Spring equinox 2024: 'Equal night' signals changing seasons across the globe today

Starting tonight, spring will have officially sprung in the Northern Hemisphere.

Today (March 19) is the March equinox, an annual astronomical event that marks the change of seasons across the globe. In the Northern Hemisphere, the March equinox signals the end of winter and the beginning of spring (which is why it's also called the vernal equinox, or spring equinox). In the Southern Hemisphere, today marks the end of summer and the beginning of autumn.

Equinox comes from the Latin words meaning "equal night." That's because, on this day and several days before and after, the length of daylight is almost exactly 12 hours in both hemispheres, with slight variations depending on your distance from the equator, according to the National Weather Service.

Equinoxes occur twice a year — in March and September — because of our planet's tilt.

Because Earth orbits the sun at a tilt of about 23.5 degrees, different parts of the planet receive more or less sunlight at various times of the year, depending on our planet's position relative to the sun. For example, on the summer solstice in June, Earth's Northern Hemisphere tilts toward the sun and receives the brunt of its light, making it the longest day of the year for northerners, while the Southern Hemisphere tilts away and experiences its longest night.

Related: Spring equinox 2024: Why March 19 is one of the best stargazing nights of the year

On equinoxes, however, Earth's tilt aligns with its orbit, and the sun appears exactly above the planet's equator rather than north or south of it. The result is "equal night" in both hemispheres — a phenomenon that is clearly visible from space in stunning photos like these.