by Wendy Connick

Created on: June 05, 2008   Last Updated: October 11, 2010

In elementary school, your teacher probably demonstrated the Solar System by showing you a model with a light bulb in the middle and several balls on wires circling around the bulb. This model is a good way to show how planets orbit the Sun, but it doesn't explain the different seasons that we experience here on Earth.

In addition to spinning around the Sun as the light-bulb model demonstrates, the Earth is also tilted on its axis by about 23.5 degrees (the axis is an imaginary line running through the center of the Earth from the North Pole to the South Pole). From mid March through mid September (in other words, between the spring and fall equinox) the Northern Hemisphere of the planet is titled towards the Sun. During the other half of the year, the Southern Hemisphere tilts towards the Sun instead.

This "axial tilt" is responsible for the seasonal changes that affect most of the Earth. March through September is the warmest season in the Northern Hemisphere because that's when the northern half of the Earth is inclined towards the Sun; the days are longer, since the tilt means that this half of the Earth is in direct sunlight for longer periods, and the increased sunlight leads to increased heat. The opposite happens in winter, when your half of the Earth is inclined away from the Sun and spends more time shadowed from the Sun. The only part of the Earth that has little seasonal temperature change is near the Equator, where axial tilt has no effect.

The summer solstice is the day when the Earth tilts most directly towards the Sun. In the Northern Hemisphere, this happens in mid-June, when the northern half of the Earth leans towards the sun; in the Southern Hemisphere the summer solstice happens in mid-December, when the southern half of the Earth inclines sunward. Whenever it occurs, the summer solstice is the longest day of the year. In fact, near the polar region the daylight lasts for 24 hours on the summer solstice, and there is no daylight at all during the winter solstice.

Ancient astronomers in many cultures were aware of the solstice, and used various means to keep track of it. Stonehenge, for example, was designed so that on the summer solstice the Sun rises directly over the Heel Stone and shines into the center of the monument through a horseshoe of stones. The Sun Dagger, located at the top of Fajada Butte in the southwestern US, is also designed to mark the solstice. Created by the Anasazi Native American tribe, it is a spiral-shaped drawing in the rock placed so that on the summer solstice a dagger of sunlight pierces the exact center of the spiral.

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