"Im not sure it this the second chamber was open to the east originally but it looks like a pretty good fit for the equinox sunrise. It may be just a coincidence but if say the east end of the chamber was open but the rest closed off it would surely light the small chamber completely on the Equinox?
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It would depend on the horizon to east if the sun shone into the chamber sufficiently to lighten it , i.e. the sun rises due east at equinox but if there are hills in the way by the time it gets above them it has traveled further west and therefore not shining straight into the chamber but from an angle .