The Lake County Astronomical Society

What's that Dark Strip on the Eastern Horizon after Sunset?

If you've ever seen a dark band hugging the eastern horizon soon after the sun has set, then you've experienced the Earth's shadow projected against the sky.

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The following is a photo of this phenomenon taken on one of our trips to New Mexico (looking toward the east after sunset).

The first requirement to see this is that your eastern horizon has to be very low and clear. You might be able to see it around here by looking eastward across Lake Michigan. As the sun sinks beneath the western horizon, the sunlight continues to be scattered in the sky -- that's what gives us twilight. But visualize how the Earth gets in the way of the direct rays of the sun, so just as you cast a shadow when the sun is at your back, the Earth's shadow is projected against our somewhat opaque atmosphere. The band becomes wider, rising as the sun sinks farther below the western horizon. As the sky becomes darker, the Earth's shadow just peters out in the advancing night.