In Search of the Venusian Shadow



When I was a young boy, I recall reading a book written by Patrick Moore in which he mentioned the fact that three normal (excluding bright fireballs and perhaps artificial satellites) bodies in the night sky were capable of casting a shadow on Earth*. The Sun and Moon are pretty obvious but it was the third that fascinated me - Venus. Last year (2004), Venus captured everyone's imagination with a spectacular transit across the Sun's surface. Technically, during such an event, we, on planet Earth, are looking at the shadow of Venus as the planet blocks a tiny portion of the light from the Sun.

* Apparently it's also possible to see a shadow cast by the light of Jupiter and Mars.

2004 Transit of Venus

However, this was not the shadow I was interested in. In extremely dark locations when the air is clear, Venus is close to maximum brightness and reasonably high in the sky, it is possible to see a shadow projected onto the ground caused by the light from the planet itself. This itself has been noted several times in the past.

Quite by chance a couple of months ago I found myself in Patrick's home talking to him about an imaging project I was intending to undertake next year. The conversation turned to things that had rarely ever been photographed and he told me that as far as he was aware there were few decent photographs of a shadow caused by the light from Venus**. So the challenge was set...

** Since posting this page, a couple of photographs have come to light including two by Brian Manning (BAA Journal June '03 p.177). However, to date, I'm still unaware of any that show the shadow of a human being cast by Venus.

The first problem for me to overcome was ambient light. I'm lucky enough to live on a peninsular with beaches to the east, south and west of me. This provided a perfect venue to make my attempt. On Friday November 18th 2005, after many of failed attempts, I stuck a sheet of white A4 paper against a concrete sea wall sheltered from any stray light and took the photograph shown below.



 

Ok - it doesn't look that impressive and to be honest, when I reviewed the night's catch, I nearly placed it in the failed pile. However, closer scrutiny revealed something odd about the photo. If you look at the paper behind the tripod, there's a distinct sharp edged shadow visible. It's faint but it is there!

Apart from a couple of boats on the distant horizon (which were much fainter than Venus at the time), the only light source that could have cast the shadow was Venus itself.

The photograph below shows a comparison of the scene illuminated by Venus and with me standing in the way of the planet's light.

 


To be a bit more scientific about the process, I designed a piece of equipment that would prove, once and for all that I had managed to capture this faint shadow. Basically, what I needed to do was to remove all possible sources of extraneous light, capture the shadow and prove that it was indeed a shadow cast by Venus itself.

My equipment consisted of a long cardboard tube mounted on a tripod (so that it could be pointed easily). An obstruction was created and pasted inside the tube. For effect, I decided to make the obstruction in the shape of the astronomical symbol for Venus as shown below.



The tube would be pointed at Venus, the light from which would pass down the tube and out the other end except where it was blocked by the shadow obstruction. A thin sheet of tissue was placed over the bottom end of the tube and a camera set up to capture whatever was shown on it.

On this night, I was assisted by my children Richard and Douglas who, as well as carrying my equipment down to the beach, were also able to verify my technique.

With everything set up, the only light that could cause an image of the obstruction to appear on the tissue screen would be that from Venus and/or from the background sky. In order to rule out the background sky, I took two control images - one with the tube pointed at the area adjacent to Venus but making sure that the brilliant planet's light did not enter the tube. The other (a shot I nearly forgot to take!) would show the shadow cast on the screen making sure that the orientation of the camera and tube was such that the open end of the tube was not behind the tube.

The photograph below shows the screen photographed once with Venus visible down the tube and once making sure just the adjacent sky was visible. Rather embarrassingly, the orientation of the tube was such that the symbol was upside down and was consequently imaged looking more more like the symbol used to represent a church on an Ordnance Survey map than that used to denote Venus! To regain some degree of authenticity, the image presented below has been rotated by 180 degrees!

 


 
  The image below shows the setup in-situ. The course semi-circular area has been taken from the same photograph and digitally stretched to reveal the faint shadow. Although the sky looks relatively bright in the image, it should be noted that the exposure was ~ 1 minute with the camera set at it's most sensitive configuration. The brilliant light visible just above the tube is a rather over-exposed Venus.

 
 

 
  The final image shown below was nearly forgotten. As a consequence, it was taken late on towards the end of the session when Venus was starting to move into the murk lurking on the south-western horizon. Visually, the planet's light was no longer brilliant white but had taken on a distinctly yellowish hue.

As a consequence, the image has had to be pushed hard to reveal the shadow. Selsey doesn't really have yellow sea and shingle - that's caused by the light from a sodium (hence the yellow-orange) street lamp positioned some distance away at the end of the road we used to access the beach. We had carefully positioned ourselves so that we were hidden from the light itself but the camera recorded it's illuminating effects out to sea. It should be noted that this lamp could not influence the experiment as there was no possibility that it's light could enter the open end of the tube.

What is being shown here is a faint (due to Venus moving into a thicker part of the Earth's atmosphere and the Venusian light dimming) shadow seen on the tissue screen at the end of the tube. The camera has been deliberately placed so that a line drawn from it directly through the screen could not 'see' the open end of the tube. The shadow as recorded could only have come from the one light source entering the tube - Venus.

 
 


 
  On the night of November 20th 2005, the sky was particularly clear after sunset. With the shadow tube results already captured, my children and I decided to have one last attempt and see whether we could capture their own shadows. As it happened this turned out to be much harder than expected as it's difficult for a cold human being to stand still long enough for the amount of time needed to catch the faint Venusian shadow.

The photo below shows the scene that greeted us when we arrived at the beach.

 
 






 
  The only human shadows that we could capture on this night were those of my children's hands. Even then, the slightest movement destroyed the distinct sharpness of the Venusian shadow (the hand on the right in the top photograph below had moved slightly during the exposure and its shadow is almost lost as a consequence). The hand shadows are just about visible in the images below...

 
 




 
  The problems we were having recording this faint shadow were evidently being aggravated by the fact that as the planet's altitude drops quite quickly, it enters a region of thicker, more murky atmosphere which attenuates some of its light. The photograph below was taken earlier in the session and by virtue of the fact that Venus was higher in the sky and that tripods can stay still for long periods of time, the shadow is much easier to see.

 
 


 
  Finally, we called it an evening and packed up for home. It was extremely cold and Venus was fading rapidly as it moved towards the horizon. A final shot was taken to close the session (including uninvited aircraft!) which is shown below.

 
 


 
  In conclusion I'd like to thank Sir Patrick Moore for reminding me to take the photograph his original text had inspired me to follow all of those years ago. As far as I can tell the human shadows are a world first but I'm sure someone will inform me if they aren't! I'd also like to thank my children - Richard (14) and Douglas (12) - for their patience and assistance on a cold, damp November evening on a Selsey beach. Thanks guys.


(c) Pete Lawrence, 2005 (DigitalSky website)