The tilt of the Earth and its impact on sun angle is the reason the Northern and Southern Hemisphere have opposite seasons. Summer occurs when a hemisphere is tipped toward the Sun and winter when it is tipped away from the Sun. The orientation of the Earth with respect to the Sun also determines the length of day. Together, the sun angle and day length determine the total amount of solar radiation incident at the Earth. To illuminate this point (pardon the pun), let's follow the Earth as it progresses through its orbit around the Sun. Watch the video below to help visualize the concepts below.
Video: How Earth's tilt Causes Seasons: How Being Earth's Tilt Causes Seasons
On about June 21st the Northern hemisphere is tipped toward the sun as shown in Figure 2.12 At noon, the subsolar point, or place where the sun lies directly overhead at noon, is located at 23 1/2o north latitude. This date is known as the summer solstice, the longest day of the year for places located north of Tropic of Cancer. The 23 1/2o parallel was so named because it is during the astrological sign Cancer when the Sun's rays strike at their highest angle of the year north of this line. The North pole tips into the Sun and tangent rays strike at the Arctic and Antarctic Circles. (A tangent ray is one that meets a curve or surface in a single point). This creates a 24 hour period of daylight ("polar day") for places located poleward of 66 1/2o north. We find the South Pole tipped away from the Sun, sending places poleward of 66 1/2o south into 24 hours of darkness ("polar night").
On Sept 23rd, the Earth has moved around the Sun such that the poles are neither pointing toward or away from the sun. On this day, the Sun is directly overhead 0 degrees, the equator, at noon. Tangent rays strike at the poles. It is the autumnal equinox and all places experience 12 hours of day light and 12 hours of darkness.
The winter solstice occurs on December 22nd when the Earth has oriented itself so the North Pole is facing away from, and the South Pole into the Sun. Again, tangent rays strike at the Arctic and Antarctic circles. Places poleward of 66 1/2o north are in the grips of the cold, polar night. Places poleward of 66 1/2o south experience the 24 hour polar day. The Sun lies directly over 23 1/2o south. Occurring during the astrological sign of Capricorn, 23 1/2o south latitude is called the Tropic of Capricorn.
Continuing to March 21st (i.e. spring equinox) the Earth has positioned itself similar to that which occurs in September, only on the other side of the Sun. Once again tangent rays strike at the North and South poles, and the perpendicular rays of the Sun strike the Equator at noon. All places have equal day length (12 hours day;12 hours of night) as the circle of illumination cuts all latitudes in half.
So over the course of a year, the Sun's rays are only perpendicular to the surface (directly over head) at places between 23 1/2o north and south. Places between the Tropic of Cancer and Capricorn experience two times when the Sun is directly over head over the course of a year. The sun angle does not vary much for places between 23 1/2o north and south, a larger range in sun angle occurs poleward of these latitudes. The greater the variation in sun angle, the greater the variation in surface heating.