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Around the late 1600's and early 1700's, there was a huge advancement in the design of instrument for measuring angles. Instrument makers were shifting their focus to optical systems based on mirrors and prisms. The critical development was made independently and almost simultaneously by John Hadley in England and by Thomas Godfrey, a Philadelphia glazier, about 1731 with the invention of the Octant.

How it works

An Octant consists of a frame (in the shape of one eighth of a circle), an index arm, two mirrors and an eyepiece. The index arm is pivoted at the circle’s center and moving over the graduation on the arc. One of the mirrors, the index glass, fully reflecting is placed on the index arm exactly above the pivot and the other, the horizon mirror, half-silvered, on one radius of the octant. The eyepiece is placed upon the other radius, opposite to the horizon mirror. 

By the Law of reflection of light, the angle of incidence is equal to the angle of reflection for a plane mirror. It follows that if the mirror is moved so that the angle of incidence is altered, the angle at which the emergent ray is reflected will be altered by an angle twice that through which the mirror has been moved. This is the main principle used by the Octant. 

When using the Octant, the instrument is held in a vertical position and the sea horizon is viewed through the horizon mirror. Light rays from the celestial body will be reflected by the index glass followed by the horizon mirror, into the eye of the observer. By rotating the index arm until the reflected image of the celestial body is aligned with the horizon, the corresponding altitude, which is twice the angle moved by the index arm, can be read from the scale.

The usage of the Sextant is the same as the Octant. The angle measured is transferred to the lunar table whereby the longitude of the observer will be known. Further reduction in the radius of the Sextant helps to reduce the weight of the instrument even more.


The Octant and Sextant were the first instruments that could measure angle with sufficient accuracy. The observer need only to look at one place while adjusting the instrument and the reading is not affected by the rolling and pitching of the ship because the horizon and the star seemed to move together.

It is difficult to use both the Octant and the Sextant at night, as the horizon will be invisible. In order to solve this problem, people tried to make use of artificial horizon. One of the instruments that made use of the artificial horizon is the Bubble Sextant whereby a spirit level is used to provide the artificial horizon but there is still some inaccuracy to this method.

Glare from the sunlight is reduced when observing the Sun using the Octant, as compared to using the Quadrant or the Cross-staff. Moreover, the observer only needs to focus on the horizon alone, thus preventing ocular parallax. However, the Octant is a more complex instrument as compared to the others mentioned earlier, hence it requires more effort to construct.