What Exactly is the Equinox, and Why Does it Happen?

We experience equinox twice in a year or every six months. During these times, the Earth experiences roughly the same amount of day and night – about 12 hours of daylight and 12 hours of nighttime. For the year 2019, the autumnal equinox will be experienced on September 23 while the vernal equinox happened last March 20th.

The vernal equinox, or sometimes called the spring equinox, marks the beginning of the summer season in the northern hemisphere while the autumnal or fall equinox ushers in the winter for the north and summer for the southern hemisphere.

Why does an equinox happen? The Earth normally has an axis tilted at 23.5 degrees as it orbits the sun. During equinoxes, the Earth’s axial tilt and its orbit merge, while the sun sits directly above the equator of the Earth. The sun illuminates the Earth’s surface at a right angle, therefore, casting a terminator or twilight zone which divides the planet with an invisible line that passes through the North and South poles.

Although the equinox is widely understood to mean “equal night” (derived from the Latin words aequus meaning “equal” and nox meaning “night”), that does not mean that our planet experiences exactly twelve hours of night and twelve hours of day. Earth isn’t perfectly divided by the terminator due to atmospheric refraction which means that the Earth’s atmosphere causes sunlight to bend by 60 kilometers or equivalent to 0.5 degrees. It is interesting to note that other planets on the solar system experience equinoxes as well. The Cassini probe has documented an equinox on Saturn in the year 2009. Compared to Earth which experiences an equinox every six months, Saturn experiences it every 15 Earth years.

In ancient times, different cultures made use of diverse methods to track the equinoxes. All of which has led to the accurate reading of seasons and the passing of the sun. Several examples include architectures like pyramids, stone engravings, and monuments. Churches even incorporated sun trackers into its architecture. Several cultures in the past have also celebrated the days when the equinoxes occur. In Mexico, the ancient Mayans had sacrificial rituals by the main pyramid (El Castillo) when the spring equinox came.

The Persian calendar has the equinoctial New Year called the Nowruz which marks the first day of the year. Easter and Passover are celebrated around the March equinox as well. Even at present, there are cultures where the equinox is honored. The Lakota Tribe from the US Midwest holds a ceremony wherein they smoke sacred tobacco. The ceremony marks the return of longer days. Meanwhile, in England, people gather to watch the sunrise over the Stonehenge.