Logan Square, one of William Penn’s original city parks, hasn’t been a true square since the early 20th century, when the city began plowing the Ben Franklin Parkway through Center City’s northwest quadrant. The heart of the square was turned into one of the world’s most elegant traffic circles, home to Alexander Stirling Calder’s Swann Fountain, while the remaining pieces were cast adrift, an archipelago of mournful traffic islands, uninhabited and bleak. The Center City District has made a mission out of reclaiming those pieces, starting a few years ago on the west side with the scooped trapezoid called Aviator Park, opposite the Franklin Institute. Now, working its way east, it has just rescued the orphan outside the chocolate-colored Cathedral Basilica of SS. Peter and Paul, a space that goes by the name of Sister Cities Park. The transformation of this tiny shard of land amounts to a small miracle.