Earth's Spring Equinox Seen from Space
Author: A Millisecond of Eternity
On March 20, 2019, a GOES satellite photographed Earth at the spring equinox. In the image, half the planet is illuminated while the other half lies in darkness, like a black-and-white cookie.
This beautiful symmetry will not surprise anyone familiar with astronomy and geography. A real photograph—especially an astronomical one—can nevertheless be more powerful than the theories we learn, inspiring wonder at the universe’s extraordinary beauty. In Latin, “equinox” means “equal night.” Geography teachers tell us that the spring and autumn equinoxes occur in March and September each year, when day and night are nearly equal in length at every latitude.
Earth’s rotation axis is tilted 23.5 degrees relative to its orbital plane. The subsolar point moves between the Tropic of Cancer and the Tropic of Capricorn, so sunlight is usually distributed unevenly across Earth. Depending on Earth’s position in its orbit around the Sun, day and night are generally unequal in length in either the Northern or Southern Hemisphere.
“At the spring and autumn equinoxes, the Northern and Southern Hemispheres receive essentially the same amount of daylight,” said C. Alex Young, associate director for science in the Heliophysics Science Division at NASA’s Goddard Space Flight Center. In other words, at noon on an equinox, the Sun is directly above the equator.
The 2019 spring equinox occurred at 5:58 a.m. Beijing time on March 21, marking the first astronomical day of spring in the Northern Hemisphere. The image above was taken by the GOES-East satellite at 8:00 p.m. Beijing time on March 20, 2019.
The GOES series—the Geostationary Operational Environmental Satellite system—is a network of Earth-observation satellites operated by the U.S. National Oceanic and Atmospheric Administration (NOAA). One satellite is positioned at 75° west longitude and another at 135° west—hence the names GOES-East and GOES-West. Together they cover roughly one-third of the globe and provide continuous meteorological observations of the Western Hemisphere 24 hours a day.
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