Curious about what exactly the "Vernal Equinox" is? Try looking at Encyclopedia Britannica: "The vernal equinox, marking the beginning of spring in the Northern Hemisphere, occurs about March 21, when the Sun moves north across the celestial equator."
The United States Naval Observatory site has lots of great astronomical information too. Pictured here is the Observatory, showing the "time ball" tower. Some of the Nautical Almanac is online, and we have the print copy of the Nautical Almanac in our Government Documents collection on the ground floor, under "D213:11."