Venusian Clanger

Venus can be a very bright object in the night sky and you can even see it during daytime. It has long been suggested that it is so bright that you can see the shadow cast by it. A couple of years ago Pete Lawrence took some images on a dark night by the sea showing reflections of Venus from the water and shadows cast by various objects and even his kids! Those pictures are pretty impressive but some people thought the shadows may simply be due to the brightness of the sky. Now, via Chris Lintott, I see that Pete Lawrence has done another fantastic experiment to show that it is possible to capture a shadow cast by the planet Venus.

Pete had the great idea to document the change of the shadow over time as Venus set in the west. This is brilliant because any shadows cast by fixed objects, such as lamp-posts or neighbours lights, will always be in the same direction. However, shadows due to objects in space would move due to the rotation of the Earth. This principle is exactly how you tell the time with a sundial. Pete has taken that idea and made what you might call a Venus-dial using a Clanger as a gnomon (the gnomon is the bit that casts the shadow and can be any object you like - even a person from Lincolnshire).

He took the resulting images, each spaced by seven minutes, and has made a fantastic animation (680kB). The advantage to an animation is that the eye can easily pick out the movement of the shadow even though the shadow may be difficult to see in any one frame. Also, in this case, the rate of the movement tells us if it is astronomical in origin and with no other bright objects in that part of the sky, it must be due to Venus (you can compare the direction of the shadow too). In the animation you can also see that the shadow of the windowsill gets higher on the image as Venus sets in the west.

This was a really simple experiment but with a great result. Great job Pete.

Posted in astro blog by Stuart on Friday 18th May 2007 (21:15 BST) | Permalink
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