Tonight, the 2012 vernal equinox takes place, officially ushering in spring. On March 20 at 1:14 a.m. EDT, the sun’s rays will shine directly overhead at the equator, which results in approximately 12 hours of daylight and darkness at all latitudes.
The vernal equinox, or spring equinox, marks the first day of spring here in the Northern Hemisphere, and the first day of fall for those in the Southern Hemisphere.
In the Northern Hemisphere, the sun is rising earlier now, and nightfall comes later. For us in the Northern Hemisphere, most people are enjoying the warmer days of spring. South of the equator, autumn is starting.
The equinox is an event that is a product of Earth’s orbit around our sun. The imaginary celestial equator is a great circle dividing the sky into northern and southern hemispheres. The celestial equator wraps the sky directly above Earth’s equator. At the equinox, the sun crosses the celestial equator, to enter the sky’s northern hemisphere.
Due to the fact that Earth doesn’t orbit the sun perfectly upright, but is instead tilted on its axis by 23-and-a-half degrees, Earth’s northern and southern hemispheres trade places in receiving the sun’s light and warmth most directly. We have an equinox twice a year – one in the spring and one in the fall – when the tilt of the Earth’s axis and Earth’s orbit around the sun combine so that the axis is inclined neither away from nor toward the sun.
At the equinox, Earth’s two hemispheres receive equal amounts of the sun’s rays. Night and day are approximately equal in length. The word equinox comes from the Latin aequus (equal) and nox (night).
The Earth never stops moving around the sun, though, so these days of equal sunlight and night will change quickly.
See the video below for more on the vernal equinox. Happy (almost) first day of spring!