Astronomy: Earth's Motions
Rotation
The Earth rotates on its axis once every day (23 hours 56 minutes). The axis is an imaginary line that extends from the North Pole, through the center of the Earth, to the South Pole. We are rotating at a rate of approximately 360º / day, or 15º / hour. The Coriolis Effect and Foucault's Pendulum prove that the Earth is rotating. The rotation of the Earth causes the apparent daily motion of celestial objects across the sky.
Revolution
The Earth revolves around the Sun once every 365.26 days (once a year). Our orbit is shaped like a slight eccentric ellipse. This brings us slightly closer to the sun in the northern hemisphere winter (January) and slightly farther from the sun in the northern hemisphere summer (July). The fact that we are actually closer to the Sun in the winter proves that it is not our distance from the Sun that causes the seasons. When we are at perihelion, the Sun has a slightly larger apparent diameter, the pull of gravity between the Earth and Sun is slightly stronger, and our orbital velocity is greater. The opposite is true at aphelion.
The Seasons
We experience seasons on Earth because, as the Earth revolves, it's axis is tilted by 23.5º relative to it's orbit around the Sun. This tilt remains constant throughout the year (axial parallelism). The result is that during some times of the year, the northern hemisphere is tilted towards the Sun, while at other times, the northern hemisphere is tilted away from the Sun. When the northern hemisphere is tilted towards the Sun, areas will experience more direct (higher angle) sunlight which is more intense and therefore hotter. The opposite is true when the northern hemisphere is tilted away from the Sun. The only place on Earth where the Sun will be directly overhead (at the zenith) are between the Topic of Cancer (23.5º North latitude) and the Tropic of Capricorn (23.5º South latitude). See a detailed chart of key changes that take place with each season.