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Well, here's the secret. When William Penn laid out the streets of Philadelphia, he did his best to get the numbered streets to be "polar", due North and South. The streets named for trees were perpendicular to that, supposedly due East and due West. However, surveying instruments were fairly crude in those days, long prior to David Rittenhouse and his vernier-adjusted surveyor's transit. So, the East-West streets are just a little off. Large flat buildings like Society Hill Towers have surfaces parallel to the street which are not exactly compass-accurate, as well. If things had been exactly as originally intended, you would be able to go uptown, stand in the middle of any street facing East, and see the sun shining down each street without any shadows. But only twice a year, of course, at the time of the equinox (September 22, and March 20). Because the Society Hill Towers cast wide shadows, it happens there is just a narrow strip -- going right through 255 South 7th Street -- where the sun is exactly framed as it rises and sets, just like the Mayan Temple in the Yucatan, or more famously at Stonehenge. Up till now, that's been a Linck family secret. But there must be dozens of similar solar phenomena in Philadelphia, if you know where to look for them, and when.