Astronomy of the Summer Solstice

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 kind of 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.

The Earth spins around the Sun just as the light-bulb model demonstrates, but it 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). At some points in its orbit, the northern half of the planet is titled towards the Sun. At other points, the northern half points away from the Sun and the southern half is nearer.

This "axial tilt" is responsible for the seasonal changes that affect most of the Earth. Summer is warmer because that's when your half of the Earth is inclined towards the Sun; the days are longer, since the tilt means that your part of the Earth is in shadow less of the time, 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 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 Earth's poles the daylight lasts for 24 hours on the summer solstice, since on that day one of the poles points directly at the Sun.

Ancient astronomers in many cultures were aware of the solstice, and used various means to track 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 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.