Reading: "The Seasons" by Chris Impey

Most of the life on Earth has its behavior moderated by the passing of the seasons. From hibernating in winter, to gathering and storing food stuffs through summer and fall, animal survival requires life to have some sort of an understanding of the rhythmic cycling of hot and cold temperatures. While the seasons have long been known, it was only around 200BC that the source of the seasons was first understood by the geometrist Eratosthenes. Having noted that the sun is directly overhead at the equator on the equinox, he used measurements of the position of the sun at noon in the winter and summer to realize that the is tilted 23.5° with respect to the direction of the Sun at summer and winter solstice.

Working when he did, Eratosthenes would have been trying to understand the Earth's tilt in the context of an earth-centered cosmology. In this scenario, the Sun travels on a tilted path about the Earth. A more elegant (and correct) explanation for the tilt comes from heliocentric solar system models.

The same data can also be explained with a tilted Earth orbiting in on a mostly flat orbit about the Sun. Today we know the Earth orbits the Sun with its equator tilted by 23.5° with respect to the plane of the Earth-Sun orbit (the path we see as the ecliptic in the sky). The Earth moves around the Sun and always keeps its north polar axis tipped and pointed at the North Star. When the Earth is on one side of the Sun - the side opposite the Sun from Polaris - the tilt of the Earth's northern hemisphere is toward the Sun. Someone in the northern hemisphere will thus see the Sun will ride high in the sky and the sunlight shining nearly straight down at midday, giving a strong heating effect. This is summer in the north, while the southern hemisphere will have winter. Six months later, when the Earth is on the other side of the Sun, the tilt of the northern hemisphere is away from the Sun. A northern observer will see the Sun cross low in the south at midday. This is winter in the north, while the southern hemisphere has more direct sunlight and is in summer.

There is a second consequence of the Earth's tilted axis. When the northern hemisphere is tipped toward the Sun, northern latitudes receive more than 12 hours of sunlight each day and southern latitudes receive less than 12 hours. When the northern hemisphere is tipped away from the Sun, northern latitudes receive less than 12 hours sunlight each day and southern latitudes receive more than 12 hours. Spring and autumn equinox are times of equal day and night everywhere. Regions between 23.5° N and 23.5° S latitude can view the Sun directly overhead. These are the Tropics. This tilt also has the effect of causing regions north of 67.5° N and south of 67.5° S latitudes to experience continuous darkness or continuous light during their respective winters and summers. These polar regions are sometimes referred to as the "land of the midnight sun."

Earth stays at nearly the same distance from the Sun throughout its orbit. In fact, the Earth is slightly closer to the Sun during winter in the northern hemisphere — proof that our seasons are not caused by the change in solar distance. The seasons are caused by the tilt of the planet’s axis, and if the axis weren't tilted, there would be no seasons. Imagine the Earth orbiting the Sun spinning with its axis straight up and down. Every part of the Earth would get 12 hours of day and 12 hours of night all year around and the Sun would reach the same elevation in the sky all year around. This particular alignment would create a boring, if consistent weather pattern. The tilt provides the seasons and our lives and our climate more interesting!