This instrument combines two important navigation instruments: an azimuth compass and an octant with telescopic sight. For a long time, both instruments were indispensable for the determination of direction and location at sea. The declination can be determined with an azimuth compass: that is the angle between the magnetic north and the geographical north, which varies with time and place. The degree of latitude can be determined by means of the octant, by measuring the angle between the horizon and a celestial body. The octant is an invention of Isaac Newton (1699), but the design of John Hadley from 1731 was eventually used in practice. Later on, the sextant was preferred for navigation: it enabled an angular measurement up to 120 degrees.
Azimuth compass with octant
1750 - 1774