Some experts are convinced that the Jomon people had some way of marking the passing of time. They used their knowledge of the movement of the celestial bodies such as the sun and moon, and certain stars in the sky. They noted summer and winter solstices and the spring and autumn equinoxes, to help them mark and predict the changes in the seasons and the increases and decreases in daylight hours and of food sources.
Some of the stone circles that were built by the Jomon people appear to have been monuments that were aligned to certain celestial bodies, and were likely used to help in the predictions. Experts suggest that the Jomon people were very aware of the changes that occurred througout the year, and had an annual calendar based on the seasons and perhaps the waxing and waning of the moon.
Sannai Maruyama was a trading center where people from all over Japan as well as from across the seas came to trade probably at an appointed time of the year. It is likely that such a calendar was used by the prehistoric peoples in various places to help them congregate to trade at a location and at specific time of the year.
Take a tour at the Kanayama megaliths (which is comprised of 3 main megalith sites) website to understand how the solar calendar works in conjunction with the megaliths. The rising sun at the summer solstice illuminates the first site, in way that is similar to the Heel Stone of the Stonehenge in the U.K. At the third megalith site, the rising sun of the winter solstice illuminates the site.
60 days after the winter solstice takes place, the way the sun rising over the mountain over the third site, as well as the way it sets over the face of the rock of the second site, is thought to function as a solar calendar similar to the way the sun’s light is seen in the chambers of the Giza’s pyramids, marking the sowing and growing period of ancient Egyptian solar calendar.