The Sky Guide challenge

Try and capture a shadow cast by the light of the brilliant planet Venus

Venus is currently unmistakable in the evening twilight after sunset. At the start of April, it’s above the horizon nearly five hours after the Sun goes down. As the sky reaches true astronomical darkness, the appearance of this mag. –4.3 planet is really something to behold.

Venus’s brilliance comes about because it is a planet covered in reflective clouds which is close to the Sun and Earth. This month’s challenge is to see whether you can capture something which has rarely been photographed or seen visually, a shadow cast by Venus’s light. Although Venus is quite brilliant, its delicate shadow is easily lost due to extraneous lighting. Creating a dark environment to isolate the shadow is quite hard and requires thought.

Try and pick a night when the sky is clear and the Moon is not about.

You’ll also need a relatively flat west to northwest horizon. For this to work you’ll need to be able to see Venus against an astronomically dark sky. This occurs around 22:00 BST (21:00 UT) at the start of April and 23:15 BST (22:15 UT) at the month’s end. A room with a west-facing window is ideal, but if one is not available, the next best thing is a cardboard box with one open end.

A shadow viewing screen can be made

out of sheets of white paper fixed to a wall or used for lining the inside of a box. The shadow casting target is your choice; a cut out of the word ‘Venus’ or perhaps its

planetary symbol ǵ. + If you’re using the box

option, ensure the target is rigidly fixed so it can’t move in a breeze.

Next, unless you’re in a really dark environment with crystal-clear skies, a camera will be required to record the shadow. A DSLR or MILC camera is ideal. Set to a high ISO, a low f/number and use a remote shutter release to avoid camera shake. A tripod allows you to point the camera easily. Illuminate the screen before your attempt, manually focusing on it as accurately as you can.

Do a test exposure of a few seconds up to tens of seconds. You may need to stretch the image using a photo editor – open levels and adjust the sliders to just encompass the histogram peak – to reveal the shadow. A useful technique is to make a time-lapse. As Venus sets, this reveals the shadow slowly creeping up the screen. The non-shadowed area may change colour as the atmosphere makes the planet’s light slightly redder as it approaches the northwest horizon.

For this to work you’ll need to see Venus against an astronomically dark sky

A view out of the window with the shadow-casting Venus symbol attached to the glass. Venus’s light had to pass through tree branches on the way

Edge of the window frame Tree branches Venus symbol As Venus is essentially a point source, the resulting shadows all appear sharp when cast on the wall. (Camera settings: ISO 12,800, 20” exposure at f/5.6)