Not to mention that in BTS, it obsoletes @ astro, not calendar, and IIRC Zara/Ethiopia aren't Vanilla civs...
Hm. I recently got BtS, and haven't gotten to do much with it, so I didn't realize that it was Astro, not Calendar.
Still, another point in favor of a Zara/Stonehenge strategy. Because on a map like Pangaea or some Fractals, you have little reason to tech Astronomy, except for the Observatory.