Astronomical Images : Surveying

Peter Apian

Astronomical Images

<p style='text-align: justify;'>This figure, the title-page of the volume, acts as a visual manifesto for the measurement of the whole world through instruments. It is notable that the scene includes plains and mountains, land and water, natural and man-made features, as well as the Earth and the heavens. Several instruments also feature: a terrestrial globe, a pair of compasses and a quadrant are all shown in the foreground on the left of the figure. However, the star of the scene has to be the cross-staff, which is depicted no fewer than four times. The cross-staff, also known as the <i>radius astronomicus</i>, was a relatively simple instrument that could be used in a wide variety of contexts. Composed of two straight rods, the cross-staff exploited the principle of similar triangles to measure distances either on Earth or in the heavens. In the terrestrial region, the instrument could be used to measure the heights of various features such as hills, trees and buildings (as shown here). In the heavens, it was used to measure the apparent distance between celestial bodies or the height of a body above the horizon, that is its altitude. This figure demonstrates the breadth of application of the cross-staff not only by showing it in use to measure both the heavens and the Earth, but also by illustrating two methods for using it to measure the height of objects on Earth. In the foreground we can see a man using the staff to measure the height of a building at a known distance from its base, which required just one observation. Further into the distance we can see two figures pointing the staff at the same building, with a known distance between them. This may either be intended to show two individuals or one individual making two observations at slightly different times, and illustrates another way of using the staff when the distance from the mark is not known. This figure provides a visual argument for the connectedness of the measurement of the Earth and the measurement of the heavens, thus presenting the arts of geography and cosmography as linked, and emphasising the mathematical nature of both.</p>