This blackest month, when the stars emerging from early darkness recall the masterful illumination of a Rembrandt, we can inspect the canvas itself--the sky between the stars. Here is an emptiness rich in both mystery and misconception.
For starters, how much of the sky is occupied? If you’ve ever camped under desert skies spangled with seemingly endless stars, you’ll remember how the profusion appears to leave almost no space unfilled. But in truth, a straight line slashed across the firmament is unlikely to touch a single star. (Watch how rarely one is occulted by a passing satellite.) Just one (usually faint) star inhabits each patch about the size of 30 full moons. Space, not stars, dominates the heavens.
This vacancy is counterintuitive because we focus on objects rather than their environment. The moon, for example, is perceived as much larger than it really is; few would guess that it would take some 100,000 moons packed together to fill the sky.
You might think that millions of stars cram a desert’s limpid sky. But incredibly, the full complement is just 3,000. You could count them all in an hour. Today’s light-polluted suburbs offer barely one-tenth the desert’s stars; five minutes suffices for tallying them. But you don’t have to count stars to compute the quality of your sky. Its color alone can tell the story.
In perfect conditions the night sky is almost black and displays a uniform shade from overhead to the horizon. Such ideal circumstances occur over remote deserts and mountains, where the stars appear only slightly less brilliant than they do from outer space.
From pristine sites the Big Dipper’s bowl brims with a half-dozen stars. But distant memories of virginal skies must yield to the reality of our modern firmament, pink or steel gray because of streetlight reflections. Today’s suburbs display a colored zenith tapering downward to a brighter horizon. No star appears within the Dipper; the Milky Way is a no-show, a museum curiosity; and the northern lights are mythical. In large cities the moon barely lightens the sky.
Even if it can’t compete with bright city lights, this month’s full moon--on the seventeenth--still deserves attention. It’s the year’s highest full moon. This month also marks the close of the twenty-fifth- anniversary year of the Apollo lunar landing. Most people are aware that communications restricted all the Apollo missions to the side of the moon that eternally faces Earth. Less well known is that all Apollo sites were closer to the moon’s middle than to its limb. This clustering makes them all simultaneously visible for one week each month--within four days of the full moon. The naked eye can easily find the now-famous Sea of Tranquility, where Neil Armstrong and Buzz Aldrin walked during their Apollo 11 adventure. The dark, smooth lava plain is to the lower right of the moon’s center. On December 7 the moon’s phase will match its Apollo 11 appearance.
So urban astronomy isn’t necessarily an oxymoron: we can maximize what we’ve got by selecting our targets with care. With the right binoculars, stellar and lunar observing is just as good in bright as in dark skies.
But if the ancient, fragile blackness beckons with its promise of deep-space glory, simply head 40 miles from town. The space between the stars still lives in the space between our cities.