October 15 is the day that the spotlight on the tip of stone b declares that the following year is a leap year.
A number of measuring stone implements were recovered from Iwaya-Iwakage in about 1968. The research team first had a look at them in February 2013. In February 2015, the functions of two of the tools were realized by Kobayashi and Tokuda.
The tool shown in the photo above was thought to be a measuring template for the leap-year spotlight of October 15 every four years.
Finally, the team could actually observe and record the event. The solar spotlight perfectly illuminated the tool on October 15 at 9:29 a.m. It was an exciting historical moment for all of us present!