Dead Reckoning


A mariner’s astrolabe measures the angle of the Sun or a star above the horizontal line of reference. This one is possibly Portuguese, 1602.
A horary quadrant is used to find the time of day by measuring the Sun’s altitude. This horary quadrant is English from the 17th century.
A mariner could use a cross staff to measure the Sun’s angle above the horizon at midday. From Jonas Moore, A New Systeme of the Mathematicks, 1681.
Nocturnal and Sundial, signed by Caspar Vopel, Cologne, 16th century
Finding the time of day with an horary quadrant by measuring the Sun’s altitude, from Gregor Reisch, Margarita Philosophica, 1504.
Using a mariner's astrolabe to measure the angle of the Sun or a star above the horizontal.