Saturday, September 14, 2013

Sharp shadows...  I was looking at some of the recent photos from the Curiosity rover on Mars, and noticed the shadow of the rock looked extraordinarily sharp.  The apparent diameter of the sun causes a fuzzy edge to shadows (the “penumbra”), and there wasn't much of one there at all.  But the apparent size of the sun on Mars is substantially smaller than it is on Earth.  So I did the math: from Earth, the sun's apparent diameter is about 32 arc-minutes, while from Mars it's about 21 arc-minutes.  That's enough to make shadows visibly sharper on Mars than they would be on Earth.  It's also enough to make the sun look considerably smaller...

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