September 23 marked first day of autumn north of the Equator and the first day of spring to the south.
Day and night were approximately of equal length worldwide at that time, and will be again next March 20.
The animation of Earth’s seasons to the right takes us, one day at a time, from September 19, 2010 to September 19, 2011 with images from Europe’s Meteosat-9 satellite.
Africa comprises most of the right part of the image, with Europe dimly visible in the upper right.
Each image was taken at 6 a.m. GMT, meaning it was near sunrise at those times over western Europe and about midnight in the eastern United States.
On March 20 and September 20, the terminator, or sunrise in this instance, is a straight north-south line with the sun shining directly above the equator.
On December 21, the sun resides directly over the Tropic of Capricorn when viewed from the ground, and sunlight spreads more widely over of the Southern Hemisphere. On June 21, the sun sits above the Tropic of Cancer, spreading more sunlight in the north.
Of course, it is not the sun that is moving north or south through the seasons, but a change in the orientation and angles between the Earth and the sun.
The axis of the Earth is tilted 23.5 degrees relative to the sun. The axis is tilted away from the Sun at the December solstice and toward the Sun at the June solstice, spreading more and less light on each hemisphere.
At the equinoxes, the tilt is at a right angle to the sun and the light is spread evenly.
Full story and images: NASA