'I'm always looking for the Hows and the Whys and the Whats,' said Muskrat, 'That is why I speak as I do. You've heard of Muskrat's Much-in-Little, of course?'
'No,' said the child. 'What is it?'
- The Mouse and his Child. Russell Hoban.

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Tuesday, 19 August 2008

Sun and Shadows

Here is a satellite photograph of Desolation Canyon.  Like all canyons the river runs at the bottom of the canyon.   The sun is striking the canyon walls from the lower side of the photo. But can you see the deep canyon?  And the shallower valleys on the right?

Or are you like me and it looks like the river is  flowing somehow right along the crest of the hills, and this is not two canyons at all but two ranges of mountains...

Now look below at the photo rotated 180ยบ.  Why does it suddenly look correct?  It's exactly the same photo.  

The answer is the position of the shadows.  When we survey our terrain, our brains expect the sun - the light source - to be above us.  If the light source is below, our brains will reverse the depth information, even to the extent of running rivers along the tops of mountain ranges.  We cannot adjust.  Our brains say the sunlight cannot be coming from below.

I'd be interested to know if the same phenomenon occurs for astronauts when they view the earth from 'upside down'.

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