Gregorian Tower
... that observations made with the sundial at the Gregorian Tower of Vatican City provided essential confirmation of the need to reform the Julian calendar?
... that observations made with the sundial at the <b id="mwB8Q"><a href="./Gregorian_Tower" id="mwB8U" rel="mw:WikiLink" title="Gregorian Tower">Gregorian Tower</a></b> of <a href="./Vatican_City" id="mwB8Y" rel="mw:WikiLink" title="Vatican City">Vatican City</a> provided essential confirmation of the need to reform the <a href="./Julian_calendar" id="mwB8c" rel="mw:WikiLink" title="Julian calendar">Julian calendar</a>?