Watercolor engraving Octant by Hevel, 1659.
Man has always tried to find answers to the many infinite questions related to life on this planet; this due to his unquenchable thirst for knowledge and the need to control his surrounding environment. Explanations as to the cause of day and night, the change in seasons, the concept of time and space; these were the first questions that needed an answer. It was then necessary to find an orderly observation of the skies, whose changes are determined by the Earth’s rotation, as well as the need to measure the passing of time therefore man needed to produce instruments capable of taking such measurements. Man also wanted to be able to study and understand these phenomenon better so he tried to transfer the imagery of what he could see onto a suitable object that could contain that imagery. In the beginning these reproductions were merely sketches representing a route to follow in order to reach a given destination and were rather primitive in form. Later on, man’s knowledge grew and with the help of more perfected instruments for observation and measurement, he was able to recreate real models of the world known to them as well as models of the celestial skies above him, sometimes using even three dimensional models. Many of us, at some point in our lifetime, have had the chance to observe the “globes” made of wood and covered in paper or those even engraved in brass. These objects can only amaze the observer because of their beauty and refinement. The “armillary spheres” are also of remarkable beauty. They are so called because of the “armillas” or armlets that characterize them and that represent the skies surrounding the Earth and Sun, according to the Ptolemic-Aristotelian theory (heliocentric). These spheres were made of brass armillas, (presumably until the end of the 17th century), and later on were made of wood and covered with paper. The creation of these models representing the known world and universe required the creation of instruments of observation and measurement capable of giving scientific value to the finished product. Such instruments have become more and more precise with the passing of time and the need to recreate realistic models has become secondary.
One of the first instruments capable of performing astronomical calculations was the astrolabe. Its ancient origins date back to the 2nd century A.D. and was found in the orient and later in Europe, crossing through Spain and thus becoming an indispensable instrument for performing both astronomical and mathematical calculations. The use of this instrument continued well into the 17th century.
The word “astrolabe” derives from the Greek meaning “to find the stars”.
It is composed of a circular shaped container, called the “mother” where one or two engraved plates are contained. These plates are known as “tympanum”; the engravings are essentially lines representing the projection of the skies at a given latitude. Then there is a “netting” above the tympanum that depicts the Zodiacal circle, the constellations and the main stars.
The back of the “mother” is detailed with graduated engravings, coupled with an alidade used for calculating angular measurements of height. The astrolabe was therefore a rather complex instrument that required an accurate astronomical expertise.
The quadrant is a much more practical and simple instrument enabling man to calculate the height of various constructions, mountains and even of the stars and the Sun. It is called quadrant because it corresponds to a quarter of a circle. It represents gradations or scales and diversified constitutive elements, depending on a given use. With the border divided in degrees and a face covered with straight lines or curves it functions as an hourly quadrant. In this case the instrument is provided with a plumb-line with a running bead and two levels. It was used by looking at the sun through two viewfinders. The plumb-line would indicate the height of the sun on the border in that precise moment and the bead then ran along until it found the line corresponding to that hour. This was how the ancients measured time but it was not the only way. Among the first instruments for measuring angles was the octant. It was created by John Hadley, (1684-1744), to measure angles by reflection, enabling him to reveal and measure the angle between two objects by making the reflection of one coincide with the other, observed in a mirror. The characteristic of this instrument employed the use of a mirror mounted on a pivot of a radial arm that moved on a graduated arch. The arch occupied only an eighth of the circle; thus the name of this instrument that, thanks to the mirror, can measure 90° angles. In the later part of the 18th century the octant was substituted by the more sophisticated sextant which could satisfy the need for greater precision in measuring altitudes. The arch of the sextant is one sixth of the circle and again uses a mirror to measure from 0° to 120°. The quadrant, octant and sextant are characterized by their beauty and harmonic delicacy and by their remarkable constitutive dimensions which were reduced in size over the centuries.
This was because it was quite difficult at that time to subdivide the circle in sufficiently precise scales and therefore the increasing of size meant the decreasing of eventual errors in reading the angular values.
This short description obviously cannot cover completely such an interesting subject which is so much vaster and richer in content, however does serves as an overture to that marvelous world made up of ancient instruments of observation and measurement.