The Earth rotated about its axis once every 24 hours, plunging about half of the globe into darkness and illuminating the other half at any one time as the Sun comes in and out of view. This is what causes night and day.
Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.