Earth's seasons are determined by the angle at which the sun passes through the sky each day. During summer, the sun rises higher in the sky and more sunlight reaches the surface, but during winter, the sun passes closer to the horizon, spends less time in the sky and imparts less energy to the ground. The relative angle of the sun is determined by the tilt of the Earth.
The Earth tilts approximately 23.5 degrees from vertical relative to the plane of its orbit around the sun. Around June 21, this tilt points the Northern Hemisphere toward the sun and gives the northern temperate latitudes more daylight than they get at any other time of year.
Around Dec. 21, the reverse is true, and the Southern Hemisphere is inclined toward the sun. This is why Australia, southern Africa and much of South America enjoy summer while Europe, Asia and North America experience winter.
The dates of these extremes are known as the summer and winter solstices. Between these two extremes, the spring and fall equinoxes fall on or about March 21 and Sept. 21, respectively. On these dates, the disc of the sun crosses the plane of the equator, and both night and day last 12 hours.