1. The Earth and Its Seasons

2. Why Do We Have Days? Day and night are determined by the Earth’s rotation. The light from the Sun is always being emitted. The Earth, turning on its axis, causes the difference in where the light shines on the Earth.

3. The Earth rotates once every 24 hours on its axis. The time it takes for the Earth to rotate completely around is what we call day. The counter-clockwise rotation of the Earth makes it appear that the Sun and Moon rise in the east and set in the west.

4. The combined effect of the Earth’s tilt and its revolution (orbital motion) around the Sun result in the seasons. Because the axis is tilted, different parts of the Earth are pointed towards the Sun at different times of the year. It takes the Earth 365¼ days to complete one revolution around the Sun.

5. Summer is warmer than winter because the rays from the Sun hit the Earth at a more direct angle during summer than during winter. During winter, the rays hit the Earth at a more oblique angle which spreads the rays over a larger area, resulting in less heating of the Earth. These effects are due to the 23½ tilt of the Earth on its axis. The tilt of the Earth remains the same anywhere along its orbit around the Sun.

6. The beginning of the seasons is determined by the tilt of the Earth and the Earth’s position compared to the Sun.

7. June 20th/21st - Summer Solstice: Northern Hemisphere is tilted toward the Sun and the Southern Hemisphere is tilted away. September 22nd/23rd – Autumnal Equinox: Neither hemisphere is tilted toward the Sun, but the equator is receiving direct sunlight. December 21st/22nd – Winter Solstice: Southern Hemisphere is tilted toward the Sun and the Northern Hemisphere is tilted away. March 21st/22nd – Vernal Equinox: Neither hemisphere is tilted toward the Sun, but the equator is receiving direct sunlight.

8. The seasons are reversed in the hemispheres. Summer in the Northern Hemisphere is winter in the Southern Hemisphere and vice-versa. The length of day will vary with the season, again due to the tilt of the Earth. Summer days are longer than winter days. The reverse is true of nights.