Yes, you can work out when a satellite picture was taken. Don't worry about the satellite's orbit; focus on length of shadows and things like that. With earth images, it's made a lot easier if there are some good well-known landmarks in the picture. Buildings under construction can be a good tipoff too, but that might be cheating. <img src="/images/icons/wink.gif" /><br /><br />Calculating the lenght of a coastline is definitely done. There's a whole specialty devoted to photographic analysis of these sorts of things. It's done for surveillance, of course, but mainly for cartography (mapmaking). It's hugely valuable information. They have to know where the satellite was relative to the ground when it took the picture -- it's best if the satellite is imaging stuff directly underneath itself, so the curvature of the Earth doesn't mess up the situation too much.<br /><br />Another way of using satellites for cartography is to put GPS devices at key locations -- the GPS satellites will give you their locations to within a known margin of error (varies depending on the device and how many satellites it can acquire at the time), and you can use those known positions to work out distances and things like that. GPS data combined with satellite photography has given the world the most accurate maps ever made. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em> -- The Tenth Doctor, "Blink"</font></p> </div>