Challenging though it was, filming the eclipse was in one sense easy: We knew when and where it was going to happen, and we knew the effect would be powerful. Endeavoring to make a 12-hour documentary on a subject that predates the invention of photography, and whose sources are written in an 18th-century vernacular, was in other respects a daunting mission. For our series on World War II and the Vietnam War, we could talk with living witnesses and access archival footage. We had tens of thousands of photographs from the Civil War. The American Revolution had none of those elements.