Rising positions of stars change very slowly over the centuries, due to the Earth's precession, so the directions of these cairn pairs can be used to project at what date they aligned best with the rising points of these stars. The FA Aldebaran alignment would have worked best between AD 1200 and AD 1700. Further, precession changes the date of first helical rise: Although today the first heliacal rise of Aldebaran is a few days after the summer solstice, between AD 1200 and 1700, the first heliacal rise of Aldebaran would have been just before the summer solstice, allowing an observer to predict the coming of this event.