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Replying to @josecastillo
fundamentally the problem space is this: I have the positions of the planets in that first struct: X/Y/Z coordinates. I want to convert that to the second struct: coordinates in the sky as seen from Earth. It’s happening in this function. This is grokable. https://github.com/gmiller123456/astrogreg/blob/ff3415d46d2b2ffccb1a65c9b5883e84325a51fa/vsop87-multilang/astrolib.js#L10
(original)