SHADOW-CASTING WITH VENUS

Play a shadow game with Venus before the Moon returns on April 24th.
Venus
Venus is the focal point of the western sky these April evenings.
Bob King
Venus has been a powerful presence at dusk for months — bright enough to cast shadows. How would you like to put it to the test? The Moon won't spoil the darkness until at least April 24th — April 25th if you wait till after moonset — making it the perfect time to see your own shadow by the light of another celestial body.
To make a successful shadow sighting you'll need two things. First, a reasonably dark viewing site without direct artificial lighting. Your window of shadow-casting begins at the end of evening twilight — about 2 hours after sunset — and ends shortly before Venus sets around 11:30 p.m. (a little earlier or later depending on your location). Second, you'll need something white against which to project your shadow. A large piece of posterboard duct-taped to a car window works well. You can also use a white bedsheet draped over the top of your car and held in place with a couple of boots or other household items.
Shadow comparisons
An object (lilac circle) completely blocks the light from a point source like Venus, creating a sharp-edged shadow boundary. Part of the light from an extended source like the Sun spills into the shadow's edges because the object incompletely blocks the source.
Bob King
For shadow-casting purposes, Venus acts more like a point source of light, like a star, compared to more extended sources such as the Sun or Moon. Each object casts a different shadow. Observe your own shadow stretching across the street the next sunny day, and you’ll notice that it has a soft, diffuse border. Your body blocks part of the Sun but not all of it. Light from the sun's periphery spills into your shadow and softens and grays its borders, creating a partial shadow or penumbra.
Penumbral and umbral shadows of a person
The fuzzy penumbra created by an extended source (the Sun) is clearly visible in this shadow self-portrait. Notice that the farther the shadow falls from the subject, the softer and wider the penumbra.
Piqui Díaz
At foot level the penumbra is absent, or nearly so, because the distance between the shadow and your shoe is mere inches, making your shoe very effective at blocking the entire Sun. But as the distance increases between your body and its shadow, less of the Sun is blocked and more light filters around the edges. In exactly the same way, Earth's penumbral shadow expands the further behind the planet you go.
Venus is different. Your body either blocks it 100% or it doesn’t. There’s no “spill” around the edges like with the Sun. That’s why Venus shadows appear laser-crisp. 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 penumbra where only some of the individual shadows overlap.
Shadow-casting setup
My setup with posterboard, stool, and shadow-casters.
Bob King

SHADOW PLAY

Armed with bedsheets and posterboards, I drove to a reasonably dark site in my quest for Venusian silhouettes and photographic evidence. First, I set up a "studio" (above). I taped the posterboard to the Venus-facing side of my car and placed a vase and a wooden "X" on a stepladder. 
Next, I  took photos of the two objects casting shadows onto the board. Once my eyes were fully dark-adapted I spread out the bedsheet and stood with my back to the planet. Sure enough, there it was — I could see my shadow! Because the rods of our eyes — the light-sensing cells responsible for seeing at night — are good at detecting motion, my silhouette was easier to see if I swayed from side to side.
Playing around, I discovered that if I pivoted at a right angle to Venus — with the planet off to one side as I faced either north or south — and extended my arm in front of the sheet, the shadow became much more conspicuous. Later, for comparison, I photographed the same items in full sunlight to demonstrate the difference in the quality of the shadows.
Difference in shadows
Can you see the difference? Shadows cast by Venus are on the left and by the Sun on the right, with the items placed at the same distance from the posterboard.
Bob King
Find a dark place the next clear night and see for yourself. To photograph your shadow, set the ISO to 8000 and expose wide open (full aperture at f/2.8 or f/4) for 20 to 30 seconds. To focus in the dark, tape a piece of dark paper to the bedsheet or posterboard and temporarily place your lens on the AF, or autofocus, setting. Shine a bright flashlight onto the sheet or board and include the piece of paper so that there's contrast in the field of view for the camera to catch focus. Switch the lens back to M (manual) and take the picture. And remember — when Venus is high shadows fall closer to the ground; when low, they stand higher up on your shadow-catcher.
Point source self portrait
Point-source self-portrait by the light of Venus using a sheet draped over the side of my car on April 10th. Details: ISO 8000, f/2.8, 25-second exposure.
Bob King
On the evening of April 25th the crescent Moon will shine at magnitude –7.9 some 10° below Venus shining at magnitude –4.7. I wonder if it might be possible to photograph the shadows of both offset from one another? A mind-bending challenge!


No comments:

Powered by Blogger.