March 20

Thursday is the March Equinox–the beginning of spring in the Northern Hemisphere and the beginning of autumn in the Southern Hemisphere. It is one of the two days in the year that the Sun is directly over the equator. It is also one of the two days in the year the the Sun rises due east and sets due west. This gives rise to the phenomenon of “Chicagohenge” here as the Sun beams through the canyons created by the buildings of Chicago’s north-south-east-west-gridded streets.

Surprisingly, though, there are slightly more than twelve hours of daylight during the equinox. The center of the Sun indeed spends 12 hours above the horizon and 12 hours below the horizon, but we get a few more minutes of daylight because sunrise and sunset are measured with the edges of the Sun’s disc. Furthermore, the Earth’s atmosphere bends the light of the Sun slightly, allowing us to see the Sun even when it is technically below the horizon line.

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