Complex sentence: In addition to the other islands of the Central Group, Graciosa was explored by Portuguese navigators during the first quarter of the 15th Century, although there exists no accounts of the first sighting of the island.
Split into simpler sentences: In addition to the other islands of the Central Group, Graciosa was explored by Portuguese navigators during the first quarter of the 15th Century. Although there exists no accounts of the first sighting of the island.