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Pedro Nunes
introduced this instrument in Chapter 6 «De instrumentis quibus astrorum
altitudines et distantiae capiuntur» of his Petri Nonii Salaciensis Opera.
The nautical ring is an instrument suited for solar observations.
The instrument
has a suspension ring in f. The artisan should drill a hole (as tiny as
possible) in g (arc
ag is
equal to the arc ab=45º)
by extracting the section kgh. Nunes advises to take out a similar amount
of material at the opposite side b so that the instrument be balanced. The
semicircle ceb is
graduated from 0º to 90º depending on the value to be taken by the observer
(direct reading or zenith distance).
The ring should
be oriented vertically to the sun so that the light could enter through g.
As Nunes states
«on this instrument, the degrees of altitude are twice as large
as they would be at the common astrolabes(…). In effect, for equal angles, those
that have their vertices on the circumference subtend arcs double in length to
those that have an angle at the centre». This mathematical property allowed to
double the precision of the instrument.
The nautical ring
did not find continued or lasting use in navigation, though there are still
references to its use until the XVIII century.
Nautical ring - E. Nairne,
London, 17th century. Ø= 275 mm.
Museo Naval de Madrid.
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