Caption: Medieval scientists probably measuring altitude with an astrolabe.
Not quite the Ancients, but the Ancients made similar measurements.
An astrolabe has an arm and a divided circle that are used to measure altitude.
An astrolabe is a distant ancestor of the mariner's sextant.
Astrolabes were actually dual-use devices. As well as using them for measuring angles, they could also be used for calculating the positions of astro-bodies. The modern planisphere is also a descendant of astrolabe.
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The Wikipedia caption for the image
cites the source as
"Psautier de saint Louis et de Blanche de Castille, BNF Paris, ms. 1186 res,
fol. 1v, ca. 1200 AD".
But the Psalter of Saint Louis (1253--1270) contains, it seems, only Old Testament scenes. So the Wikipedia caption CANNOT be right, unless there is an Old Testament scene that can be interpreted as an astronomical observation by monks. Wikipedia caption date CANNOT be right either.
On the other hand, maybe the Wikipedia caption refers to the Leiden Saint Louis Psalter (1190--1200), but that is in Leiden (in the Netherlands), NOT Paris.
Image link: Wikipedia: File:Ms1186 astrolabe.jpg.
Local file: local link: astrolabe_medieval.html.
File: Art_a file: astrolabe_medieval.html.