THE BEGINNINGS OF MEDIEVAL INFORMATION VISUALIZATION - Information Visualisation: Techniques, Usability and Evaluation - page 15

It is also interesting to note that Isidore's rota carries with it a less
explicitly communicated layer of meaning related to the Roman-Egyptian
calendars. Roman calendars represented a computistical approach to fixing
time, based on a government's authority at preordaining important dates
for its population to observe; while Egyptian calendars represented time as
a natural phenomenon, experienced as a progression of the sun's
movements through the seasons. In the end, the Church would take a
computistical approach [34] to accommodate both methods for measuring
the passage of time in its calculation of important feast days, and thus
affirm its magisterial authority over its followers.
The second of Isidore's rotae is displayed in Fig. 1.7. It is a rota anni
from a chapter entitled “On the seasons,” here taken from a version of De
natura rerum created in Salzburg c. 800 ( MS. Clm 14300, fol. 14, Munich
Bayerische Staatsbibliothek) [35]. It is designed to show how the four
seasons harmonize.
Fig. 1.7 Annus diagram, Isidore of Seville, c. 800 ( MS. Clm 14300, fol. 14,
Munich Bayerische Staatsbibliothek).
The year (annus) is rendered as a circle, around which the four cardinal
directions organize the seasons within the overall structure of the globe.
Oddly this rota places spring (ver) at its top, which corresponds to the east
(oriens), a position typically reserved for north. The yearly cycle follows