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Digital version of the Perpetual Calendar found in the fantastic [This Book is a Planetarium](https://bookshop.org/books/this-book-is-a-planetarium-and-other-extraordinary-pop-up-contraptions-popup-book-for-kids-and-adults-interactive-planetarium-book-cool-books-for/9781452136219)
This Pen uses: HTML, CSS, JavaScript, and