Equinox

Astronomical event

The word equinox comes from Latin æquinoctium of Aequus (equal) and nox, noctis (night).
This astronomical event occurs when the Sun's center is exactly vertical to the Earth's equator.
The Sun is not a single point of light seen from Earth, the full term for which he remains above the equator is reached in 33 h.
There are two equinoxes each year occurring between 19 and 21 March and between 21 and 24 September. Those moments that mark the passage of the Sun at the zenith, are by convention the early spring and fall. September 22, 2010 at 3:09 Universal Time, the Sun crossed the celestial equator from the North.
This is the first day of autumn in the northern hemisphere and the first day of spring in the south. Days and nights are equinox exactly 12 hours.
After Sept. 22, in the northern hemisphere, the days get shorter, the sun rises less high on the southern horizon as winter approaches.

At the poles, the equinox marks the transition between six months and six months day and night. Beyond the Arctic Circle, after the spring equinox, the town of Longyearbyen in Norway gets 15 minutes of daylight every day while in Singapore about 1 ° north, this variation is only a few seconds.

Image: Ultraviolet image of the Sun was obtained by the satellite SDO (Solar Dynamics Observatory), September 22, 2010.
It shows false color, the emission of highly ionized iron atoms. Arcs trace the path of the plasma in the loops of magnetic field above the solar regions.
Credit: NASA / Goddard / SDO AIA Team

Spring equinox and autumn

Image: The dates of the equinoxes and the seasons are on the official website of IMCCE (Institute of Celestial Mechanics).
The Institute of Celestial Mechanics and Ephemeris Calculation built, provides and publishes ephemerides of all solar system bodies. It is the source ephemeris own public official to settle all calendars.

Image: Illumination of Earth by Sun on the day of the vernal equinox and autumnal.
The dates of the equinoxes vary from year to year because the Earth's orbit is not circular.
Credit NASA