In Search of the Venusian Shadow - Part 2



Back in November 2005 I carried out a number of photographic experiments to see whether I could capture the shadow cast by the planet Venus.

Click here to view the 2005 Venus Shadow project page...

However, the project was not without controversy and a number of people doubted the validity of the results obtained, stating that the shadow could have been caused by the brightness of the sky.

At the current time (May 2007), Venus hangs like a brilliant orb in the west. Easily visible just after sunset, the planet dramatically brightens as the sky becomes dark, once again giving rise to an opportunity to capture its elusive shadow.

As I found out last time around, still frames of the shadow have been captured a number of times in the past well before my November '05 project became live (although I believe that the photographs of the shadow cast by my children's hands are probably the first record of a human shadow cast from the light of another planet - I'm sure someone will correct me if I'm wrong though!).

This time around the conditions for capturing the shadow were a little serendipitous and gave me the opportunity to do something new which I do believe is unique.

Serendipity

Our family house has a west facing wall. In the middle of that wall is a window which lays adjacent to a mini-landing at the top of the main stairs and at the bottom of another shorter flight of stairs leading to the main bedroom landing. One evening, I happened to be climbing up the main stairs in the dark and spotted Venus in my line of sight. Immediately above my head was a smaller wall the plane of which was parallel with the window.

As the house was in complete darkness, the only thing that could illuminate this small wall was the glow from the sky, a sodium street light in the road along the front of the house and Venus. Fortunately, the location of the street light meant that what little light from it did get through the window only illuminated part of another main wall at right angles to the small one that I was now intending to use as a screen. The first test shot was taken and a shadow was seen - but was it caused by Venus?

Then a simple experiment dawned on me that would prove once and for all that the shadow was indeed caused by Earth's brilliant twin. By taking a number of sequential shots on the same night, as long as the camera and shadow casting object remained stationary, the apparent movement of Venus in the sky should cause the shadow to move on the wall being used as a screen.

Shot Mechanics

The single test shot I took showed me that the shadow was pretty faint. For a good shadow to be recorded, I needed a 4 minute exposure at ISO1600 using a 28mm lens at f/3.5. This is quite a long exposure and Venus, moving at the sidereal rate, would move by 1 degree over this period. The net effect of the movement would blur the otherwise sharply defined shadow from Venus as a point source. In the end I deemed this acceptable because this time around I was only interested in the movement of the shadow over time rather than sharp definition. The animation, which is shown below, is the result of 5 separate frames. Each frame was exposed for 4 minutes with an interval gap between frames of 3 minutes. The frames were taken at the following times (UT)...

21h41m51s
21h48m51s
21h55m51s
22h02h51s
22h09m51s




The result is pretty conclusive and what's interesting is that the faint colour cast of the shadow also changes throughout the sequence. This was caused by Venus moving ever closer to the horizon and reddening as its altitude dropped.

If you're wondering what the shadows are...



Update: The animation below has been contrast stretched and resized to make it a little clearer.





The main target was chosen after a chat with the person that started me on the path to capturing the shadow cast by Venus - Sir Patrick Moore. I suggested the target and he agreed that it was totally and wholly appropriate for the task. If you've wondering what it is, then wonder no further. It's a Clanger that was made for one of my children by my mum!



For the technically minded, the Clanger was positioned 2.4m from the 'screen' wall.

Summary

The purpose of this simple experiment was to categorically prove that it was possible to capture the shadow of Venus in less than ideal conditions (in an urban UK twilight sky). Apart from the odd plane passing by (which was much too fast), there were no other light sources capable of generating the motion recorded other than Venus.

Pete Lawrence, Selsey, UK
17th May 2007