On the instrumentation of measurement, with particular regard to the problems faced by those who travel by Sea

by Bartolomeo Gagliardi

In writing this treatise I seek to develop upon my earlier work titled "On the principles and methodology of the measurement of the Earth and the Heavens", in which I wrote of an apparatus of my manufacture which I have chosen to name the Jacob's Ladder, a mechanism of greatest utility in the measurement of terrestrial longitude. That instrument, simple in manufacture yet powerful in application, requires for its function the enjoyment of a stable platform from which lunar observations may be taken. Although its efficacy has already recommended itself to many who explore the mysteries of cartography and navigation it has proved of less use to those who travel by sea, for whom a stable platform is a necessity oft difficult to obtain. With that consideration before me, I here present a further device of yet greater assistance to those who seek to measure the latitude and longitude of the celestial sphere and thus determine the means by which the Earth itself may safely be navigated.

My creation is a development of the common astrolabe, that most marvellous of tools which permits the accurate knowing of time both by day and night and the judgement of location and direction so useful to those engaged in expeditions and similar ventures.

The astrolabe of common design consists of many parts, first of which is a flat discus with a raised rim. The rim of this discus is titled the limb and is marked in degrees and to show the hours of both day and night. Into the recessed area of the discus, known as the mater, plates are inserted. Each such plate, collectively known as the tympani, is engraved for a given latitude upon which a user may find himself. These engravings are both curved, in representation of the arcs of altitude and azimuth, and straight, in representation of North, South, East and West. Once the tympanum is inserted into the mater, a further device known as the rete is placed upon it. The rete is a further disk, but pierced so that the tympanum underlying it may yet be seen. All parts of the rete are cut away, save for an offset circle which represents the plane of the ecliptic and a number of points which in use will identify the positions of the stars. For the greater assistance of the lay reader I have attached to this humble submission a diagram illustrating the interaction of the common parts.

That the astrolabe is a device of great value to navigators and cartographers is unquestioned, but its use is limited by the requirement that each latitude of observation have its own tympanum. The user must carry a great many such tympani, particularly if his voyages will carry him across a multitude of latitudes as is often the case when engaged in exploration for the greater glory of His Holiness and that illustrious family of which he is the most glittering part.

To this end, I have devised my universal Astrolabe, named in honour of His Holiness the Astrolabium Leonis, Leo’s Astrolabe. The Astrolabium Leonis on the one side performs as a traditional astrolabe, permitting the uses of such instrument to be enjoyed by its holder. The facing side, traditionally illustrated with angles and scales, instead is engraved with a projection of the starry sky which projection originates at the line of the vernal equinox, in contrast to the polar projection more traditionally employed.

With the point of projection established, next one must determine the plane upon which it is made. I have chosen the Solstitial Colure, that great circle which passes through the celestial poles and thus lies at right angles to the plane of the equinox. By this means, it can be seen that the diameters of the instrument are fashioned by the celestial equator and the horizon of the observer, the horizon of the observer of course being determined in turn by the latitude at which he stands.

Furthermore, the celestial poles are now of necessity to be found on opposite points of the edge of the instrument, as are the points directly above and below the place at which measurement is taken, known in their turn as the zenith and the nadir.

The implications of this subtle arrangement are no doubt obvious to those of mathematic or astronomic persuasion, but for the more practical reader I here set out the change in usage this design permits. Whereas in the traditional astrolabe one must insert a diverse tympanum for each latitude, on my device the latitude is determined by the placing of the point and plane of the projection and no replacement of tympani is required. If one simply moves the rete around the mater so that the observable distance between the zenith and the celestial pole is reset one may use the instrument with the greatest precision, determining such matters as the length of any day at any latitude. Of course, this arrangement precludes the accurate measurement of time, which is among the reasons for the more usual astrolabe mechanism remaining on the obverse face.

Truly then, it can be said that this device is a universal astrolabe, an astrolabium catholicum. With one side one may measure hours with finest accuracy and carry out all those operations for which the astrolabe is so well suited. When altered location demands use of new tympani, one may simply reverse the device and use that projection of my own design to determine matters without need for utilisation of numerous disks each engraved for its own particular latitude. The device requires no stable platform, and thus may be used at sea, and although admittedly this creation requires no small expense for its manufacture the cost should not be beyond that of a well equipped sea expedition.

Together with my Jacob’s Ladder, it is my belief that this device will prove of the greatest assistance to astronomers, navigators and explorers of all kind and I pray that its signal utility permits its adoption such that it may be taken and used to find new lands that we may spread the light of the true Word of the Lord to the heathen darknesses awaiting our most merciful mission.