March 20, 2004
The above METEOSAT visible satellite image is a colored composite of several images taken yesterday (March 19). This year, the Sun was directly above the Equator at 06:49 Universal Time or UT (1:49 a.m. Eastern Standard Time or EST) on March 20. Note that this is the earliest date for the start of spring since 1896, when it began on March 20 at 02:24 (UT).
The actual definition of the vernal equinox is when the ascending node of the ecliptic (the path the Sun appears to take across the sky) intersects the Celestial Equator (the position the Equator would assume if it was projected against the sky). Anyway, the important thing to remember here is that this marks the start of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. Since our orbit around the Sun isn't a perfect circle, the date of the vernal equinox changes from one year to the next year.
On the above image notice that it's light near both poles -- a sure sign that it was acquired near the time of the equinox. In addition to both poles being illuminated, there's also a hint of sunglint at the Equator (circular smudge in Indian Ocean at center of photo), which also indicates the photo was taken on or very near an equinox. One other inference that it's near the time of the vernal equinox is the cloud mass over the Middle East. Since the Middle East region is very hot and dry in late summer, an extensive cloud cover rarely occurs.
Also noteworthy on this photo is the relative lack of clouds. With the exception of the eastern Indian Ocean, the steppes of central Asia, parts of the Middle East and China, most of the entire hemisphere is cloud free on this date.