The Sciences

Out of the Blue

By Bob BermanJun 1, 1993 5:00 AM


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You might reasonably expect a column called Night Watchman to say something about the night sky. But this month? Full darkness doesn’t even exist in a half-dozen European countries and most of Canada. In the northern half of the United States it lasts for less than seven hours.

One solution: forget the darkness. Surprising discoveries obligingly await us even when the 9 P.M. night sky is a sunny blue. Let’s start with the moon. Some people express amazement at seeing it in daylight, as if it were a vision of the Madonna. But a diurnal moon hardly deserves incredulity; it appears just as often as the nighttime version. Most noticeable are the half-moon and football-shaped gibbous moon, phases separated by two or three days. In the southern sky, they’re prominent in the daytime after sunrise in midmonth, and before sunset during June’s final week.

A more intriguing target is Venus. Check out the dazzling UFO- like star low in the east before dawn. Keep your eye on it through sunrise and you’ll find it refuses to vanish. A star in the daytime! That’s when seasoned telescope users prefer to observe Venus, because it’s higher above the blurry air near the horizon and displays a sharper image.

Jupiter is out before sunset if you look in the right spot: about halfway up the southern sky, near the moon, on June 26. A helpful trick for finding these daystars is to avoid direct sunlight, which makes your pupils contract. Stay in the shadow of a building while gazing upward. And hope, if a policeman passes, he believes your story.

Some observers report that the brightest true star, Sirius, is marginally seen in daylight, but anything fainter is doubtful. (Staring into the blue expanse is also the best way to see floaters, specks or wormlike shapes caused by debris inside your eyeball. Common and usually harmless, they drift slightly out of step with the eye’s own movements.) If you can’t spot any daytime stars, you can still entertain yourself by looking at the sky with polarizing sunglasses. Ordinary light consists of electromagnetic waves vibrating in all directions. But air molecules polarize light, filtering it so that only light vibrating with certain orientations reaches our eyes. Polarizing sunglasses block out some of this light. And since water droplets in clouds don’t polarize light as much as air molecules do, the right sunglasses can enhance the contrast between clouds and sky and reveal subtle color differences.

Even without sunglasses you can see that the sky’s blueness obviously deepens in certain sections of the heavens. It’s always lightest down near the horizon. Pollution is partly to blame, but you can see a lighter horizon even in the cleanest air. When sunlight enters Earth’s atmosphere, nitrogen and oxygen molecules scatter the light toward our eyes. The shorter wavelengths of light--which we see as blue--are the most readily scattered. So when you look straight up, through the least atmosphere, you see scattered blue light. But as you shift your view closer to the horizon--through more air--other, longer wavelengths have been scattered as well, whitewashing the sky.

Considering that Earth has perhaps the only surface facing a blue sky (Mercury’s sky is black, Venus’s gray, Mars’ pink . . . ), our vault of day is uniquely beautiful, deserving of occasional attention. Especially now, when it’s just about the only show in town.

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