The March equinox, one of two points in the Earth’s orbit around the Sun when both the Northern and Southern Hemispheres receive equal amounts of daylight, arrived on Monday, March 20 at 6:28 AM EDT.
Its Latin meaning is “equal night,” and it marks the start of spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
Earth’s rotation axis is tilted approximately 23.5 degrees to its orbital plane, also known as the ecliptic. As a result, the amount of daylight during its 24-hour rotation period is unequally distributed across the planet.
Both hemispheres take turns facing toward and away from the Sun, which is the reason for the planet’s four seasons.
“During two special times a year, the tilt is actually perpendicular to the Sun, which means that Earth is equally illuminated in the Northern and Southern Hemispheres,” explained C. Alex Young, associate director for science at NASA Goddard Space Flight Center’s Heliophysics Science Division.
Earth’s solstices and equinoxes have been recognized and celebrated since ancient times. Modern civilizations’ ability to put satellites into orbit provides a unique view of the phenomena from a distance, Young said.
“Earth’s tilt has an effect during other times of the year–you see more light in the Northern Hemisphere or more light in the Southern Hemisphere, depending on the time of year. And then during the equinoxes, you see that the Northern and Southern Hemispheres are equally lit up.”
Earth-orbiting satellites enable scientists to study Earth’s ecosystem in unprecedented detail, with views of global cloud cover, landmasses, seas, and oceans.
Today, NASA’s Heliophysics System Observatory (HSO) studies the interaction between the Earth and Sun using 23 separate spacecraft.
In 2018, NASA’s Solar Probe Plus (SPP) will fly within four million miles (six million km) of the Sun, the closest ever for a solar probe, to study the Sun’s corona as well as its periodic emissions of high-energy particles.