The distance from the Old North Church in Boston to Charlestown is approximately 1,410 meters . Even on fast horse , that distance would take several minutes to travel . On April 18 , 1775 , lanterns were shown from the steeple of the Old North Church across the Charles River to warn American patriots that British soldiers were travelling Inland via water . The speed of light is approximately 3 x 10 to the power of 8 meters per second . How many seconds did it take for the light to be visible in Charlestown ?