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With a little bit of Google Maps and/or looking up any of the identifiable buildings in the photo, you can figure out that this photo was taken facing East.
The relevant rainbow fact hint #1 alluded to is that rainbows always form directly opposite the sun (as the xkcd comic in hint #2 explains more in depth). 9/11 is very close to the fall equinox, so the sun will rise close to due East and set close to due West during this time.
Since the rainbow is in the East, the sun is in the West, therefore this photo was taken very late in the day.