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Earth from Orbit: Vernal Equinox

March 25, 2022
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Spring has officially sprung here in the Northern Hemisphere—while in the Southern Hemisphere, autumn has begun!

The vernal equinox—the official start of astronomical spring—occurred on Sunday, March 20, 2022 at 11:33 a.m. EDT.  

There are only two times of the year when the Earth's axis is tilted neither toward nor away from the sun, resulting in a nearly equal amount of daylight and darkness at all latitudes.These events are referred to as equinoxes and occur in the spring (vernal) and fall (autumnal). 

The word “equinox” is derived from two Latin words—aequus (equal) and nox (night). At the equator, the sun is directly overhead at noon on these two equinoxes. The "nearly" equal hours of day and night is due to refraction of sunlight, or a bending of the light's rays that causes the sun to appear above the horizon when the actual position of the sun is below the horizon. Additionally, the days become a little longer at higher latitudes farther from the equator, because it takes the sun longer to rise and set.

The seasons change at various times of the year due to the planet’s axial tilt of 23.5 degrees as it orbits the sun. This tilt means the two hemispheres are exposed to different sun angles and variable lengths of daylight throughout the year. The equinoxes are the only two times a year that the sun rises exactly due east and sets due west for nearly every place on Earth.

While astronomical spring began on March 20, meteorological spring began on March 1. Astronomical seasons are based on the position of the Earth in relation to the sun, while meteorological seasons are based on the annual temperature cycle. Meteorological seasons begin on the first day of the months that include the equinoxes and solstices.Thus, meteorological spring began on March 1 and will end on May 31. Astronomical spring will officially end, and astronomical summer will begin, on the summer solstice on Tuesday, June 21 at 5:13 a.m. EDT. 

From their position 22,236 miles above the equator, NOAA’s GOES-16 and GOES-17 satellites orbit at the same rate Earth rotates, so they can keep constant watch over the same region. This allows them to view the terminatorthe edge between the shadows of nightfall and the sunlight of dusk and dawnas it moves across the Western Hemisphere.The slope of the terminator curve changes with the seasons. During an equinox, the terminator is a straight north-south line over the equator. Throughout the year, NOAA satellites observe this marker of seasonal change.