemisphere. On that day, the Northern Hemisphere leans away from the sun the most, and the sun’s rays do not strike head-on. They spread over a large area. Rays that spread out are called ‘indirect’ rays, which are weak and heat up the earth the least.

Use the following figure to answer the questions below:

1. Which rays (A or B) are direct? _____

2. Which rays (A or B) are indirect? ____

3. Which rays spread out (direct or indirect)? ___________

4. Which rays are strongest (direct or indirect)? ___________

5. Which rays heat a place most (direct or indirect)? ___________

6. Winter rays are _____________ (direct or indirect) rays.

7. Summer rays are _______________ (direct or indirect) rays.

8. Which rays can stand for winter rays (A or B)? _____

9. Which rays can stand for summer rays (A or B)? _____

Trip around the Sun

Underline the appropriate words to make the following statements true.

1. The earth revolves around the sun in a (clockwise/counter-clockwise) direction. As the earth revolves around the sun, the Earth’s axis (does/does not) change.

2. On June 21, in the Northern Hemisphere faces (toward/away from) the sun and has mainly (cold/warm) weather. It has (more/fewer) hours of daylight than the Southern Hemisphere and receives (direct/indirect) rays. These are (strong/weak) rays. In the Northern Hemisphere, June 21 is the first day of (spring/summer/autumn/winter).

3. Between June 21 and September 20, the Northern Hemisphere still leans toward the sun, but every day the lean becomes (greater/less). The rays become (more/less) direct and spread out (more/less). The days become (shorter/longer) and the nights become (shorter/longer).

4. On September 21, the earth’s axis is tilted (toward/away/neither toward or away from) the sun. The sun’s rays are (direct/indirect/neither direct nor indirect). Every place on Earth has _____ hours of day and _____ hours of night. (Spring/Summer/Autumn/ Winter) starts in the Northern Hemisphere.

5. Between September 21 and December 20, the Northern Hemisphere starts to lean (towards/away from) the sun. The rays become (more/less) direct and become (stronger/weaker). The days become (shorter/longer) and the nights (shorter/longer).

6. On December 21, in the Northern Hemisphere faces (toward/away from) the sun and has mainly (cold/warm) weather. It has (more/fewer) hours of daylight than the Southern Hemisphere and receives (direct/indirect) rays. These are (strong/weak) rays. In the Northern Hemisphere, December 21 is the first day of (spring/summer/autumn/winter).

7. Between December 21 and March 20, the Northern Hemisphere leans away from the sun, but every day the lean becomes (greater/less). The rays become (more/less) slanting. The days become (shorter/longer) and the nights become (shorter/longer).

8. On March 21, the earth’s axis is tilted (toward/away/neither toward or away from) the sun. The sun’s rays are (direct/indirect/neither direct nor indirect). (Spring/Summer/Autumn/ Winter) starts in the Northern Hemisphere.

9. Between March 21 and June 20, the Northern Hemisphere starts to lean (towards/away from) the sun. The rays become more and more (direct/indirect) and become (stronger/weaker). The days become (shorter/longer) and the nights become (shorter/longer).

10. On June 21, it is the first day of (summer/winter) in the Southern Hemisphere. The Southern hemisphere faces (toward/away) from the sun and has mainly (cold/warm) weather.

11. On Decoember 21, it is the first day of (summer/winter) in the Southern Hemisphere. The Southern hemisphere faces (toward/away) from the sun has has mainly (cold/warm) weather.