Seasons and Length of Day

What causes the seasons? Why is it hotter in summer than winter? Why are the days long in summer and short in winter? Why is the Sun high in the summer sky and low in the winter sky? This short tutorial will look at these questions.

Seasons are NOT caused by the earth's distance from the Sun changing. As a first step to showing yourself this is true, ask yourself the following.

1. When it is winter in North America and Europe, is it winter in Australia, southern Africa, and southern South America?
A).. Yes

The seasons are opposite in the northern and southern hemisphere and thus can have nothing to do with the Earth's diistance from the Sun.

To understand what causes seasons, think of how you warm your hands in front of a fire or radiator. You hold them flat on to the heat source, not edge on. That way, your hands absorb more of the heat. So too a given part of the Earth gets more heat when its surface is "tilted" more directly at the Sun. See the Figure below.

The tilt that causes the seasons arises because the Earth spins around an axis tilted at 23.5 degrees with respect to its orbit around the Sun. See Figure below.

Sketch Earth's Rotation axis is tilted with respect to its orbit around Sun

The direction of the Earth's rotation axis (the line about which it spins) remains approximately constant. To understand why this happens, think of the following:

2. Which is easier to make stand on edge: A coin at rest or a spinning coin?
A)...Coin at rest.
B)...Spinning coin.

Spin stabilizes the orientation of objects. Think of a thrown football, a top, even a bicycle.

The Earth's spin keeps it oriented in approximately the same direction even as the Earth orbits the Sun, as the sketch below shows.

Movie showing Earth's fixed tilt as it orbits Sun

To see why the tilt makes one hemisphere hotter than the other, look at the figure below. It shows the Earth in two different points along its orbit and a beam of sunlight striking the Earth. Notice the difference in the amount of each beam striking each hemisphere.

Sketch showing sunbeams hitting Earth at different times of year.

3. Which position (A or B) puts the most solar energy on the northern hemiphere?
A)... A
B)... B

The two pictures below show how the Earth would appear from the Sun on June 21 and December 21.

View of the Earth as seen from the Sun on June 21 and January 21

4. Which picture (Right or left) is June 21?
A)... Left
B)... Right

5. What location on the Earth is exactly in the middle of the image on June 21?
A)... The Equator.
B)... The Tropic of Capricorn.
C)... The Tropic of Cancer.
D)...The Analemma.
E)...The Greenwich Meridian.

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