Equinox

The term equinox (in Latin aequinoctium) derives from two Latin words: aequus, whose meaning is “equal,” and noctium (nox, noctis), referring to “night.” Its definition involves that of the celestial sphere (which is the infinite apparent sphere around the Earth that is imagined to have our planet as its center and that contains the sun, the moon, and all the planets and stars), the ecliptic (the plane on which the orbit of the Earth around the sun lies), and the celestial equator (the huge imaginary circle on the celestial sphere that lies in the same plane as our planet's equator): An equinox, in fact, can be explained as either of two points (equinoctial points) on the celestial sphere where an intersection between the ecliptic and the celestial equator occurs. On the other hand, it can also be defined as either of the two yearly times when the sun crosses the celestial equator. At those specific moments, the sun rises exactly east and sets exactly west, and as can be inferred from the etymology of the word underlined at the beginning, the length of the day (sunlight) and that of the night (darkness) are nearly the same, irrespective of latitude. Vernal equinox is around March 20, and it marks the beginning of spring in the Northern Hemisphere, whereas autumnal equinox is around September 22, and it is the beginning of the cold season. Just the opposite occurs in the Southern Hemisphere. From an anthropological point of view, extremely interesting are those myths and rituals that refer to the equinoxes and are related to ancient cultures all over the world.

Equinoxes are said to have been discovered in ancient times by the Milesian Anaximander, who applied the gnomon (a style fixed on a horizontal plane that projects a shadow) that allowed him to determine them together with the solstices and the inclination of the ecliptic; a few centuries later, the Grecian astronomer and geographer Hipparchus of Nicaea was credited with the discovery of the precession of the equinoxes, which he studied by comparing the results of his observations of the sky with those of his predecessors. He stated that equinoxes are not always the same as they move westward along the ecliptic if compared with the assumed fixed stars on the celestial sphere. This phenomenon is due to the slow movement [Page 1010](precession) of Earth's rotational axis caused by the gravitational forces of the sun and the moon on the equatorial bulge of our planet. Considering that precession is approximately 1° in 72 yrs. (years), the equinox line moves clockwise and completes its 360° round in 25,800 yrs. As a consequence, every year the dates of equinoxes come slightly earlier.