““We do not know who realized that there was an element missing but we owe whoever it was a great debt. We can imagine an astronomer, perhaps in Babylon or ancient Egypt, suddenly realizing that there were not just two periodic motions to consider but three. Perhaps it was a sage, who after decades of study knew the data by heart. Perhaps it was some young rebel, not yet brainwashed into thinking that you had to explain what was seen only in terms of observable objects. Whatever the case, this innovator uncovered a mysterious third oscillation in the data, occurring not once a month or once a year but approximately every eighteen and two-thirds years. It turns out that the points where the two paths cross on the sky are not fixed: They rotate as well, taking those eighteen-plus years to make a complete cycle.””
![“[In the Field Museum of Natural History] we could see…” quote by Warren MacKenzie](https://sms4smile.com/quotes/img-quotes/8fc/warren-mackenzie-4654_ancient_in-the-field-museum-of-natural-history-w_183538.jpg)
