The September equinox occurred this morning at 5:05 am EDT, marking the official start of Fall for those of us in the Northern Hemisphere. Twice a year, during the latter halves of March and September, every person on Earth experiences the effects of an equinox in the form of a 12-hour day followed by a 12-hour night.
To observe this perfect division of light and dark over a 24-hour period here on Earth is one thing, but seeing it from 36,000 km up helps one appreciate, in an instant, the awesome symmetry of the event, as light from the Sun spreads evenly over our planet's Northern and Southern Hemispheres.
The equinoxes are the only times of year when Earth's axis tilts perpendicular to the Sun's rays; in the weeks and months separating the equinoxes, Earth's axis will tilt progressively farther away from or toward the sun, exposing the Northern and Southern Hemispheres of the planet to varying degrees of sunlight in the process.
The whole cycle — from last year's winter solstice, to today's fall equinox — is captured beautifully in the time-lapse video of Earth featured here. The video comprises images of Earth captured over the last 12 months by EUMETSTAT's Meteosat-9, including the one pictured up top.
You'll notice that the Earth does not appear to be rotating in the video. That's because the Meteosat-9 satellite is in what is called "geostationary orbit" around the Earth. The satellite is located directly above the equator, and is orbiting Earth at the exact same speed that the planet is rotating about its axis, such that the satellite does not move at all relative to Earth's surface.
The video helps illustrate how the angle of the Earth's axis relative to the Sun affects the amount of sunlight each hemisphere receives (and, by extension, the changing of the seasons) throughout the year. According to the NASA Earth Observatory:
On March 20 and September 20 [at approximately 00:06 and 00:12, respectively], the terminator [where the shadows of nightfall meet the sunlight of dusk and dawn] is a straight north-south line, and the Sun is said to sit directly above the equator. On December 21 [at approximately 00:03], the Sun resides directly over the Tropic of Capricorn when viewed from the ground, and sunlight spreads over more of the Southern Hemisphere. On June 21 [at approximately 00:06], the Sun sits above the Tropic of Cancer, spreading more sunlight in the north and turning the tables on the south. The bulge of our spherical Earth blocks sunlight from the far hemisphere at the solstices; that same curvature allows the Sun's rays to spread over more area near the top and bottom of the globe.
[Via NASA Earth Observatory]
Top image and video via NASA earth Observatory