Capture a shadow cast by the planet Venus

An indoor astronomy project with a huge payoff: capturing a shadow cast by the light of the brilliant planet Venus.

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. Credit: Pete Lawrence

Venus’s brilliance comes about because it is a planet covered in reflective clouds which is close to the Sun and Earth. A good challenge is to see whether you can capture something that has rarely been photographed or seen visually: a shadow cast by Venus’s light.

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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.

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For this to work you’ll need to be able to see Venus against an astronomically dark sky. This will occcur around 22:00 BST (21:00 UT) at the start of April 2020 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 + .

If you’re using the box option, ensure the target is rigidly fixed so it can’t move in a breeze.

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). Credit: Pete Lawrence
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). Credit: Pete Lawrence

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.

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Pete Lawrence is an experienced astronomer and a co-presenter of The Sky at Night. This article originally appeared in the April 2020 issue of BBC Sky at Night Magazine.