The seasons on Earth are caused by the 23.5° tilt of the axis of rotation. The summer begins on or about June 21, when the Sun is directly overhead at local noon on the Tropic of Cancer (23.5°N latitude). This is the
Summer Solstice. Solstice means “Sun stands still” in Latin. All points above the Arctic Circle (66.5°N latitude) have 24 hours of sunlight; above the Antarctic Circle (66.5°S) all points have 24 hours of darkness. In the United States, the Sun rises north of due east and sets north of due west. As it reaches the noon position, it is in the southern part of the sky. For an observer in the United States, the Sun is never directly overhead. The opposite occurs at about December 21, the first day of winter (
Winter Solstice). The Sun is directly overhead at the Tropic of Capricorn (23.5° S latitude). The North Pole experiences total darkness, whereas the South Pole is in total light. For an observer in the United States, the Sun rises south of due east and sets south of due west.
The equinoxes (“equal night”) fall on or about March 21
(Vernal Equinox) and September 23
(Autumnal Equinox). The Sun is directly overhead at the Equator and the entire Earth has 12 hours of day and night. The Sun rises directly in the east and sets directly in the west. Contrary to a popular myth, you can balance an egg on its end on any day of the year, not just when the day and night are in balance. It is just a function of gravity. Figure
1 shows the positions for each season.
Figure 1
The positions of the Earth at the start of each season.
Insolation and Seasons on Earth
Weather
