Equinoxes – Spring Equinox (March 20, 2011)

Equinoxes - Spring Equinox (March 20, 2011)

Spring Equinox from the space. This NASA image is acquired on March 20, 2011.

During two Equinoxes, around March 20 and September 23, the terminator (the edge between the shadows of nightfall and the sunlight of dusk and dawn) is a straight north-south line, and the Sun is said to sit directly above the equator. As a result, the northern and southern hemispheres are equally illuminated and daylight and nighttime are of approximately equal duration all over the planet. The word comes from Latin equi or “equal” and nox meaning “night”. In other words, the equinoxes are the only times when the sub-solar point is on the equator. The sub-solar point on a planet is where its sun is perceived to be directly overhead). To learn more about the local noon and the sub solar point, see the article titled “How Earth Moves“.

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