As any observer will tell you, the atmosphere is a very messy thing. Even on nights that seem very calm and crystal clear, wind currents, temperature changes, and other structures in the atmosphere cause any image to blur. Many astronomers consider this blurring (or "seeing") to be acceptable if the size of the blurry spot made by a star is less than one-half to one arcsecond. (One arcsecond is 1/3600th of a degree, or the thickness of a small finishing nail seen from a mile away.)
During the day, the seeing is usually much worse than one arcsecond, as heating from the sun sets up currents in the atmosphere.
This is what makes the above picture so amazing. It was taken by Thierry Legault in Normandy, France, and shows the Space Shuttle Atlantis leaving the International Space Station and silhouetted against the sun. You can clearly see the shape of the space shuttle and the station, even though they were 550 kilometers (345 miles) away! I did a quick calculation, and I estimate that the width of the shuttle's cargo bay is about 2 arcseconds in this image, and yet you can see even sharper details!
Some astrophotographers now use video cameras to create sharp images. Although the atmosphere is constantly roiling, there are occasional, very short periods of very steady seeing, much better than an arcsecond. These astrophotographers look through thousands of frames of video for the handful that are crystal clear, and add these together to get a very sharp image.
Thierry Legault didn't have that option, though, because the space station only took a little over half a second to cross the sun. So, Legault just took some very short exposures (1/8000) of a second. Such a short exposure "freezes" the atmosphere, which changes on the order of every hundredth of a second. That, with a little patience, preparation and luck, and you get a nice clear picture of two objects 550 kilometers away.
Thierry Legault has several more very impressive pictures on this website.