Happy first day of Spring!

At 11:44 am UT this morning, (4:44 if you're in California, 6:44 if you're in Chicago or Texas, 7:44 if you're in Virginia, Florida, or Ohio), the Earth crossed that mystical point in space such that the Sun crossed the celestial equator as seen from Earth. This denoted that the Sun is now "in" the Northern Hemisphere and marks the first day of Spring.

What is particularly interesting is that, had the Sun crossed the celestial equator just sixteen minutes later, you could have "drawn" a perfectly straight line through the center of the earth, the intersection of the Prime Meridian and equator, the center of the Sun, and the intersection of zero hours right ascension and 0° declination on the celestial sphere.

While the Prime Meridian and equator of the Earth coincide with the celestial Prime Meridian and Equator once every 23 hours and 56 minutes (or about 366 times a year), the 

celestial prime meridian is defined as the First Point of Ares or the vernal point, i.e., that point that the Sun occupies in the sky during the vernal equinox.

What this means is that at noon UT today, the Sun was directly above 0°N, 0°E, and was also at 0 hours right ascension and 0° declination. (The Sun, afterall, is a disk not a point, so while the exact center of the Sun was not directly overhead, part of the Sun was.)

In Celestial terms, this is what it looks like:

Notice how the Sun covers the intersection of the celestial Prime Meridian and equator.

Other interesting points:

While today is the equinox, it is not the day that day time and night time are equal. That was the equilux and occured several days ago (Tuesday, here in So Cal). The reason day and night are not the same length today is because the Sun's disk spans several arcminutes of the sky, and the Earth's atmosphere bends the sunlight around the curve of the Earth slightly. These two factors combine to make the day slightly longer than night.

Secondly, since the Sun is a disc, it actually takes it about 33 hours to completely cross the celestial equator. That means starting yesterday and going until sometime tomorrow, the Sun will be covering the vernal point.