It’s easy to be a Venusaholic. The brilliant, pale yellow planet commands the western sky at dusk. On March 24 it will stand farthest from the sun at 46 degrees and sets late, well after 10 o’clock for skywatchers in mid-northern latitudes. Venus is prominent because it’s approaching its greatest apparent distance from the sun from our perspective (see diagram below) and also occupies the same space the sun will as it approaches the summer solstice.
The planet is illuminating in other ways. Did you know it’s bright enough to cast shadows? Venus is a point source compared to the sun which is an extended source. One is a point of light, the other a disk. That affects the kinds of shadows they cast. Look at your own shadow on a sunny day, and you’ll observe that it has a soft, diffuse edge, not a hard, sharp outline. Tree and building shadows are the same. Your body blocks part of the sun but not all of it. Light from the unblocked parts spills into your shadow and softens and grays its borders, creating a partial shadow or penumbra.
Venus is very different. Your body either blocks it 100 percent or it doesn’t. There’s no “spill” around the edges like the sun. That’s why Venus shadows appear crisp, as if cut with a laser. Your body hides the planet completely. Another way to compare a point to an extended source is to think of the sun as hundreds of point sources, each casting its own narrow beam. When all these point-source shadows overlap, there will be total darkness (the shadow’s umbra) surrounded by a lighter region called the penumbra where only some of the individual shadows overlap.
Have you ever seen your shadow by Venus-light? It’s very cool. You need two things:
- A dark-sky site with little light pollution. It’s doesn’t have to be perfect, but the darker the better. Start watching about 2 hours after sunset at the end of twilight. Remember: When Venus is high your shadow is low, and when Venus is low your shadow is closer to eye level.
- Either a white bedsheet you can drape over the side of your car or a piece of white poster board you can attach with duct tape.
I checked this out about 2 weeks ago using both a bed sheet and poster board. Once my eyes were completely dark adapted I stood with my back to Venus and faintly saw my shadow on the sheet. Playing around I discovered that if I turned at a right angle from Venus — with the planet now off to one side — and extended my arm in front of the sheet the shadow was MUCH more obvious. To make the shadow more obvious move your arm around. Our eyes’ rod cells are good at detecting things that move in the dark.
I also set up a tripod and brought along a vase and a wooden letter X, set them on a stool and photographed their shadows on the poster board to demonstrate how different they are from the shadows cast by these same objects in sunlight at the same distance from the car.
Find a dark place the next clear night and see if you can spot your Venusian shadow! You’ve got a couple days of attempts before the moon interferes.