EQUINOX

An equinox is an event in which a planet’s subsolar point passes through its Equator. The equinoxes are the only time when both the Northern and Southern Hemispheres experience roughly equal amounts of daytime and nighttime.

On Earth, there are two equinoxes every year: one around March 21 and another around September 22. 

Sometimes, the equinoxes are nicknamed the “vernal equinox” (spring equinox) and the “autumnal equinox” (fall equinox), although these have different dates in the Northern and Southern Hemispheres.  The March equinox is the vernal equinox in the Northern Hemisphere, and the autumnal equinox in the Southern. The September equinox is the autumnal equinox in the Northern Hemisphere and the vernal in the Southern.

During the equinoxes, solar declination is 0°. Solar declination describes the latitude of the Earth where the sun is directly overhead at noon. (The Equator, of course, is 0° latitude.)

So, equinoxes are the only times of the year when the subsolar point is directly on the Equator. The subsolar point is an area where the sun's rays shine perpendicular to the Earth's surface—a right angle. Only during an equinox is the Earth's 23.5° axis not tilting toward or away from the sun: the perceived center of the Sun’s disk is in the same plane as the Equator. 

Before and after the equinox, the subsolar point migrates north or south. After the March equinox, the subsolar point migrates north as the Northern Hemisphere tilts toward the sun. Around June 21, the subsolar point hits the Tropic of Cancer, (23.5°N). This is the June solstice, after which the subsolar point begins to migrate south. After the September equinox, the subsolar point continues to move south as the Southern Hemisphere tilts toward the sun. Around December 21, the subsolar point hits the Tropic of Capricorn(23.5°S). This is the December solstice.

As its name suggests, an equinox indicates equally illuminated hemispheres, with the solar terminator equally dividing the Earth from north to south. (The solar terminator is the shadowed line indicating daylight and sunlight on a globe.)

A true equinox would indicate 12 hours of both day and night. Although the equinoxes are as close to this phenomenon as happens on Earth, even during the equinoxes day and night aren’t exactly equal. This is largely due to atmospheric refraction.

Atmospheric refraction describes the way light seems to bend or deviatefrom a straight line as it passes through Earth’s atmosphere. Atmospheric refraction is a result of increasing air density, which decreases the velocity of light through the air. Due to atmospheric refraction, we are able to see the sun minutes before it actually rises and sets. 

The Culture of the Equinoxes 

March Equinox

The March equinox, unofficially marking the spring season, is traditionally observed as a time of rebirth and renewal. For this reason, many cultures have celebrated the March equinox as the first day of the new year. The ancient Babylonian calendar began on the first full moon after the March equinox, and today, many cultural and religious calendars continue to celebrate new year in the spring. 

September Equinox

Fewer events mark the September equinox. Perhaps the most familiar of these are Rosh Hashanah and Yom Kippur, Judaism’s “High Holy Days.”

Rosh Hashanah is the Jewish new year, marked around the September equinox. Rosh Hashanah is calculated as 163 days after the first day of Passover (which itself is calculated by the March equinox.) The sounding of the shofar, a ram’s horn used as a trumpet for thousands of years of Jewish ritual, welcomes Rosh Hashanah.

Yom Kippur, the holiest day in the Jewish calendar, falls about 10 days after Rosh Hashanah. Yom Kippur is a solemn holiday known as the “Day of Atonement.” It is traditionally observed with a daylong fast and prayers for forgiveness.

Most cultural events associated with the September equinox are tied to autumnal harvest festivals. Chuseok, celebrated over a three-day period in the Koreas, is one of the most familiar of these folk festivals. Sometimes nicknamed “Korean Thanksgiving,” Chuseok is a celebration of family and Korea’s rich agricultural heritage.