During the summer observational season, the sun clears the top of Stone C, as seen from Stone A, near the middle. As page 31 of the Guidebook shows, the sun is at the exact center on summer solstice day, 6/21. That is as far north, i.e., to the left, as it can go.
The three photos above were taken by Chika-san on May 23 from 8:25 to 8:26 am. The sun on that day appeared slightly to the right of center because there are still around 29 days to go before summer solstice.
These photos were taken by Chika-san at sunrise on May 23, 2017 at Senkoku-ishi. We are looking at Stone A, standing near Stone C with our back to the rising sun. The first two photos were taken at 6:22 and at 6:28 am. Notice that the sun has risen and shines on the upper right part of Stone A, but the grottos are not yet illuminated. The third and fourth photos were taken at 6:36 and at 6:42 am. The shadow of Stone C on A is now clearly seen, and the lower grotto is still in shade at 6:36 but it is lit at 6:42am. The final photo is a close-up of the opening of the lower grotto at 6:42.
Please refer to page 30 in the Guidebook. The three photos shown on that page are dated 4/22, 6/21, and 8/20. The date of 4/22 is about 60 days before the summer solstice. The lower grotto is in deep shadow. The date of 6/21 is the summer solstice when the grotto is fully illuminated.
The photos we show above were taken around thirty days before the summer solstice. They show us how the lower grotto is gradually illuminated, minute by minute.