Scientists Say: Equinox and Solstice

Equinoxes and solstices mark the start of Earth’s seasons

Hundreds of millions of people in Central Asia, the Middle East and elsewhere around the world celebrate the new year, or Nowruz, on the spring equinox. Here, a table is set up with symbolic Nowruz items, such as an apple, garlic and coins.

Germán Vogel/Getty Images

By Maria Temming

March 20, 2023 at 6:30 am

Equinox (noun, “EEK-win-ox”) and Solstice (noun, “SOUL-stiss”)

An equinox is a time of year when the amount of daytime and nighttime hours per day are about equal. On Earth, we experience two equinoxes each year. One equinox happens around March 20 or 21. It marks the start of spring in the northern hemisphere. And it marks the start of fall in the southern hemisphere. The other equinox falls around September 22 or 23. It marks the start of autumn in the northern hemisphere. And it marks the start of spring in the southern hemisphere.

The solstices are the two times a year with the most or least amount of daylight per day. One solstice happens around June 21. It marks the start of summer in the northern hemisphere. And it marks the start of winter in the southern hemisphere. The other solstice happens around December 21 or 22. It marks the start of winter in the northern hemisphere. And it marks the start of summer in the southern hemisphere.

Earth has equinoxes and solstices for the same reason it has different seasons. Earth is tilted relative to the sun. So, over the course of a year, the northern and southern hemispheres take turns facing the sun more directly. The two equinoxes and two solstices each year mark the start of the four seasons.

an illustration shows earth at four different points in its orbit around the sun, with earth's tilt constant, so that at two points earth's north pole is tilted toward or away from the sun, and at two points the north pole is neither tilted toward nor away from the sun — The solstices and equinoxes mark points throughout the year when Earth’s northern and southern hemispheres are pointed more toward or away from the sun. These changes affect the number of hours per day each hemisphere spends in sunlight. eliflamra/Getty Images

Let’s look at the northern hemisphere. At the June solstice, the Earth’s northern hemisphere is most directly facing the sun. So, this hemisphere spends a maximum number of hours per day bathed in very direct sunlight. The result is long, warm summer days. At the December solstice, the northern hemisphere is tilted away from the sun. So, that hemisphere gets less direct sunlight and spends more hours per day in darkness. This results in long, cold winter nights. At the equinoxes, the northern hemisphere is not pointed toward or away from the sun. The result is medium amounts of daylight and mild spring and autumn temps.