What Causes the Earth's Seasons?

The four seasons that are experienced on earth are caused by an interaction between the earth's tilt and the angle of sunlight that reaches the earth. To break this down it can be simply explained by saying the earth spins on an axis around the sun. In addition, the earth is also tilted at various degrees in relation to the sun. At certain times of the year the earth is tilted closer to the sun, which results in a higher portion of sunlight reaching the earth's surface. When this happens the northern hemisphere (which is where we live) is experiencing Summer! This means we have longer days and shorter nights. The sun is higher in the sky and more direct light reaches the earth. The opposite would be when the earth is tilted away from the sun. Then less sunlight would reach the earth, and this light is more spread out and not as direct. This would mean we are experiencing Winter! This means we have shorter days and longer nights. In between the two we have Spring and Fall. This is the time when the earth's tilt is starting to shift to one end of the extreme. In Spring earth's tilt is tracking back toward the sun, and in Fall the earth's tilt is starting to tilt away from the sun.

Cool Facts: The earth, which orbits around the sun once each year is tilted 23.5 degrees.
The sun crosses the celestial equator twice a year.
The spring equinox typically occurs around March 20-21.
The fall equinox typically occurs around September 22-23.
The summer solstice is June 21.
The winter solstice is December 21-22.

Typical Misconceptions: 1) Seasons have to do with the distance of earth from the sun. The seasons have nothing to do with the distance of earth from the sun! 2) Seasons are caused by earth's rotation on it's own axis ( the 24 hour cycle bringing night and day). Seasons are not caused by the earth's 24 hour rotation (based on which side of the earth is facing the sun)! 3) the sun rises and sets. Actually, instead the spot on earth where you are rotates to the morning!

A neat classroom activity:

Materials needed: apple, pencil, a large ball (ex: basketball), chart paper, and magic markers.

Demonstration: Place the large ball on a table. Tell students this represents the sun. Hold up the apple (stem facing upward), draw the equator around the middle of the apple with a magic marker. Tell students the apple represents the earth. Now poke the pencil through the apple from top to bottom and explain that the top where the stem of the apple is, is the area of the Northern Hemisphere. Ask students "If the earth is tilted on it's axis how should I tilt the (earth) apple?" When your apple (earth) is property tilted being sure to keep the pencil pointed in the same direction as you start to walk it (orbit) around the bigger ball (the sun). It is easier to keep the pencil facing the same wall. Do one full full orbit , then another and stop when the Northern Hemisphere is leaning toward the sun. Ask students "where is the Northern Hemisphere pointed?" Do you think the top of the apple (Northern Hemisphere) is warmer at this time of year?" If so why? Next, keep orbiting the apple until the Northern Hemisphere is pointed away from the sun (ball). Ask students "what do you think happens when the Northern Hemisphere points away from the sun?" "Is it colder?" Last have students think about the Southern Hemisphere ( the bottom part of the apple). Doing this experiment can be a way to provide a visual aid for teaching the reason for the seasons in the classroom!