Chapter 2: Early Astronomy

Description of relations between Axial tilt (or Obliquity), rotation axis, plane of orbit, celestial equator and ecliptic. Earth is shown as viewed from the Sun; the orbit direction is counter-clockwise (to the left).

Most of the life on Earth has its behavior moderated by the passing of the seasons. From hibernating in winter to gathering and storing foodstuffs 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 200 B.C. that the source of the seasons was first understood by the geometer 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.

Illustration of the position of the Earth relative to the Sun during specific times of the year, indicating seasons.

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.

These images show the sunlight on the Earth on the equinoxes and solstices at exactly the same time of day (UT). The equinoxes have equal sunlight in the northern and southern hemispheres while the solstices have a maximum sunlight in alternate hemispheres.

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."