How To See Your Venus Shadow

Venus dominates the western sky at dusk this month. Bob King

It’s easy to be a Venusaholic. The brilliant, pale yellow planet commands the western sky at dusk. On March 24th it will stand farthest from the sun at 46° 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

As Venus orbits the Sun interior to Earth’s orbit, its apparent distance from the Sun (and phase) changes. It’s currently way off to the left (east) of the sun from our perspective, one of the reasons it appears so far from it in the sky, and sets late. Wikipedia with additions by the author

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.

Because Venus is a point source, objects it illuminates cast sharp-edged shadows — the object either blocks the light from Venus or not. This is different from the sun which is an extended source. Imagine it as hundreds of individual point sources, each casting a slightly different shadow. Where the shadows overlap completely, they create a dark, inner shadow called the umbra. Where they partially overlap they create a penumbra or outer shadow. Imagine yourself behind the pink obstacle. If you’re close to it and in a straight line with the sun you’ll stay inside the umbra, and the sun will be hidden. But if you stray above or below the obstacle a little, it will only partially block the sun from your perspective. Light from the unblocked portion spills into the shadow and creates a gray, partial shadow called the penumbra — a mix of shadow and sunlight.  Bob King

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.

You can see the fuzzy-edged penumbra in this shadow self-portrait by Piqui Díaz of Buenos Aires. Notice that the farther the shadow falls from her, the softer and wider the penumbra. As the distance increases, her body covers less of the sun, and the penumbra enlarges. Piqui Díaz

Have you ever seen your shadow by Venus-light? It’s very cool. You need two things:

  1. 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.
  2. 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.
Can you see the difference? Venus-cast shadows on the left; sun on the right at the same distance from the poster board. Bob King

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.

Here’s my setup with board, stool and shadow-casting cast. Bob King

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 about a week of attempts before the moon interferes.

4 Responses

  1. embollemboll

    I’m now in on the Y4. If it was clear tonight I could get my first view. The way it has behaved so far suggest to me that this could be a good or even great comet. Perihelion puts it probably between magnitude minus 10 and minus 1. I am personally thinking minus 4, much as Venus would be. But there is the real possibility that it could flare to minus 8! If it does I hope it survives Perihelion. And if it gets that bright, even though it will be close to the Sun, I don’t have the exact coordinates could we see it at Perihelion or would it be too close to the Sun? If I am not mistaken it passes about 12 degrees north of the Sun? Comet Atlas shadow anyone?

    1. astrobob

      If it’s even 12° n. of the sun and -5 it will be visible in properly shielded telescopes in the daytime. Fingers crossed!

  2. Patrick

    Hi Bob! Thanks for continuing your updates during this difficult time. I have a question about the upcoming Venusian conjunction with the Pleiades on April 3. I read in S&T that Earth and Venus have an orbital resonance of 8:13, which is why Venus comes close to the Pleiades every 8 years, nearly to the day. Does this apply to all objects in the apparent path of Venus through the sky? In other words, on any given day when I look up and see Venus would I expect it to be in almost the same spot against the background stars eight years later?

    Thanks again for your posts and helping us all out during this time.

    1. astrobob

      Hi Patrick,
      Yes! Bascially. There is some drift due to other factors, but whenever you see Venus it will near the same location in the sky 8 years later.

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