Ring of Time — Andrew M. Seddon

In the 27th century, humanity's greatest technological achievement is the massive, star-powered Temporal Displacement Ring: a portal to the past. Professor Robert Cragg, reeling from his own personal losses, volunteers to be the first-ever time-traveling historian, fleeing into the shadow of the Roman Empire. Instead of dry, dusty bygones, he encounters real people. Commoners and nobility, sailors and businessmen, zealots and legionaries, druids, gladiators and philosophers all cross his path. The past, he finds, is not dead and gone, but very much alive... alive with wonder, fear, and, perhaps, love...