Sky watch: Purple arc appears during sunset, sunrise

Science is best learned when something occurs to make us question our eyes or, in some cases, even our sanity. If we're dedicated enough to follow through on what we've experienced, chances are we'll make an amazing discovery.

It's in just this way that I enjoy helping people learn about the heavens. Often when I'm out watching a beautiful sunset, I'll do something that confuses anyone nearby. Just as the sky show becomes really good, I turn around and face east.

The reaction I usually get is a puzzled look, followed by a gentle reminder: "Sunset's the other way." My response? "While you're watching the sun set, I'm watching night rise." It's usually at this point that people begin to step away from me with a nervous look. But I'm quite serious about what I'm doing . . . I'm watching night rise. The thing is that nearly everyone has noticed the phenomenon, but few have ever realized what it is they were seeing.

The next time you have a cloudless sky, try it yourself. Face east just as the sun is setting in the west. Low against the eastern horizon, you'll see an immense purple arc, bordered by a fringe of pink just above it. That is the shadow of our Earth.

When the sun sets, we are on the boundary between daylight and nighttime. Sunlight continues to illuminate the atmosphere in the west, but our solid planet blocks its light from reaching the air far to the east, so it appears a dark blue or purple color. And between the darker and brighter parts of the atmosphere lies a fringe of pink - also known as the "Belt of Venus" or the "anti-twilight arc" - lit up by the reddened sunset light that's passing through the atmosphere itself.

Early risers can see the same phenomenon during sunrise - only then, it appears low in the west. Your best chance to spot it is when you've got a cloudless sky with a low horizon, such as the ocean or desert.

Depending on the clarity of the air, the Earth's shadow usually appears most prominently 10 minutes or so after sunset. Eventually, this shadow rises high enough that it completely engulfs us. And that is what we call nighttime!