Prism level

Tilt errors of the sextant

The measurement error at 5° inclination and 45° celestial altitude is e.g. 7sm

1) To avoid this, the sextant was swiveled around the telescope axis until now.
The star describes an arc over the horizon. The lowest point of this arc is the correct celestial altitude.

2) The vertical reference is a glass prism.
When the sextant is tilted, the horizon line seen through the prism, appears offset from the one seen directly. The measurement is then correct
if the eyepiece line passes through the field of view without step/offset.