Four times a year, the season officially changes. And each change is marked by a specific point known as an equinox or a solstice. One term is used for the transition from winter to spring and summer to fall, and one is used for the switch from spring to summer and fall to winter.
Join us on a journey through the seasons as we break down the difference between equinoxes and solstices, tell you when they occur, and explain why this is different depending on what hemisphere you’re in.
What is an equinox?
An equinox is one of the two times of the year when the amount of daylight and nighttime hours are just about of equal length. The two equinoxes occur around March 20–21 and September 22–23, marking the onset of both spring and autumn.
During the equinox, the sun crosses the plane of Earth’s equator, making nighttime and daytime (roughly) equal length all over the world. In the Northern Hemisphere, the spring equinox, or vernal equinox, occurs around March 21, when the sun moves northward across the celestial equator. The autumnal equinox occurs around September 22 or 23, when the sun crosses the celestial equator going south. In the Southern Hemisphere, it’s the reverse. For this reason, the terms March equinox and September equinox are sometimes used to avoid a Northern Hemisphere bias.
Contrary to popular usage, an equinox doesn’t last for a full 24 hours. Technically speaking, an equinox is one of the two specific moments in time when the sun is exactly above the celestial equator. The word equinox can also refer to the position of the sun at this moment.
Where does the word equinox come from?
The word equinox comes from the Latin aequinoctium, meaning “the time of equal days and nights,” from equi-, meaning “equal,” and nocti-, meaning “night.”
What is a solstice?
A solstice is one of the two times of the year when the positioning and tilt of Earth relative to the sun results in the most amount of daylight time or the least amount of daylight time in a single day.
Technically speaking, a solstice is one of the two the exact moments in the year when the sun reaches its northernmost point (around June 21, when the North Pole tilts closest to the sun) or southernmost point (around December 22, during the winter solstice) from Earth’s equator.
The solstices are traditionally considered to mark the start of summer and winter. But which season begins with each solstice depends on which hemisphere you’re in. In the Northern Hemisphere, the summer solstice occurs in June and the winter solstice occurs in December. In the Southern Hemisphere, it’s the opposite.
The summer solstice results in the longest day of the year (sometimes called midsummer), meaning it has the most time of daylight, and the winter solstice results in the shortest day of the year, meaning it has the longest period of darkness.
The winter solstice corresponds with some major cultural observances, including Christmas. Even in ancient times, Germanic peoples celebrated a 12-day festival to mark the winter solstice. Also known as yule, this festive period is considered one of the oldest celebrations of wintertime in the world.
In the days of the Incan Empire, the winter solstice (in June in the Southern Hemisphere) was honored with Inti Raymi, or Festival of the Sun. It involved a ceremony in which an Inca priest would “tie” the sun to a column stone in a symbolic effort to keep it from escaping.
Where does the word solstice come from?
The word solstice ultimately derives from the Latin sōlstitium, which comes from the parts sōl, “sun,” and sistere, “to stand still.” This means that sōlstitium literally translates to something like “the standing still of the sun.”
What is the difference between an equinox and a solstice?
When the equinox occurs, as the equi- prefix might suggest, day and night are of equal length, and it marks the onset of spring (vernal equinox) and autumn (autumnal equinox).
The solstice, meanwhile, marks the beginnings of summer and winter, and it’s either the longest day of the year (summer solstice) or the shortest (winter solstice).
Remember, sol means sun, and solstices are the days with the most or the least amount of daylight.