Pat Shingleton: "Equinox..."
The swallows have returned to Capistrano, the buzzards have arrived in Hinckley, Ohio and we're on the look-out for the purple martins in Baton Rouge. Just after lunchtime today, Spring springs. The official time for the start of Spring is determined by the earth's motion around the Sun. Sky and Telescope provides some interesting items on the start of Spring. For the Northern Hemisphere it's defined as the moment when the Sun passes over Earth's equator on its journey north, officially called the vernal equinox. The word, equinox, originates from the Latin for "equal night." Day and night are not exactly 12 hours long at equinox for a couple of reasons. Sunrise and sunset are determined when the Sun's upper edge, not the center, crosses the horizon. The second reason involves the Sun's position near the horizon, causing refraction by the Earth's atmosphere which shifts its position slightly upward.