The instrument is in the form of a book and carries a swivelling suspension ring on its spine.
The top cover carries a universal solar and lunar equinoctial dial with an alidade for its gnomon. The sights on the alidade are modern replacements. The outer scale of hours runs I to XII, I to XII and is provided with half as well as hour divisions. It surrounds the lunar scale 0 to 291/2, each division of which is occupied by a star. The central portion of the face is occupied by a lunar volvelle with radiating hour lines 4 to 12, 1 to 8 and an aperture to display the phases of the moon.
The reverse carries a table of the latitudes of 32 places. Around the left and upper edges is the inscription 'Rologio Vniuersal De Sol, e Lua, e Estrela De Norte'. Down the right edge is a latitude scale 0 to 80, divided to 10 and 2, and numbered by 10. The folding support for use in setting up the dial is housed in the opposite face which carries the compass.
The plate of the compass bowl is silvered and indicates the four cardinal points with a fleur-de-lis, 'OR', 'M' and 'OC'. A circular table surrounds the compass, its three concentric rings marked 'Dias de lua', 'Horas' and 'Quitos'. Each corner of the face bears a wind-blowing cherub head.
The reverse carries a lunar table 'Taboa da Lua' with rows marked 1 to 19 (presumably for the 19 year lunar cycle) and columns headed with the sequence of initials of the months of the year.
The inside back cover carries a quadrant with an arc of degrees 0 to 90, divided to 5 and 1, and numbered by 10. Inside the quadrant is a shadow square marked 'Umbra Recta' and 'Umbra Versa', each portion of which runs 0 to 12, divided to 3 and 1, and numbered by 3. Inscribed within the square is a circular table of dominical letters and golden numbers ('Nombre dor'). Both are marked to begin in 1588.
Two fixed pinhole sights are mounted along one edge of the quadrant, and a plumb bob and string are attached at the vertex of the quadrant. In the unused corner of the quadrant plate is the signature 'Ego Sebastianus Thibault Dreux Fesit [sic]'.
The back cover carries a nocturnal with a folding 'INDEX' constructed for a star in about 12? Scorpio. The toothed volvelle has a radiating scale of hour lines numbered 1 to 12, 1 to 12. The setting pointer at 12 moves over both a calendar and a zodiac scale. The inner calendar scale identifies the months by initial letters, each month being divided by 5 and ending with number of its days. Each sign of the zodiac scale is identified by its symbol and divided by 5.
The corners of the covers are decorated with fruit and foliage engraving but the edges supplement the table of latitudes by providing an additional series of 25 place names and latitudes. The places are all outside Europe, and largely or exclusively represent Portuguese possessions in Africa and Asia.