Kanayama Megalithic Solar Calendar

Kanayama Megalithic Solar Calendar

Six-season chartThe ancient people of the Japanese archipelago called their land Hinomoto (Hi-no-moto), the Essence of the Sun. The Kanayama Megaliths are located in the mountains region called Hida, the Land of the Sun. The Jomon people who constructed the Kanayama Megaliths were sun-watchers primarily and star-watchers partially. It is difficult to have a good view of the night sky when one is in a mountain forest. Instead, the Kanayama astronomers focused on acquiring careful knowledge of the movement of the sun in the sky. The Japanese term is Hinomichi (Hi-no-michi), the Path of the Sun.

The Jomon sun-watchers developed a highly sophisticated megalithic system for accurately following the Path of the Sun.

The Kanayama Megalithic Solar Calendar took into account the 365 full solar days in a solar year. Plus they included leap-day corrections on a four-year leap-year cycle as well as a longer 128-year leap-year cycle. Therefore, this calendar is supremely accurate to one day in 51,000 years. Compare this with our modern Gregorian calendar which will accrue a one-day error after only 3236 years. Since 435 years have already elapsed, there are only 2801 years left before we will encounter the one-day error. On the other hand, while the megaliths may have been functioning for 5,000 years already, they still have another 46,000 years to go.

sine function

The Kanayama Calendar is based on the Jomon’s knowledge of Simple Harmonic Motion or SHM. SHM is what describes the periodic motion of pendulums, playground swings, and the sun in the sky over the course of the year. (Hint: it is a sine function.) The solstices (solstice means the standstill of the sun) occurs at winter and summer when the sun’s noonday position in our sky seems to change little day by day. On the other hand, at time of equinoxes, spring and fall, the sun’s noontime position changes rapidly in the sky. All sunwatchers know this. They try to form their own calendars by observing the sun’s position at the two solstices (6/21 and 12/22) and the two equinoxes (3/21 and 9/23). However, it is extremely difficult to know just when the sun is at solstice, by the very fact that it doesn’t “move” much in the sky.

The half-angle dates. The Kanayama sunwatchers were highly creative. They asked themselves, “When is the sun’s noontime position half-way in direction (in angle) (in the sky) between the solstice and the next equinox?” We modern people would probably say, “Half-way between the seasons, so I would say about ~45 days after the solstice, right?” If that is your answer, you are wrong! The “half-way in the sky” date is ~60 days after the solstice, and ~30 days before the equinox.

In case you think this “half-angle date” will vary with your location (namely, latitude) on earth, you are again wrong. This date can be calculated from SHM (which is after all, just a sine function). You will calculate the two dates before and after the summer solstice as:

4/22 and 8/20


and the pair for the winter solstice as:

10/23 and 2/19.


Therefore, the solstices are bracketed and are in the midpoint of the two summer half-angle dates and the two winter half-angle dates. Observations of solar position are made on those four dates at the Kanayama Megaliths. We wonder if other astronomers in other parts of the world took special observations on those four dates.

Pinpointing the Summer Solstice.  The determination of summer solstice day is further refined 31 days before and after the solstice day. A megalithic grouping (which we call the Senkoku-Ishi) was arranged (and it still is) so that a spectacular spotlight pattern appeares on a rough panel for a few days beginning, and a few days ending, on:

5/21 and 7/22


Leap-year Determination.  Careful observation of the shift of the light patterns from year to year tells us that the solar year is not exactly 365 days long, that a day has to be added from time to time. Some times, though, we can expect to add a day by a four-year cycle but find that the extra day is not needed this year. That happens every 128 years. So there are, within a couple of human lifetimes, two leap-year cycles of four years and 128 years. There are higher order cycles as well, if one lives long enough.

How are the exact years for adding an extra day to be determined? Well, the Kanayama astronomers decided to make a special observation on 10/14 (if a normal year). If a leap-day is needed, the observation will extend to 10/15. This is how they would know. The “other side” of the winter solstice is 2/28. So another pair of important dates is:

10/14 and 2/28.

And so there would be a corroborating solar observation and then an extra day would be inserted the next day. How clever!

DSC00501 (1)



3 thoughts on “Kanayama Megalithic Solar Calendar

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  2. Pingback: Higashinoyama: Sixty Days Before Winter Solstice 2018 | Iwaya-Iwakage of Kanayama Megaliths

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