The Mathematics of the Longitude - Department of Mathematics

Chapter 5
How to Measure the Altitude of a
Celestial Body
5.1 Introduction
The
astrolabe invented by the Greeks was the first instrument used to
measure altitude of a celestial body. The astrolabe was mostly used in the
Medieval Ages and by the Islamic world. However, it was cumbersome
because it had to be suspended and often produced large, unavoidable
errors in rough weather.
Figure 5.1 A man using the
astrolabe.
The
development of the cross-staff supplanted the use of the astrolabe. The
cross-staff consisted of a long piece of wood with a crosspiece that could
slide along the staff. The navigator would sight a celestial body along the
top of the crosspiece and move it so that the bottom tip met the horizon.
Navigators often used the sun as a sight, and the cross-staff often caused
eye damage for longtime navigators.