Solstices & Equinoxes
The astronomical seasons are defined by four precise moments in Earth's orbit when the Sun reaches key positions relative to the celestial equator. Each is an exact instant in time, not a date range, and marks the beginning of a new season in the standard astronomical calendar.
Spring Equinox
Around March 20–21. The Sun crosses the celestial equator moving northward, and the ecliptic longitude of the Sun is 0°. Day and night are approximately equal in length worldwide. Also called the Vernal Equinox, it marks the start of spring in the Northern Hemisphere and autumn in the Southern.
Summer Solstice
Around June 20–21 in the Northern Hemisphere. The Sun reaches its northernmost declination (+23.5°) and appears to "stand still" before reversing direction. The longest day and shortest night of the year in the Northern Hemisphere occur at this moment.
Autumn Equinox
Around September 22–23. The Sun again crosses the celestial equator, this time moving southward. Day and night return to near-equal length globally. Marks the start of autumn in the Northern Hemisphere and spring in the Southern.
Winter Solstice
Around December 21–22 in the Northern Hemisphere. The Sun reaches its southernmost declination (−23.5°), producing the shortest day and longest night of the year. After this point, days begin to lengthen again toward spring.
Season Duration
The number of days between one solstice or equinox and the next. The four seasons are not equal in length due to Earth's elliptical orbit: Northern Hemisphere summer is the longest (~93 days) because Earth is near aphelion and moves more slowly; winter is the shortest (~89 days) when Earth is near perihelion.