The Sun is in constant motion in the sky. We see it every day rising in the east, soaring overhead, and sinking slowly in the west. This daily motion is a consequence of the Earth spinning on its axis.
But as it turns out, the Sun also slowly meanders north and south in the sky, inching a little bit each day over the entire course of the year. This motion is a consequence of the Earth moving along its orbit, combined with the fact that it is slightly tipped over. Sometimes the north pole of the Earth is tipped toward the Sun; this is during the summer in the Northern Hemisphere, and the Sun climbs very high in the sky. Sometimes the north pole of the Earth is tipped away from the Sun; this is during the winter in the Northern Hemisphere, and the Sun doesn't climb very high in the sky at all.
You can track the north and south motion of the Sun in a variety of ways, but the easiest way is to observe the location of the sunrise and sunset every day. On June 21 (the "summer solstice") the Sun rises and sets at the most northern point on the horizon it ever reaches, and on December 21 (the "winter solstice") it rises and sets at the most southern point on the horizon it ever reaches.
Between the solstices, the sunrise and sunset locations change. From December to June, the sunrise and sunset creeps north along the horizon, and from June to December it slowly creeps farther south. The equinoxes (March 21 and September 22) are special --- on those days, the Sun rises directly in the East, and sets directly in the West.
In cities like Chicago, where the orientation of our street grid is nearly aligned with the compass points, the days around the equinox afford us a special opportunity to see the rising and setting sun framed down the avenues and boulevards of our city. We call this opportune alignment of the sunrise and sunset with the city "Chicagohenge." For the next week or so, spend some time watching the Sun rise and set down the streets of Chicago. Take your camera with you, and see if you can capture some great pictures!
Learn more about cityhenges!
Written by Dr. Shane Larson, astronomer at the Adler Planetarium.