Seasons change according to where the Earth is in relation to the Sun.
Credit: Elenamiv | Shutterstock
This year (2012), spring starts March 20 with the vernal equinox. Here's the science behind the seasons:
If you live near the equator, then you're used to virtually no seasons (except summer, of course). If you live at the North or South Pole, then you get two very long seasons: the sun stays up all summer, then stays down all winter. At the North Pole, the sun stays down for 182 straight days during winter.
If you live in between, you get four seasons.
The Earth, which orbits around the sun once each year, is tilted 23.5 degrees. When the top of the Earth is inclined toward the sun, it's summer in the Northern Hemisphere, where the sun rises highest in the sky and the days are longest. A quarter of the way around in the orbit, fall sets in. When the top of the Earth points away from the sun it's winter in the Northern Hemisphere. Another quarter of the orbit brings spring.
If you can imaging Earth's equator being projected into the sky, that plane forms what is called the celestial equator. The sun, remember, changes position in our sky during the year; during Northern Hemisphere summer it is above this celestial equator, and for the half the year it is below the equator.
The sun crosses the celestial equator twice a year, at the spring and fall equinoxes. They occur near, though not necessarily on, the day when day and night are of equal length.
The spring equinox typically occurs around March 20-21. The fall equinox comes around Sept. 22-23.
The summer solstice (June 21) winter solstice (Dec. 21-22) occur when the sun is farthest from the celestial equator.
Seasons have nothing to do with Earth's distance from the sun, which varies slightly because our planet's orbit is not a perfect circle. And season's are not caused by Earth's rotation on its own axis, a 24-hour cycle that brings night and day (based on which side of the Earth is facing the sun). By now, if you didn't already know, perhaps you can guess that the sun never really "rises." Instead, the spot on Earth where you are rotates into the morning.