Reading the sextant is very simple:

• Degrees are read directly from the graduated arc opposite the index mark on the index arm.
• Minutes are read from the micrometer drum opposite the vernier index mark.
• Seconds are read from the vernier where one of the vernier graduations lines up with one of the
  micrometer graduations.

The sextant is designed to be held in the right hand whilst the forefinger and thumb of the left hand is used to move the index arm (by disengaging the clamp) and turn the micrometer drum. By moving the arm away from you, the angle on the arc is increased. Bringing it towards you, the angle is decreased. Turning the drum clockwise (as seen from the rear of the drum) the angle is increased. Turning it anti-clockwise it is decreased.

Sextant reading 0° 01’ 10” on the arc.

Reading the sextant “On” and “Off” the arc.   When determining the index error it is necessary that one is able to read the sextant both on and off the arc. How is this done?  First set the index arm and the micrometer drum to zero.  By turning the drum clockwise, the graduations on the drum will increase above zero, ie 5’ , 10’, etc.  By turning it anti-clockwise the reading will decrease, ie 55’, 50’, etc.  In the first case the reading is positive and the angle is read directly from the drum, ie 5’ on the arc.  In the second case the reading is negative and should be deducted from 60’, ie 60’-55’= 5’ off the arc.

Sextant reading 0° 01’ 10” off the arc.