For thousands of years, humans have marked specific times of year as especially important, whether for religious reasons, agricultural reasons, or simply out of curiosity. Ancient observers of the sky noticed a yearly cycle of very long, warm days to...

The time of year when the sun is directly overhead at the equator. These events happen twice per year at around 21 March (the VERNAL equinox) and 23 September (the autumnal equinox). See also:...

Illumination of the Earth by the Sun on the day of equinox, (ignoring twilight). The Earth in its orbit around the Sun causes the Sun to appear on the celestial sphere moving over the ecliptic (red), which is tilted on the equator (blue). Diagram of...