Huge quantities of information are never more than a few clicks away on the Web, but it's not always easy to see what things were like yesterday. News stories and blog posts might be archived, but other information often gets lost. For instance, while it's trivial to find a book's sales ranking on Amazon today, it's less simple to see what it was last week. And for anyone curious about how news evolves, it might not be obvious how a story's prominence has changed--did it get top billing on news sites the day it broke, or was it buried at the bottom of the page? A new tool called Zoetrope is designed to help track such information by letting users browse backward through time.

Other projects, such as the Internet Archive, already preserve historical versions of websites. But Mira Dontcheva, a research scientist in the Advanced Technologies Lab at Adobe Systems, where Zoetrope was developed, says the new tool makes it much easier to browse through this kind of data. "Having access to temporal information can help us come up with more compelling stories of what's going on around us," she says.

A user can peer back in time through Zoetrope in several ways. Simply pulling a scrollbar at the bottom of the browser winds a Web page back to show what it looked like hours, days, or months ago. Or, if the user is interested in one specific piece of information, like the price of a certain product, he or she can draw a "lens" over that area of the page to see how it changes.

An experienced user can perform even more-advanced analysis. For example, configured correctly, Zoetrope will recognize a price as it goes up or down and will show the results as a graph. It's also possible to draw lenses on different websites and sync them in order to carry out a historical comparison. For example, a user could use one lens to track weather information and another lens to track movie-attendance figures. Looking at how both lenses change over time might reveal a correlation between bad weather and high movie turnout. Zoetrope can also track some pieces of data as they move about a page over time.