This week's weather prediction was not very encouraging, so late last week I returned to Miantonomi's Cave.
The slanted boulder on top of the structure blocked the sunlight from the cave's two windows, so there were no sun daggers such as those seen on equinox and August 13. The cave was dark the entire time.
If there were any significant patterns at the cave entrance, I couldn't tell, due to vandalism and shadows from trees.
I returned to the cave roof, where there is a prominent triangle carved into the stone. I have often suspected that this large triangle is part of a "user's manual" for this observatory. The triangle does seem to point roughly in the direction of winter solstice sunset.
Note that there is also a small notch in the slanted boulder at the top of the structure, to the upper right in the photo. The problem with any outdoor observatory is determining the vantage point. An alignment can look completely different depending on where the observer is standing. Looking around behind the cave, I found a faint triangle mark on top of a boulder.
So I sat on it, waited a short time, and saw this:
The sun did seem to sink into the notch on top of the slanted boulder around 3:43 PM. Sunset was at 4:15. This notch could be a winter solstice mark, but it is not precise due to differences in observer position and the difficulty of determining anything by looking into the sun. The large triangle mark could also refer to this large, peaked boulder, which is about 975 feet to the southwest on the line for winter solstice.
Perhaps an observer at Miantonomi's Cave was to see if the sun set directly behind this massive boulder. Unfortunately, this boulder is now obscured by trees. The triangle also could also denote a sacred place, similar to a manitou.
If the weather improves, I'll return later this week.