Astronomers like to describe the equinox within the conceptual celestial sphere. Here the heavens are projected around the Earth, like an enormous planetarium. The model is bisected by the celestial equator, a projection of Earth’s own equator. The equinox occurs at the point at which the sun’s path or ecliptic crosses the celestial equator. In spring it is known as the vernal equinox, and in fall the autumnal equinox.

The other two seasonal points on the sun’s path are the two solstices; in the northern hemisphere, the summer solstice marks the longest day of the year, while the winter solstice marks the shortest. The seasonal aligning of the sun has been more than just a unique celestial event for humankind throughout history. Ancient sites like Stonehenge in England and Machu Picchu in Peru have well documented solar alignments during the solstices.

Similarly, the equinoxes have been associated with some amazing man-made phenomena. In the ancient Maya city of Chichén Itzá, the great pyramid known as El Castillo is oriented along cardinal axis. During the equinoxes, shadows cast by the railings create the illusion of a writhing serpent body, descending the northern steps where it joins the carved serpent’s head at the base of the stairway. The cultural significance of the fall equinox and changing of seasons continues today, especially in the northern hemisphere, where the autumnal equinox occurs around harvest season.

In fact, the full moon nearest the autumnal equinox is commonly referred to as the harvest moon. In China and other Asian countries, this time is celebrated with the mid-autumnal festival; the origins are linked to the birth of the moon goddess and festival traditions revolve around families with reunions and feasts and special moon cakes. In Jewish culture, thanks for the harvest is given during the week-long feast of the tabernacles or Sukkot. Families eat meals in temporary shelters outside, recalling the Israelites' days in the wilderness after the Exodus from Egypt.