Iwaya Rockbat published the May 21 dashed spotlight report for 2016, https://iwakage.wordpress.com/2016/05/26/dashed-spotlight-of-21-may-2016/
The appearance of this spotlight in the grotto of Senkoko-Ishi heralds the coming of summer solstice 31 days hence, in other words June 21, 2016 at Kanayama Megaliths. And 31 days after the solstice, July 23, the dashed spotlight will make its last appearance of the year. There are 62 days between the two dates, and the solstice is in the middle. Remember, between spring and autumn, the sun daily moves northward until the solstice and then moves southward, retracing is path. This dashed phenomenon only lasts for a few days each time.
What is Lahaina Noon? It is the popular term used in the Hawaiian Islands for the two days per summer when the noontime sun is directly overhead any given spot in the Islands (Okunomichi link 1). On those two days, the zenith sun casts no shadow of thin vertical objects such as poles and stop signs (Okunomichi link 2). Those two dates in 2016 for four Hawaii cities are listed in the first of the two posts. For Honolulu, Oahu, the dates are May 26 and July 15; for Hilo, Hawaii, they are May 18 and July 24. On the Tropic of Cancer, there is only the one date of the summer solstice itself, June 20.
It occurred to Iwaya Rockbat that stop signs such as the one shown by Okunomichi can be used as indicators of summer solstice date, just as the dashed spotlight does. Using the dates given for Honolulu, May 26 and July 15, we counted the number of days between them and found there were 50 days. The middle date will occur 25 days after May 26. It will be June 20. It is exactly right for Honolulu!
For Hilo, the two dates are May 18 and July 24. The designated summer solstice date is June 20. Again, this is perfectly right!
So, what prevents us from using signposts in Hawaii to determine by observation the date of the summer solstice? The big issue is the accuracy of observation. The day before the photo was taken, Okunomichi had taken a similar photo of the same signpost. The shadow was close to non-existent. It was nearly the same. We conclude that this is not a very accurate way of determining the date of summer solstice. A place closer to the equator would be better, but how much better?
On the other hand, the megaliths of Senkoku allow a special beam of light to strike the side of stone A’ in the grotto. Most of the time, after clearing that side panel, the sunbeam lands on the floor of the grotto. Only on a few special days a year does it illuminate the “bumps” on the triangular face that the ancients had carved especially for this purpose. The arrival and end of the dashed spotlight is a delicate determinator of the summer solstice date.
We can marvel at the ingenuity of the ancient people who created with wonderful solar observatory using megaliths!