The Sky Guide Challenge
Capturing the shadow of Venus
From Earth, Venus appears intensely bright shining away in the evening or morning twilight sky. If you catch it right, it can also be seen against a dark sky, but the conditions for this can be tricky to achieve. If the sky is dark enough, the intense light from Venus can do something obvious but still rather amazing – it can cast a shadow. This month’s challenge is to photograph the shadow cast by Venus.
Venus’s light may be intense, but there’s not much of it. Consequently, any other light will swamp it completely. One way to overcome this is to create an environment where the light from Venus is dominant. One way of doing this is to make use of a room or an area inside a house where there’s a window facing in the correct direction and which will let light from Venus enter and fall on a building wall. If this isn’t feasible, another technique is to create an artificial ‘room’ – say with a cardboard box – that can be positioned in such a way that a single opening points at Venus. Both methods require a target that will be used to make the shadow. This can be anything you like, such as a shape cut from card attached to a window or, in the case of the cardboard box, suspended in the opening. The shape we opted for was the symbol for Venus.
Finally, the ‘screen’ wall will work better if it is smooth and white. A piece of white
paper or card is ideal for this. This can be temporarily attached to a wall or stuck to the surface of the box opposite the opening.
To show the shadow at its best, a good technique is to create an animation of the shadow moving as Venus appears to set. A DSLR camera on a tripod is good for this, but any camera that allows a high ISO and multi-second exposure should work too.
Set everything up on a clear evening. Imagine where the light from Venus will be falling and casting the shadow – basically a straight line from Venus through the target and onto the screen. Set a high ISO, fully open the lens and aim to use an exposure of 10–30 seconds. Focus on the screen carefully. You may want to temporarily illuminate it to do this accurately.
Take the first image. Wait one minute and then repeat. Flicking between the two images should be enough to reveal the weak shadow. The shadow is incredibly sharp because Venus is effectively a point source of light. However, the longer your exposure, the fuzzier the shadow will become because its edges are essentially moving.
Shadow An enhanced image of the shadow cast by Venus onto the internal wall of a house
The target for our shadow image was the symbol for Venus cut out of card. Distant trees also created a mottled effect in the final animation