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The Seasons
 

This model illustrates the change of seasons in the Northern hemisphere. The Earth's rotation around the Sun is simulated in center of the screen. Its axis of rotation, as well as North (N) and South (S) Poles, are outlined.

The Earth's rotational axis is pitched from the orbit axis at an angle of approximately 23.5?. Without such pitch, the change of seasons would have been impossible. The change of seasons is caused by the Earth's rotation around the Sun, and the Earth's rotational axis' pitch to the orbital plane.

Summer begins in the Northern hemisphere when the Sun shines on the Earth's North Pole. Meanwhile, the planet's South Pole is completely overshadowed, and winter begins in the Southern hemisphere. Spring in the Northern hemisphere coincides with autumn in the Southern hemisphere. Autumn in the Northern hemisphere coincides with spring in the Southern hemisphere. As you can see, the seasons in Southern and Northern hemispheres are always exactly opposite.

On March 21 and September 23, day and night are equal in length, lasting for 12 hours each. These days are known as spring and autumn equinoxes. During summer, the days last longer than during winter. Consequently, the Earth's Northern hemisphere receives far more heat during spring and summer (from March 21 to September 23) than during fall and winter (from September 23 to March 21).

The Earth's rate of motion along its orbit is not constant. It changes from 29.5 km/s in aphelion (July) to 30.3 km/s in perihelion (January). Accordingly, the Earth passes the distance from the autumnal to the spring equinox along its orbit quicker than it passes the orbital distance from the spring to the autumnal equinox. Thus, the spring through autumn time segment in the Northern hemisphere is 6 days longer than the autumn through springtime segment.

The Earth rotates around the Sun by an elliptical rather than a circular orbit. During the aphelion, the Earth is 5 million kilometers farther away from the Sun than during the perihelion. These differences are apparent from the noticeable angular variations in the solar path, as well as from the fact that winters in the Northern hemisphere are less severe than in the Southern hemisphere, while summers in the Northern hemisphere are relatively more chilly.

Press "Run" button to initiate the animation. Press "Stop" to pause the model, and press "Reset" to return the model to its initial state. A small window in the model's bottom-right corner displays the landscapes that are characteristic for Russia's mid-latitude zone during the appropriate seasons. The timer on the left keeps track of the virtual years and months that have elapsed since the moment of the model's activation.

 
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