For astronomers, the main difference between midtown Manhattan and rural Mongolia—other than the availability of a good latte—is the darkness of the night sky. In dense urban areas the sky is 25 to 50 times brighter than in less populated places, blotting out all but the heartiest celestial sights. Stray light is not just an urban problem, however. Even far from Times Square and Broadway, a pervasive glow fills the spaces between the stars. The intensity of this background varies with the season and with solar activity, but the sky itself always outshines all the visible stars combined.
The sky’s natural luminescence has four primary components:
* Airglow, a kind of permanent aurora. During the day, ultraviolet rays from the sun split oxygen, nitrogen, and other molecules in the upper atmosphere. At night these molecular fragments recombine, emitting a mostly green light with a tint that is normally too faint for the retina to perceive. Airglow noticeably dims when the sun is relatively quiescent, as is the case from now through 2007.
* The scattering of light passing through the atmosphere. Most familiar during the daytime, this effect can have dramatic consequences at night as well: Look for a dull blue cast to the entire sky around the time of the full moon, from April 22 to 26, due to scattered moonbeams.
* Sunshine reflecting off dust particles orbiting between the sun and Jupiter. This is the zodiacal light, the “false dawn” described by the poet Omar Khayyám. It appears as a transient, white, cone-shaped pillar tapering up from the western horizon after nightfall (and in the east before dawn). The zodiacal light can be obvious in rural areas, especially in the southern states and in the tropics.
* Glimmering caused by the swarms of faraway stars in our galaxy. Although the eye cannot pick them out individually, these faint stars cumulatively blend together into the eerie, shimmering band known since ancient times as the Milky Way. In April this diffuse luminosity vanishes around midnight because the Milky Way then lies along the horizon. Look overhead at that hour and you are peering straight out of our galaxy into comparatively open intergalactic space.
Part of the sky’s optical noise can be eliminated by going to a remote mountaintop, but you cannot escape from scattered sunshine and sprinkled starlight. That radiance is everywhere—including outer space. The Hubble Space Telescope, orbiting 370 miles above the ground, sees a background that is nearly twice as dark as a desert sky. Still, the zodiacal light illuminates the heavens there and obscures faint objects; it is the main diffuse background at visible wavelengths. Hubble must also contend with the dazzling brilliance of Earth below, which fills nearly half the sky. (From the moon, 650 times farther away, a full Earth still appears 40 times brighter than the full moon does from here.)
In some ways, though, it is a marvel that we can see any sky at all. Two centuries ago, German astronomer Heinrich Olbers concluded that—logically—the night sky should be bright rather than dark. If we live in an infinite universe containing an infinite number of stars, he reasoned, then we should see a star along all lines of sight. Every point of the sky, no matter how small, should be filled with starlight, so the whole firmament should look like the surface of a star. Staring up in any random direction should therefore be exactly like staring at the sun, and day should be just like night.
Fortunately this is not the case; otherwise a nocturnal stroll would require SPF 600,000 sunscreen. So why is the sky dark? Olbers was stumped by his seeming paradox, but today we know the answer: The sky is dark because the universe is young. Its 13.7-billion-year age has just not allowed enough time for the light from most of the cosmos to reach us.
The first 12 days of April are an ideal time to get away from the city lights and relish the darkness. Around midnight the Milky Way will be largely out of view, the moon will have set, and airglow will be near its minimum. The result should be the blackest skies since the new century began. It could be the most dazzling nondazzling sky event of the year.
What's up on the April sky?
April 3: Jupiter, now at opposition, is the closest and brightest of the year. Out all night in Virgo, it dominates the dark sky like a lighthouse.
April 3: Daylight saving time begins. We move clocks forward because Earth’s orbital motion is causing the sun to rise earlier in the Northern Hemisphere.
April 8: A partial solar eclipse is visible in southern states. Use eye protection, such as shade 14 welder’s goggles—never stare at the sun without protection.
April 12-13: Mars and Neptune draw together in the east. Binoculars reveal Neptune in the same field of view with a much brighter mars.
All month: Saturn is high in the southwest at nightfall, glorious though a small telescope.
