Sky watch: This bright star is no UFO

Don't be surprised if you find yourself picking up the phone to report a strange "UFO" in the southeastern sky at dusk in a few days. It will appear as a brilliant light that flickers and sparkles in a multitude of colors during the week of Jan. 24. And if you watch it long enough, it'll seem to dart around in random directions.

The spectacle you'll see after dark over the southeastern horizon in a few days is no UFO: It's simply the bright star Sirius.

When a star appears low in the heavens, its light must travel a long path through our atmosphere before it arrives at our eye. This air rapidly distorts the star's light in different directions and creates the phenomenon "twinkling." The brighter the star is, the more easily we can detect its twinkling. Sirius, being the brightest star of the entire nighttime sky, certainly qualifies.

"But wait," you say, "This is no twinkling star! It's radiating colors like a Fourth of July sparkler!"

That's because the star's white light is made of every color of the spectrum, and each is refracted differently by the atmosphere. This makes the star appear to sparkle with many colors - especially when viewed through binoculars or a telescope.

"OK," you counter, "but this still doesn't explain why it appears to bounce around."

This is true. But this phenomenon has little to do with astronomy, and much to do with the physiology of the human eye.

The eyes' muscles are constantly moving these light detectors. When a star appears in the sky with few reference points around it - like Sirius appearing low in the sky - the eyes' movements cause the brain to interpret their movements as those of the star itself.

It's a phenomenon called the "autokinetic effect," and it has helped many times to turn the appearance of Sirius (and Venus, Jupiter and other bright objects) into UFO reports throughout the years.

Now put the phone down and go outside to enjoy the show.