علم الفلك في القرون الوسطى (بالإنجليزي)

{# The local PDF, when we have one, is always the iframe's primary source: it's ours, so it always embeds cleanly. An external ebook_url is fundamentally unreliable to embed — most real sites set X-Frame-Options/CSP frame-ancestors and refuse to be framed at all — so it's only ever attempted in the iframe when there's no local file to fall back on; otherwise it's exposed purely as the always-visible "open in new tab" link. #}