Astronomy

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Video

Materials

• 1 globe on a stand (Earth's axis must tilt)
• 1 table lamp

Short explanation

Earth rotates around its own axis, giving rise to day and night. Earth also moves in an orbit around the Sun. Since Earth's axis tilts 23.5 degrees, the length of a day depends on where in the orbit Earth is. This variation in solar radiation gives rise to the seasons.

Long explanation

What you have created is a simple model of the solar system, where all celestial bodies except Earth and the Sun have been removed. The model shows what took humanity hundreds of years to understand, namely that Earth orbits the Sun and not the other way around.

Earth orbits the Sun in 1 year, and at the same time rotates 365 times around its own axis. The model shows that it's Earth's rotation around its own axis that gives rise to day and night. When one half of the globe is facing the Sun, it's day there, but at the same time it's night in the other hemisphere.

The model also shows that Earth's axis (the imaginary "stick" straight through Earth around which Earth rotates) is tilted and that it's this tilt that gives rise to the seasons. When the North Pole tilts away from the Sun, it's winter in the northern hemisphere. This is because the days are short and the Sun is low in the sky - and this in turn means that only some solar energy reaches Earth's surface every day. This makes it cold. But at the same time, it's summer in the southern hemisphere. The days are long and the sun is high in the sky. A lot of solar energy reaches Earth's surface every day. This makes it warm. When the North Pole, on the other hand, tilts towards the Sun, it's summer in the northern hemisphere, while it's winter in the southern hemisphere. In between, when the North Pole tilts to the side seen from the Sun, it's spring and autumn respectively. Then the days and nights are about the same length - both in the southern and northern hemisphere.

When the North Pole tilts the most towards the Sun, it's the summer solstice in the northern hemisphere. This is the longest day of the year, and the Sun also reaches its highest point in the sky. At the Arctic Circle, the Sun shines all day and all night on the day of the summer solstice. The further north of the Arctic Circle you are, the more such days you have in a year. When the North Pole, on the other hand, tilts the most away from the Sun, it's the winter solstice in the northern hemisphere. Then the day is the shortest and the Sun makes its lowest passage in the sky. At the Arctic Circle, the Sun doesn't even shine at all on the day of the winter solstice. And the further north of the Arctic Circle you are, the more such days you have in a year. When the North Pole tilts straight to the side seen from the Sun, which happens once in the spring and once in the autumn, it's the vernal equinox and the autumnal equinox, respectively. That day, day and night are just as long - and this is true wherever you are on the globe!

Experiment

You can turn this model and demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What would happen to day and night if Earth rotated faster around its own axis?
• What would happen to day and night if Earth did not rotate at all around its own axis?
• What would happen to the seasons if Earth did not rotate at all around its own axis?
• What would happen to the seasons if Earth's axis did not tilt?
• What would happen to the seasons if Earth's axis tilted even more?
• What would happen to the seasons if Earth moved faster through space in its orbit around the Sun?
• What would happen to the seasons if Earth did not move at all in its orbit around the Sun?

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