What is an Equinox?

What is an Equinox?

Generally we think that the days are longer than nights in summer and shorter in winter. However, this is not entirely true during an astronomical event called the Equinox, but what’s that?

An Equinox is a point in the year when the daytime and nighttime are of equal lengths, which is 12 hours each. This situation occurs twice in a year, usually on March 21th and September 23rd, when the sun is exactly above the Earth’s equator, an imaginary line around the middle of a planet or a celestial body.

This causes the terminator or Twilight zone, a moving line that divides the day lit side and the dark night side of a planetary body to pass through the Earth’s north and south poles. This unique angle causes all the areas of the Earth and other planets to experience almost the same amount of daylight and darkness twice an every year.

There are two types of Equinoxes that occur, in the Northern Hemisphere, the March Equinox is called the vernal equinox, while in the September, it is called the Autumnal Equinox.  In the Southern Hemisphere, the names are opposite because the seasons are switched. Example: autumn and the autumnal equinox occur in the Southern Hemisphere in March, when it is spring in the Northern Hemisphere.

What is the significance of this event? Like the Solstices equinoxes are historical markers of seasonal change. The March equinox unofficially marks the spring season and is traditional perceived as a rebirth and renewal time. For this reason, many cultures have celebrated the March Equinox as the first day of the New Year. Comparatively, the September equinox marks the fewer events generally associated with autumnal harvest festivals. Chuseok, the huge Korean holiday around the September equinox is a time for family gathering and gift-giving. In addition to traditional autumn foods such as apples and nuts, one of the most popular Chuseok gifts are boxed sets of Spam that is a favorite food of the Koreans.

It is said that the ancient Mayan architects may have honored the Equinoxes in the design of the enormous stone pyramid named El Castillo at Chichen-Itza, Mexico. Every equinox, the light of the sun casts shadows down the pyramid’s steps making look like a slithering serpent. “El Castillo” is a temple honoring the serpent god Kukulcan. But nobody knows if it was planned or just a happy accident.
Did you know, the terminator doesn’t perfectly divide the planet into dark and light? The Earth’s atmosphere bends sunlight by 37 miles, which equals half a degree which means one half of the planet is still a little more lit than the other even on equinox.

And other interesting thing about equinoxes is that it’s just not limited to Earth. Every planet in our solar system experiences it. In 2009, the Cassini probe in orbit around Saturn captured an equinox on the Earth on the ringed planet. As on Earth, equinoxes occur every half-year on Saturn but that equals 15 years on the Earth. Making Cassini’s photo session a unique event.