Google Earth and Google Earth Pro are free applications (they require local download and installation) that provide viewing access to historic aerial photography in the United States and around the world via the ‘Show historical imagery’ button, which looks like a clock with a green arrow around it. The ‘Show historical imagery’ button launches a time-slider with tic-marks for each date of available imagery. Historical imagery in Google Earth is limited for some locations although Google has assembled an impressive global collection of historic aerial photographs. For a good example of the capabilities of the ‘Show historical imagery’ button, visit some of their example applications (e.g. Beijing, Las Vegas, London, Tacoma Bridge, and Warsaw).