To those of us who live on Earth, the most important astronomical object by far is the sun. It provides light and warmth. Its motions through our sky cause day and night, the passage of the seasons, and Earth's varied climates. But what causes the seasons to change? In this activity, you will learn why we have different seasons.

Have you ever heard the word solstice? The word basically means that the sun stops doing something. In the Northern Hemisphere (this is all of the area north of the equator), the summer solstice happens on or around June 21st. This is the longest day of the year in terms of the amount of sunlight we get, and it marks the start of summer. The sun stops getting higher in the sky. The winter solstice happens on or around December 21st. This is the shortest day of the year in terms of sunlight, and it marks the beginning of winter. The sun stops getting lower in the sky.

So now you know how summer and winter are determined, but what about spring or fall? Have you ever heard the word eqinox? Equinox means that there is an equal amount of day and night. In fact, the word equinox is Latin for "equal nights." This is when the sun is directly above the equator. This happens on or around March 21st. This is known as the start of spring in the Northern Hemisphere. It happens again on or around September 22nd. This is known as the start of fall in the Northern Hemisphere.

In the Southern Hemisphere (this is all of the area south of the equator), all of the seasons are the opposite of what they are in the Northern Hemisphere. For example, when it is winter here, it is summer there; when it is fall here, it is spring there; and so on.

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So what actually causes the seasons? Think of a spinning top. The Earth is like the top as it rotates and orbits the sun, except it is not straight up and down. The Earth is tilted at an angle of 23.5 degrees.

The seasons are caused by the tilt of the Earth's rotational axis away or toward the sun as it travels through its year-long path around the sun. The Earth has a tilt of 23.5 degrees relative to the "ecliptic plane" (the imaginary surface formed by its almost-circular path around the sun). The tilt toward the sun is maximized during the Northern Hemisphere summer in late June (the summer solstice). At this time, the amount of sunlight reaching the Northern Hemisphere is at a maximum. In late December, on the date of the winter solstice, the tilt away from the sun is maximized, leading to a minimum of sunlight reaching the Northern Hemisphere. The seasons, of course, are reversed in the Southern Hemisphere.