The equinoxes are astronomically defined as either of the two moments in the year when the Sun is exactly above the equator (i.e. when the plane of the Earth’s equator passes the center of the Sun).
The Vernal Equinox, marking the beginning of spring in the Northern Hemisphere, occurs about March 21,when the Sun moves north across the celestial equator.
The Autumnal Equinox falls about September 23, as the Sun crosses the celestial equator going south.
- The word “equinox” derives from the concept of equal day/night.
- However, pre-technological cultures had no way of timing the exact length of the day and night, nor knowing the exact moment when the ecliptic and celestial equator intersected, nor did they know about refraction from the Earth’s atmosphere.
- They were quite capable of knowing on which day of the year the Sun rose exactly east and set exactly west. Times of sunset and sunrise vary with an observer’s location (longitude and latitude), so the dates when the Sun rises due east and sets due west likewise depend on location.
- Depending on location, the dates of Sun rising and setting due east and west can be as much as 2-3 days from the officially designated equinox.