Sunset in Washington on Sept. 15. (Kevin Wolf via Flickr) (Kevin Wolf) By David Policansky

It is late September, and the autumnal equinox has arrived. That gets me to thinking even more than usual about sun angle. Noticing differences in sun angle during different times of year and locations helps our knowledge of weather, climate and the natural world. Subscribe for unlimited access to The Post You can cancel anytime. Subscribe

We have seasons because the sun angle varies over the course of the year, and it varies because the Earth’s plane of rotation is tilted by about 23.5 degrees from the plane of its orbit around the sun. As a result of this tilt, the sun is high in the northern hemisphere in May, June and July and low in November, December and January.

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