The least count of a sextant, which refers to the smallest angle it can measure accurately, is typically 1 minute of arc (1′). However, with the aid of a vernier scale, many modern sextants can achieve a least count of 10 seconds of arc (10″).
This high precision allows sextants to be crucial tools for navigation, particularly in determining a ship’s position at sea by measuring the angle between a celestial object (like the Sun or a star) and the horizon. Their accuracy and reliability have made them indispensable for centuries in maritime navigation, even in the era of GPS.