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Physics

From our observations of the stars we know that their patterns in the sky repeat over a
period of time. The same thing happens with the seasons. This has helped us to conclude
that the Earth goes through a cycle. We call this a year. One year is the time it takes for the
Earth to orbit once around the Sun.
We also know that this path is not a perfect circle. It is an ellipse, and although the
intensity of sunlight changes because the distance from the Sun changes, this is not the
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cause of the seasons. The different hemispheres still experience different and opposing
seasons throughout the year. The change of seasons must be caused by something else.

The Earth rotates around its axis at an angle of 23.5 degrees in relation to its orbital plane.
This is the main cause of the seasons.

WHAT CAUSES SEASONS?

Spring in the northern hemisphere
Autumn in the southern hemisphere

North Pole Winter in the northern hemisphere
Summer in the southern hemisphere
North Pole

North Pole South Pole
North Pole
23 /2°
1
South Pole

South Pole

Summer in the northern hemisphere
Winter in the southern hemisphere South Pole

Autumn in the northern hemisphere
Spring in the southern hemisphere

ACT How can you demonstrate and explain the change of seasons in the different
hemispheres? How would the seasons appear to people living near the Equator? How will
the intensity of light change depending on the tilt of the planet? Why?

OVER TO YOU!

1. What is the duration of an Earth day? What 4. Why are the seasons important to humans?
impact does this have on how our yearly 5. Imagine that the Earth had no tilt on its axis.
calendars work?
What seasons would we observe then? Give
2. Why is there a change from day to night? reasons for your answer.
What would happen if the Earth stopped
6. Why do we have calendars to measure time?
rotating around its axis?
Research how different civilizations have used
3. Describe and explain how the change of calendars to measure time and explain how they
seasons happens. were developed.

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