San Diego light pollution making Venus-shadow observations tough

The cloud-mantled planet Venus reflects 76 percent of the sunlight falling on it. Less than one-billionth of the light that bounces away is intercepted by Earth. Most strikes Earth’s day side, but some falls on a crescent-shaped slice of Earth’s night side. Anyone inside that crescent may see — as we now see after sundown — Venus hanging lantern-like in the sky, far brighter than any other planet or star.

On a February evening some years ago, less than one-billionth of all the Venus-light shining on Earth was softly, almost imperceptibly illuminating the landscape around my desert campsite. A faint, sharply defined shadow, cast by Venus, moved as I moved across the white sand. When it was time to crawl into my sleeping bag, Venus had sunk to ridgeline in the west. I noticed a gossamer gloom advancing from the same direction. The leading edge of the shadow crept across the sand, jumped upon my Mylar ground cover, and climbed up my face. In two seconds, Venus faded and vanished from sight.

Right now, as then, the celestial circumstances are ideal for stalking the Venus-shadow. For the next three weeks. Venus stands high in the western sky at dusk and remains visible for the next three hours. Greatest brilliancy occurs on February 24, when Venus will be almost as bright as it ever becomes in the Earth sky.

Seek out a very dark location in the mountains or the desert, far from any kind of artificial light. Bright surfaces work best; if you’re not on light-colored sand, spread a white sheet on the ground. Venus-shadows are sharp – not like the mushy shadows cast by the sun or moon- because Venus itself is practically a point source of light. A Venus- shadow cast by a mountaintop one-quarter mile away has a fuzzy edge only about one inch wide. A sun- or moon-shadow at the same distance has a fuzzy edge about 100 times wider.

Credit for our ability to discern Venus-shadows goes to the retinal rod in our eyes, which get remarkably sensitized under truly dark skies. The dark skies last until February 23, when the moon reappears in the evening sky as a very thin crescent below Venus. On the 24th, the slightly thicker crescent moon will lie very close to Venus, outshining it by a factor of 13.