Outdoor robots equipped with vision sensors can gather a lot of information from the sky, say CMU researchers in a new CMU Robotics Institute technical report, What Do the Sun and the Sky Tell Us About the Camera? (PDF format). The authors examine only two pieces of information, the position of the Sun and the sky appearance. By combining this information with a physically-based model of the sky, they can deduce information about the camera and lens. This allows their system to work with images from almost any convention camera (such as the one above by law_keven). The researchers collected sky images from online webcams to use as test data. They also produce a camera-invariant model of the sky that can be used in future work to understand scene illumination. This research was done by Jean-François Lalonde, Srinivasa Narasimhan, and Alexei A. Efros.