Opposite the Sun

By Bob Berman|Friday, April 01, 1994
April finds the returning sun perched over the Northern Hemisphere. But let’s be perverse and gaze where few winter-weary sky watchers are looking: to the region of sky opposite the sun.

Fools may ignore it, but this antisolar point has been a place of special interest since antiquity: the ancients figured out what constellation stood precisely opposite the sun on any given date. That’s not surprising, for the spot holds marvels not seen anywhere else. Let’s begin with our planet’s shadow.

Day ends with Earth’s shadow darkening the eastern sky. There it is, showing itself as a dusky band hugging the horizon. This curious apparition, easy to see if your low eastern sky is unobstructed, goes by an alluring name--the twilight wedge. Broadening as twilight deepens, the curved upper margin of this vast umbra grows more indistinct as the spinning Earth carries you ever farther into darkness. Immersed within this enormous shadow, you no longer sight cleanly along its edge.

Is the esoteric twilight wedge inadequate motivation for you to turn your back to the sun? Perhaps you want something more challenging.

A good candidate is the gegenschein, a faint oval glow sitting smack at the antisolar point, created by reflections from distant dust in the solar system’s plane. These tiny grains bounce sunlight straight back to our eyes the way a movie screen throws its brightest image to viewers most in line with the projector.

Good places to view the gegenschein--one of the most elusive apparitions in the heavens--are increasingly rare today. If you live in any light-polluted region, forget it.

Time, then, for easier quarry: Jupiter, the brightest star of the midnight sky. Every 13 months our speedier Earth catches up with the giant planet, and when Jupiter is closest, it naturally appears at its brightest and, through any telescope, biggest. This alignment happens only when Jupiter occupies the antisolar point. That’s why such close encounters are called oppositions.

To hard-core planet watchers, oppositions beg for binocular or, even better, telescopic exploration. While it will blaze spectacularly from now through summer, Jupiter reaches exact opposition on April 30, lighting the antisolar point like a flare. This marks that awesome world’s closest encounter with Earth until June 1995.

Be it serious or casual sky gazing, the premier antisolar spectacle is not even dazzling Jove, but a total lunar eclipse. The next occurs in two years, on April 4, 1996. Eclipses of the moon occur only at the sky’s antisolar spot.

Skeptical readers who have enjoyed this smorgasbord of antisolar delicacies may not swallow the next one: the L3 Lagrangian point. L3 is a gravitationally stable position situated opposite the sun where, someday, a permanent space station and colony may be established. Imagine living--and paying taxes--in the ultimate in upscale neighborhoods! Some space enthusiasts predict that because a satellite at any of the five Lagrangian points would need relatively little energy to maintain its post, commercial resorts might eventually be built at each. But before you send in a deposit, get some valuable free advice by going back over this article and linking the first word of each paragraph. (Note: Those little black squares at the beginning of the story mark new paragraphs.)
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