Solstice and Equinox
The solstices and equinoxes are four key astronomical events that structure the Earth’s year and mark the seasonal transitions. They result directly from the tilt of the Earth’s axis by 23.5 degrees relative to the plane of the Earth’s orbit, known as the ecliptic.
The solstices
A solstice occurs twice a year, when the Earth’s axial tilt reaches its maximum relative to the Sun. The summer solstice (around June 21 in the Northern Hemisphere) marks the longest day and the shortest night. The Sun reaches its highest point in the sky and remains above the horizon longer than on any other day. The winter solstice (around December 21) is the opposite: the shortest day, the Sun at its lowest point. In the Southern Hemisphere, the seasons are reversed.
The equinoxes
An equinox also occurs twice a year, when the Sun crosses the celestial equator. The word comes from the Latin aequus nox, meaning “equal night”—on this day, day and night are approximately the same length throughout the world. The spring equinox (around March 20) marks the beginning of astronomical spring in the Northern Hemisphere. The autumnal equinox (around September 22) marks the beginning of fall.
Why Do We Have Seasons?
The seasons are not caused by the distance between the Earth and the Sun—this distance varies by only 3% over the course of the year. They result from the axial tilt, which changes the angle at which the Sun’s rays strike the Earth and the duration of daylight. During the northern summer solstice, the northern hemisphere is tilted toward the Sun: the rays strike more directly, and the day is longer. Six months later, the opposite is true.
Significance in Astronomy
Solstices and equinoxes have been observed and marked by all ancient civilizations. Stonehenge, the pyramids of Giza, the Templo Mayor, and Chichén Itzá are aligned with these astronomical events. These dates still structure our civil and religious calendars today. The spring equinox also serves as a reference point for the equatorial coordinate system in astronomy: the vernal point determines the origin of right ascension.
Concrete example
At the northern summer solstice, the Sun reaches its zenith exactly above the Tropic of Cancer (23.5° north). At the winter solstice, it is at the zenith above the Tropic of Capricorn (23.5° south). At the equinoxes, it is at the zenith above the equator. These tropics take their names directly from these astronomical events.
Did you know?
Beyond the polar circles (66.5° latitude), the Sun does not set for several days around the summer solstice (midnight sun) and does not rise for several days around the winter solstice (polar night). At the pole itself, the Sun rises only once a year—at the spring equinox—and remains visible for six months.
Frequently Asked Questions
Are the dates of the solstices and equinoxes fixed?
No, they vary by one to two daysone year to the next due to the discrepancy between the tropical year (365.2422 days) and our 365-day calendar. Leap years partially compensate for this discrepancy.
Is the day really equal to the night during an equinox?
Not quite. Atmospheric refraction makes the Sun appear above thehorizon even when it is geometrically below it, and the Sun is not a point but a disk. As a result, the day is slightly longer than the night at the equinoxes. Exact equality (equilux) occurs a few days before or after.
Do all planets have solstices and equinoxes?
All planets with a tilted axis have seasons. Mars, with a tilt of 25 degrees, has seasons similar to those on Earth but twice as long. Uranus, with an extreme tilt of 98 degrees, experiences seasons lasting 21 Earth years, during which its poles are either in full sunlight or in total darkness.