Shadows on Mars...

Shadows on Mars...  I was browsing through some recent photos from the Curiosity rover on Mars, and this one caught my eye.  There are two places in the photo where you can see a sharp shadow of a rock.  It's the shadows that I found interesting, precisely because they're so sharp – very little of the penumbra (the fuzzy edge) that we see with shadows here on Earth.  Why is that?

It's because Mars is further from the sun than Earth is, and therefore appears to be smaller.  Penumbra happen because the source of light isn't a perfect point, but instead is a circle of perceptible size (you see the sun as a bright circle in the sky, not a single point).  The sun's angular size from Earth is about 0.5° – and from Mars it's about 0.35°.  That means that the sun, as seen from Mars, is just over half the size in the sky of the sun as seen from Earth.  That means the penumbra of shadows on Mars are just over half the size of the penumbra of shadows on Earth.  Much sharper!