More on the Portuguese

[timelets]

The general idea was that if the altura, or height above the horizon, of the sun or a star (normally the Pole Star) could be determined and compared with the known altura of the port of destination, then the ship could sail north or south until it reached that latitude, and then sail, as appropriate, east or west in the certainty of finding its point of destination. The measurement of the altura was possible with the use of either the quadrant or the astrolabe.



Both devices were standard university instruments of astronomy and astrology that carried a great deal of information that was both unnecessary to the calculation of latitude and simply incomprehensible to the layman. On the back of the astrolabe there was, however, an alidade, which was a rule on a swivel with two pinhole sights. The observer held the instrument upright by a swivel suspension ring, peered along the alidade, and measured the altura of the star by reading off the position of the alidade on a scale marked on the rim ( figure 3 ). The quadrant was an instrument with similar functions.



But instruments, inscriptions, and stars were not enough. Part of the association of elements to convert stars into latitudes lay in the practices of the mariners, and it was this element that was the most prone to distortion. It was difficult, although not ultimately impossible, to create a new social group necessary for closure: the astronomical navigator.

- John Law, ibid

The navigator embodied a (cyclical?) monoid, i.e. he could start applying technology arrows (device-based mappings) from any object in the graph: Stars -> Latitude -> Port -> Course -> Stars.