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A diagram showing how the Sun moves through the sky on an equinox.
Click on image for full size

The Vernal and Autumnal Equinoxes

We're still pretending that you're the person standing on the Earth in the picture to the left, living in Topeka, Kansas, around 40 N latitude. The picture on the left shows the view from the solar system (upper panel), and from on the surface of the earth (lower panel). Notice that some of the same features are labelled on each panel. In the upper panel, the Earth's axis is pointing into your computer screen.

The upper panel shows that on an equinox (which occurs around March and September 21), neither half of the Earth points directly towards the Sun. In fact, the Sun is at the equator, so both halves of the Earth are getting about the same amount of sunlight. For you in Topeka, the altitude of the Sun is about 50, in between its altitude on the solstices. The bottom panel shows how this looks to someone standing on the ground in Topeka.

Equinox literally means "equal night". On the vernal (spring) and autumnal (fall) equinoxes, day and night are nearly the same length (the date on which day and night are actually closest to the same length is called the equilux, and occurs a few days towards the winter "side" of each equinox). Neither hemisphere gets more sunlight than the other, so both have similar seasons (fall in one hemisphere and spring in the other).

How high the Sun gets in your sky, and how long it is above the horizon during the day, depend not only on the season, but also on your latitude.

Last modified September 3, 2010 by Randy Russell.

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