Got it.
The sun is kind of in the Sphinx’s neck.
It didn’t recognise the Equinox sunset the first time, then it did.
Got back to the helicopter to check the time:
17:28/15:28 UTC Sept 22.
(so the photo was c. 17.26/15.26 UTC)
By the way. Chicken Itza. Infuriating.
Triggers at 12.43 local time, not at 16.43 when the sun makes a snake shape on the stairs. 
