Sky Watch: Earth's shadow at nightrise

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 a wonderful discovery.

It's in just this way that I enjoy helping people learn about the heavens, and anyone who has ever been stargazing with me knows this quite well. In fact, more than once during a night, people will hear me say: "Turn around!" That's because many stargazers become so fixated on what's in front of them that they miss what's around them, and sometimes that's even more amazing or beautiful.

It's not only while stargazing that I do this. Often when I'm out watching a beautiful sunset, I'll do something that confuses anyone who happens to be nearby. Just as the sky show becomes really colorful, I turn around and face east.

The reaction I get is usually a puzzled look, followed by a gentle reminder: "Uh, what are you doing? Sunset's the other way."

Perhaps even more perplexing to people is my response: "Well, while you're watching the sun set, and I'm watching night rise."

It's usually at this point that people get a nervous look in their eye and step slowly away from me. 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 after the sun sets in the west. Low against the eastern horizon you'll see an immense purple arc, bordered by a fringe of pink just above it. Many people think it's just haze or pollution. Not true. This is the shadow of our Earth.

Early risers can see the same phenomenon around sunrise, only then it appears low in the west. Just before sunrise, face west, and you'll see the arc as it sets behind the terrain. Your best chance to spot it is when you've got a cloudless sky with a low horizon, such as the ocean or desert.

This occurs because our planet is a solid body that casts its shadow in the direction away from the sun. When the sun sets, for example, we find ourselves on the boundary between daytime and nighttime. Sunlight continues to illuminate the atmosphere in the west -- that's what gives the sky a light blue color -- but our solid planet blocks the sunlight from reaching the air far to the east, so that part of the sky appears a darker 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 actually passing through the atmosphere itself.

Depending on the clarity of the air, Earth's shadow usually appears most prominent ten minutes or so after sunset. Eventually, this shadow rises high enough at sunset that it completely engulfs us.

And that is what we call . . . you guessed it . . . nighttime!