1. Click on the clock in the Toolbar. You'll see a time slider appear with tick marks wherever there is historical imagery in Google Earth for the area you're viewing.
(Note: the further zoomed in you are and the less tilt you have, the fewer tick marks you'll see.)



The time slider contains several tools that allow you to view archived historical imagery.




2. Try it out for yourself! Zoom into San Francisco.

3. Drag the time slider to the left to view imagery from various time periods, such as:

    

See how the city has changed over time!