In an unscientific era when maps were rarities, how did ancient Romans envisage their far flung empire? This was done by various means for certain, including with the aid of an ingenious type of portable sundial that has barely attracted notice. As the Romans understood before the first century BCE, to track the passage of the sun across the sky hour-by-hour one needed to know one's latitude and the time of year, and that, furthermore, sundials did not have to be fixed objects. These portable instruments, crafted in bronze, were adjustable for the changes of latitude to be expected on long...