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Weather results from the irregular interaction of the sun with Earth's atmosphere. Significant variations in the amount of solar energy reaching our atmosphere occur as the Earth spins on its axis and, at the same time, completes its annual journey roughly the sun.

Earth, the "third rock" from the sun, follows an elliptical orbit that takes one year to complete. The length of Earth's year was determined by the time it takes to complete one orbit. Because Earth's orbit is an ellipse, it actually is closer to the sun during assured parts of the year. Even though more solar heat falls on the Earth during these approaches, this does not cause seasons.

While the Earth travels through space in its orbit, it also spins like a top on its axis. It completes one spin about every 24 hours. Earth's rotation causes the alternation between day and night. The differences between weather conditions during day and night (like high and low temperatures) also affect weather patterns.

The real cause of Earth's seasons is the fact that the planet is tilted about 23.five degrees on its axis. As the tilted Earth travels in its orbit around the sun, different parts of the Earth are tilted closer to the sun. So different parts of the Earth receive different amounts of heat during certain parts of the year.

The North Pole is tilted away from the sun in December, so the Northern Hemisphere receives less light. This results in lower temperatures and shorter days.

As the Earth moves along its orbital path, the Earth's tilt puts the North Pole around the sun. It continues to mover nearer to the sun until June 21st, the summer solstice. The sun is directly over the Tropic of Cancer on this day. The Earth continues to move around the sun until the winter solstice on December 21st, completing the cycle.

The Southern Hemisphere experiences the same cycle, but the seasons are opposite. The Southern Hemisphere is closest to the sun in December, so the southern latitudes experience summer-like weather during this period. Likewise, the Southern Hemisphere's winter solstice occurs on June 21st.

Only the middle and high latitudes (above 23.Five degrees north and below 23.5 degrees south) experience 4 seasons. The tropics generally experience only 2 seasonal variations: wet and dry.