A common misconception is that the Sun is closer to Earth in the summer and farther away from it during the winter. Instead, the seasons are caused by the 23.5 o tilt of Earth’s axis of rotation relative to its plane of orbit around the Sun ( Figure below ). Solstice refers to the position of the Sun when it is closest to one of the poles. At summer solstice, June 21 or 22, Earth’s axis points toward the Sun and so the Sun is directly overhead at its furthest north point of the year, the Tropic of Cancer (23.5 o N).
The Earth’s tilt on its axis leads to one hemisphere facing the Sun more than the other hemisphere and gives rise to seasons.
During the summer, areas north of the Equator experience longer days and shorter nights. In the Southern Hemisphere, the Sun is as far away as it will be and so it is their winter. Locations will have longer nights and shorter days. The opposite occurs on winter solstice, which begins on December 21. More about seasons can be found in the Atmospheric Processes chapter.
Solar Radiation on Earth
Different parts of the Earth receive different amounts of solar radiation. Which part of the planet receives the most solar radiation? The Sun's rays strike the surface most directly at the Equator.
Different areas also receive different amounts of sunlight in different seasons. What causes the seasons? The seasons are caused by the direction Earth's axis is pointing relative to the Sun.
The Earth revolves around the Sun once each year and spins on its axis of rotation once each day. This axis of rotation is tilted 23.5 o relative to its plane of orbit around the Sun. The axis of rotation is pointed toward Polaris, the North Star. As the Earth orbits the Sun, the tilt of Earth's axis stays lined up with the North Star.
Northern Hemisphere Summer
The North Pole is tilted towards the Sun and the Sun's rays strike the Northern Hemisphere more directly in summer ( Figure below ). At the summer solstice, June 21 or 22, the Sun's rays hit the Earth most directly along the Tropic of Cancer (23.5 o N); that is, the angle of incidence of the Sun’s rays there is zero (the angle of incidence is the deviation in the angle of an incoming ray from straight on). When it is summer solstice in the Northern Hemisphere, it is winter solstice in the Southern Hemisphere.
Summer solstice in the Northern Hemisphere.
Northern Hemisphere Winter
Winter solstice for the Northern Hemisphere happens on December 21 or 22. The tilt of Earth's axis points away from the Sun ( Figure below ). Light from the Sun is spread out over a larger area, so that area isn't heated as much. With fewer daylight hours in winter, there is also less time for the Sun to warm the area. When it is winter in the Northern Hemisphere, it is summer in the Southern Hemisphere.
In Southern Hemisphere summer, the Sun’s rays directly strike the Tropic of Capricorn (23.5 o S). Sunlight is spread across a large area near the South Pole. No sunlight reaches the North Pole.
An animation of the seasons from the University of Illinois is seen here: http://projects.astro.illinois.edu/data/Seasons/seasons.html . Notice the area of solar radiation, or insolation, in the lower right of the screen.
Halfway between the two solstices, the Sun's rays shine most directly at the Equator, called an equinox ( Figure below ). The daylight and nighttime hours are exactly equal on an equinox. The autumnal equinox happens on September 22 or 23 and the vernal, or spring, equinox happens March 21 or 22 in the Northern Hemisphere.
Where sunlight reaches on spring equinox, summer solstice, vernal equinox, and winter solstice. The time is 9:00 p.m. Universal Time, at Greenwich, England.