NOAA's GOES-13 satellite captured this image of the Earth at the spring equinox, on Wednesday, March 20, 2013 at 7:45 a.m. EDT.

(NOAA)

On Wednesday, the first day of spring, the length of day and night were about the same for most of the planet. The amount of solar energy delivered to the Northern and Southern Hemisphere was also equal.

You can see that effect in an image taken Wednesday morning (March 20) at 7:45 a.m. EDT by the National Oceanographic and Atmospheric Administration's GOES-13 satellite. The photo shows both hemispheres equally lit. As spring wears on, the Northern Hemisphere will receive more sun than the Southern Hemisphere, creating the familiar seasons in each region (summer and winter, respectively).

The dawning of spring comes on different dates (from March 19-21) and different times each year for two reasons. First, the year is not an exact number of whole days; it takes the Earth about 365 and one-quarter days to orbit the sun (which is why we have a leap day every four years).

Second, Earth circles the sun in a slightly non-elliptical orbit, and that, in addition to the gravitational pull of the other planets, changes Earth's orientation to the sun from year to year. Equinoxes (which mark the onset of spring and autumn) and solstices (which mark the beginnings of summer and winter) are points in time and space that designate a transition in the planet's annual trip around the sun.

In the higher latitudes of the Northern Hemisphere, the time of "equal day and night" occurs a few days before the spring equinox, while in the Southern Hemisphere that date comes after the March equinox, according to the National Weather Service.

The sun in this image was created artificially, though the GOES spacecraft does have sensors which constantly monitor the sun for solar activity, NOAA reports.

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Afghan young men jump over a fire as they celebrate Nowruz, the Persian New Year festival in Herat on March 19, 2013. The festival is celebrated in Iran, Turkey, Central Asian republics, Iraq, Azerbaijan as well as war-torn Afghanistan and coincides with the astronomical vernal equinox. (Aref Karimi/AFP/Getty Images)