equinozi

Equinozi, or equinoxes, are two moments in the year when the Sun crosses the celestial equator, making the length of day and night approximately equal. The term comes from Latin aequinoctium, meaning “equal night.” At these times the Sun is on the celestial equator as seen from Earth.

There are two equinoxes each year. The vernal or spring equinox occurs around March 20–21 in the Northern Hemisphere, marking the start of astronomical spring there. The autumnal or fall equinox occurs around September 22–23 in the Northern Hemisphere, marking the start of astronomical autumn. In the Southern Hemisphere, the seasons are opposite, so the dates correspond to its autumn and spring respectively.

Astronomically, an equinox happens when the Sun’s apparent path crosses the celestial equator, and the Sun’s declination is about zero degrees. This means the Sun rises approximately due east and sets approximately due west. However, due to atmospheric refraction and the finite size of the solar disk, daylight duration on the equinox is not always exactly 12 hours in all locations.

Cultural and practical significance varies. Many cultures tie seasonal festivals or calendars to the vernal equinox, such as Nowruz and other springtime observances, while others align calendars with the solstices. In modern astronomy, equinoxes help define the boundaries of seasons and serve as reference points in ephemerides and navigation.