This animation loop shows 3 images of the southeast horizon just before sunset, then 4 minutes later and 8 minutes later.   Look at the sky just above the horizon on the left half of the picture.  You should see a grayish-blue band that gets progressively thicker (that is, reaches higher into the sky) in each image.  This dark band is the earth’s shadow projected against the sky.  As the sun drops further and further below the western horizon, the blurry edge of the earth’s shadow rises correspondingly higher and higher above the eastern horizon.

This phenomenon can be seen just after sunset on almost any clear evening if you are at a location with a good view of the eastern horizon.

Image details:  The three images are 1/30-second exposures at ISO 100, 160, and 400, respectively, taken with a Canon 400D camera and zoom lens at 18 mm.

August 2008