Low angle view of hybrid tulips in the Spring, equinox
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Introduction

The vernal (or spring) equinox takes place around March 20 or March 21 and signals the start of spring in the Northern Hemisphere. During the equinox, the amount of daylight and darkness is nearly the same in length. The word equinox comes from the Latin “aequus,” meaning equal, and “nox,” meaning night. The Earth tilts at an angle of 23.5 degrees on its axis relative to its plane of orbit around the sun. As the Earth orbits the sun over the course of a year, different places get sunlight for different amounts of time. An equinox occurs at the moment when the Earth’s axis doesn’t tilt toward or away from the sun. Someone standing on the equator on an equinox can observe the sun passing directly overhead. Additionally, equinoxes are the only two times a year that the sun rises due east and sets due west.

The vernal equinox takes place around March 20 or March 21 and signals the start of beginning in the Northern Hemisphere and fall in the Southern Hemisphere. (The Northern Hemisphere is the half of the planet that’s north of the equator and includes North America, Central America, Europe, most of Asia, part of Africa and a small portion of South America. The Southern Hemisphere is the half of the Earth that’s south of the equator.) Six months after the March equinox, another equinox occurs around September 22 or 23 and marks the beginning of fall in the Northern Hemisphere and spring in the Southern Hemisphere. Since the Earth actually takes about 365.24 days to orbit the sun, equinoxes happen around six hours later from year to year, before moving back a day on leap years.

In addition to two annual equinoxes, there are two solstices every year. The summer solstice, around June 20 or June 21 in the Northern Hemisphere, occurs when the sun is farthest north of the equator; it’s the longest day of the year in terms of daylight. The winter solstice, around December 21 or December 22 in the Northern Hemisphere, happens when the sun is farthest south of the equator; it’s the shortest day of the year. The word solstice comes from the Latin “solstitium,” meaning “stopped sun.”

For centuries, people have celebrated the vernal equinox. At the ruins of Chichen Itza, the ancient Maya city in Mexico, crowds now gather on the spring (and fall) equinox to watch as the afternoon sun creates shadows that resemble a snake moving along the stairs of the 79-foot-tall Pyramid of Kukulkan, also called El Castillo. On the spring equinox, the snake descends the pyramid until it merges with a large, serpent head sculpture at the base of the structure. While the Maya were skilled astronomers, it’s unknown whether they specifically designed the pyramid to align with the equinox and create this visual effect.

At Stonehenge, the prehistoric monument in England featuring the remains of a circle of huge standing stones, druids and pagans congregate to watch the sun rise on the equinox and welcome spring. However, it’s unclear what, if any, meaning the equinox held for those who constructed the ancient monument, as they left no written record about why or even how it was built.

Among various spring holidays is Nowruz, the Persian New Year, which starts on the vernal equinox. The centuries-old holiday is observed by millions of people around the world and lasts 13 days. In Japan, the day of the spring equinox is a national holiday called Shunbun no Hi. Some people commemorate the day by tending to the graves of their ancestors.