Equinoxes

The equinoxes are the two specific times of the year when the sun crosses the celestial equator. On these days, both the Northern and Southern Hemispheres of Earth receive approximately equal amounts of daylight and darkness. The term equinox comes from the Latin words aequus (equal) and nox (night). The vernal, or spring, equinox occurs around March 20 and marks the beginning of spring in the Northern Hemisphere. The autumnal, or fall, equinox occurs around September 22 and signifies the start of autumn. In the Southern Hemisphere, the equinoxes mark the beginning of seasons opposite those in the north, with fall beginning in March and spring in September.

Background

Earth moves around the sun at a speed of about 67,000 miles per hour (108,826 kilometers per hour). From the surface, however, humans do not perceive this speed. To us, the sun appears to be moving through the sky while Earth remains stationary. From our perspective, the sun seems to trace a path along the background of star patterns, or constellations. This path through the sky is called the ecliptic. Earth makes one complete journey around the sun in about 365 and a quarter days. That is also the time it takes for the sun to make one trip through the ecliptic.

Ancient astronomers believed that the sky was an unmoving great celestial sphere. They projected the geographical points of Earth up onto this celestial sphere. For example, they designated a celestial north pole to correspond to the direction of Earth’s North Pole. They did the same with the South Pole. They also projected Earth’s equator up onto the celestial sphere. Like the equator on Earth, the celestial equator divides the celestial sphere in half. Modern astronomers have long known the true nature of the sky and Earth’s place in the solar system, but they continue to use the concept of the celestial sphere as a reference point when referring to the movement of the sun and other celestial bodies.

Overview

Earth orbits around the sun, and its axis—an imaginary line about which the planet rotates—is tilted at an angle of about 23.4 degrees. The direction of Earth’s tilt does not change over the course of a year; however, the position of the planet does change as it moves around the sun. This means that parts of the planet receive more or less daylight at certain times of the year. During the summer months, for example, the Northern Hemisphere is tilted toward the sun, so it receives more daylight. This results in later sunsets and warmer temperatures. At the same time, the Southern Hemisphere is tilted away from the sun, so it receives less daylight. During winter, the Northern Hemisphere tilts away from the sun, leading to less daylight, earlier sunsets, and colder temperatures. Winter in the Northern Hemisphere corresponds to summer in the Southern Hemisphere.

Twice a year, during the equinoxes, Earth reaches a point in its orbit where it does not tilt toward or away from the sun. On these days, the sun is directly overhead at noon at the planet’s equator. Both the Northern and Southern Hemispheres receive about the same amount of daylight. Because of the way Earth’s atmosphere bends sunlight as it passes through the air, some places on the planet may get a few minutes more daylight on the equinoxes than others.

From the perspective of someone standing on Earth, the sun appears to be higher in the sky during the summer months and lower during the winter. At the time of the equinoxes, the sun’s journey through the ecliptic crosses over the celestial equator. During the vernal equinox in March, the sun is moving in a northward direction; during September’s autumnal equinox, the sun is headed south.

The equinoxes occur at the specific time the sun intersects with the celestial equator. Typically, the date of the vernal equinox is March 20, although it can occur as early as March 19 or as late as March 21. The autumnal equinox occurs most often on September 22 or 23. On rare occasions, it can occur on September 21 or 24. The reason for the shifting dates is that the standard calendar system and the time it takes for Earth to make a complete orbit around the sun do not match up exactly. A standard calendar year is 365 days, while Earth takes about 365 days and six hours to orbit the sun. As a result, the time of the equinoxes shifts by about six hours a year. An extra “leap” day added to February every four years helps correct the issue so calendars do not get out of synch.

The equinoxes were important to many ancient humans, who used the heavens as a guide to let them know when to plant and harvest their crops. Spring was commonly seen as a time of planting and rebirth, while autumn was a time of harvest. Many cultures developed rituals and celebrations to mark those times of the year. Some of these celebrations are still held today or have evolved into modern holidays. For example, Iranians celebrate Nowruz, their new year, at the time of the spring equinox. Some Native American peoples hold rituals at the spring equinox to welcome life back to the earth. The vernal equinox is also used in conjunction with the phases of the moon as a benchmark in determining the dates of Christian Easter and Jewish Passover.

In Japan, both the spring and fall equinoxes are national holidays corresponding with Higan, a weeklong Buddhist celebration of the transition of the spirits to the afterlife. Many Chinese and Vietnamese people celebrate the Mid-Autumn Festival, or Moon Festival, near the time of the autumnal equinox. This festival is a celebration of the harvest and is noted for its mooncakes, a pastry filled with lotus seeds, sesame seeds, egg yolks, or dried fruit.

Bibliography

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