Google Earth in 4D from Googling Google is a very nice catch about how Google Earth now allows you to view historical maps within the software. Want to see how people thought the world used to look, before all those satellites were taking pictures? A new layer makes this possible.
For this to work, you have to have the latest version of Google Earth, Google Earth 4 (which will kindly change your default search engine in Internet Explorer to use Google, unless you untick that box -- so pay attention if you don't want that change. For more on Google being worried about changing IE's search defaults, see my Searching Via Internet Explorer 7 & The Battle To Be The Default Search Engine post).
Next, within Google Earth, you need to activate the Rumsey Historical Maps feature. Google gives instructions here. The screenshot below also shows what to change:
Select a map, and it's placed over the existing world. For example, here's the Lewis & Clark map:
You can then zoom in for more detail, though I wish it were easier to blend the real world more with the historical map. Terrain boundaries, along with roads if you switch them on, are shown in a hard-to-see yellow. Even better would be if actual satellite images could some how be lightly ghosted behind the maps.
Here's a close up on the Lewis & Clark map:
Here's part of San Francisco from 1853:
Sadly, one of my favorite old maps, that of California as an island, isn't offered. Not to fear! The David Rumsey site, which provides maps to Google Earth, allows you to browse a much wider collection of maps. You don't need Google earth to do so, either. Of course, you can't then overlay the maps on to of current plots of geographic features and roads. Still, it's well worth a visit. Here's an example of California as island, that I was talking about:
Postscript: Garett Rogers from Googling Google sent how to get the underlying "real" world information to show through. Look at the Places sidebar. You should see an unnamed slider bar at the bottom. Don't see it? Try minimizing that window, then reopening it. Still don't see it? Go to the Layers window and select the map you've enabled. Then click off that to any other map, then quickly click back to the map. That should make the slider appear (I didn't see it at first because of this bug). Move the slider to the left slowly and the underlying "real" information will start to bleed through.