The Reason for the Season
And Earth's tilt
The Earth:
The Earth is on a tilt due to it's axis. The Earth rotates the Sun, taking 365 and one fourth days, (each day taking approximately 23 hours, 56 minutes, and 4 seconds). The tilt is how the temp. changes through out the year.
The Tilt
The Earth tilts at a 23.5 degree lean. As it spins on its axis, the light hits it causing day, and lack of light causing night. The tilt remain's the same through the year, but as the Earth rotates as well, the Earth tilts towards and away from the sun depending on the time of the year.
Light Distribution
The light hits the Earth at a constant rate, but because our Earth tilts, the light can be indirect, or direct. An example of direct light is when the Earth's Northern hemisphere tilts towards the sun, and light hits more directly then on the southern hemisphere where it points away, which is called indirect light.
Seasons
The Earth's receiving of indirect, and direct sunlight is what causes the season. At the mos direct light, you are experiencing Summer, which is hotter, and has longer days. Indirect light gives you winter, shorter days, and colder weather. Between the two polar opposites, are Spring and Fall which receive less extents of the direct light (spring) and indirect (fall.)
Equinox & Solstice
In Winter and in Summer, you experience solstices. A Solstice is when the light is either at its most direct, or least direct point. In Summer it happens on June 21st, the longest day of the year, while winter has it on December 21st, the shortest day of the year. Spring and Fall both have equinoxes, where the light is equal, so the days and nights are both 12 hours even. Spring experiences its equinox on March 21st, while Fall's is on September 21st.