An early attempt to analyse the positions of the Aubrey holes was undertaken by Gerald Hawkins, a professor of astronomy at Boston University in the 1960s, using an IBM 7090 computer. In his book "Stonehenge Decoded", Hawkins argued that the various features at the Stonehenge monument were arranged in such a way to predict a variety of astronomical events. He believed that the key to the holes' purpose was the lunar eclipse, which occurs on average about once a year on a 346.62 day cycle. Lunar eclipses are not always visible as the moon may be below the horizon as it moves across the sky, but over 18 to 19 years (18.61 years to be precise) the date and position of a visible eclipse will return to its beginning point on the horizon again. As the motion of the moon's orbit also causes it to work its way across the sky on an 18.61 year cycle in what is known as the journey between major and minor standstill and back, the theory that this period was both measurable and useful to Neolithic peoples seemed attractive.