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William Penn first advertised the layout of his new city in a book by Thomas Holmes published in 1683. Records are lacking about how these plans developed, how much of the idea came from Holmes, how much of the rest was carefully planned, versus how much just worked out. There's not much doubt the streets were laid out in a square grid. And the "numbered" streets all run North and South, following the compass path. These two ideas make it inevitable that the cross streets named after trees would run due East and West. That's enough for a sketch, and whether anyone thought about it further is not clear.
However, the consequences of this layout are that if you looked East at the sunrise on March 21, the rising sun would be exactly framed within the streets, whether they are lined with trees or lined with skyscrapers. That would make quite an Easter morning, with every cross street in the city pointed exactly but briefly at the rising sun.
Let your imagination run on this idea, a little. The exact day of Easter varies a little because of the Biblical definition of Passover to which it is linked, and the Eastern Orthodox Easter varies from that. With Philadelphia providing a forceful example of the utility of using an astronomical definition which was probably the original basis for all the religious traditions, maybe it would even become easier to remember just what date Easter is, each year. Or, perhaps things would go haywire the way things did in Brazil. Magellan sailed into what looked like the mouth of a big river on January 1, 1520, so he named it the Rio de Janeiro. They have an enormous Mummers-like celebration down there in Rio every New Year's, in spite of the awkward fact that it's just a big bay, there is no river there at all, and the dates for Christmas are astronomically a little mixed-up too.
Well, to get back to William Penn, his streets run North and South from Magnetic North. Even though it's quite a natural mistake to make in the Seventeenth Century, that's six degrees off from True North. So the astronomical transformation of Philadelphia into one big humongous Easter sunrise celebration never happened.
We'll never know for certain, but, on reflection, that may be just as well.