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
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.
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
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.
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
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.