At A Glance: Sextant

A sextant is an instrument that measures a celestial object’s altitude above the horizon. It is a device that measures the angular distance between two visible objects. Experts give this measurement process using various terminologies like taking a sight, shooting the object, or sighting the object. A sextant is used to calculate an object’s angle. These calculations can be expressed on a chart, known as an aeronautical chart or simply a nautical chart. Keep on reading to understand the sextant meaning in detail. Here, we shall also take a look at who invented the sextant.

Sextant Meaning

A sextant is an instrument by which one can determine the angle between the horizon and a celestial body. This celestial body can be a star, moon, sun, etc. One can estimate the latitude and longitude with this angle using celestial navigation.

The device consists of an arc of a circle with degrees marked on it. You will find a movable radial pivoted arm at the circle’s centre . Attached to the framework is a telescope lined up with the horizon. This telescope provides a clear view of the celestial objects.

To carry a measurement, one must move the radial arm of the sextant. The radial arm must be moved until the reflection of the celestial object is shown on a half-silvered mirror. This way, the reflection of the object will coincide with the horizon. When this happens, you can make the necessary calculations.

The graduated arc of the sextant is used to read the angular distance above the horizon. Finally, one can determine the latitude from this angle and the day’s exact time, which the chronometer registers. This is done using published tables.

Who Invented Sextant?

The implementation of the principle of the sextant in 1731 is attributed to the following two personalities:

•     John Hadley
•     Thomas Godfrey

Later on, this principle was discovered in certain unpublished writings of Isaac Newton.

The credit for the production of the first sextant goes to John Bird in 1759. This is the earliest known model of the sextant. Currently, it is in the Nederlands Scheepvaart Museum in Amsterdam. The size of this instrument is so large that it requires support to be placed into a socket.

In 1922, Gago Coutinho, a Portuguese navigator, modified the instrument for aeronautical navigation.

Inside a sextant

The sextant is shaped in the form of a sector that is equivalent to 1/6th of a circle. The frame refers to the part that is sector-shaped. There is an attachment to the frame of a horizontal mirror. There are several parts attached to a sextant which are as follows:

• Glass filter
• The horizon or horizontal mirror
• Glass filter
• Screw to regulate
• Small mirror
• Index arm
• Drum
• Index mirror
• Telescope
• Telescope clamp
• Eyepiece
• Telescope printing
• Frame
• Graduated arc
• Locking device

Important pointers when using a sextant

While using a sextant, make sure to remember these important pointers:

•   Before use, always make sure to check the errors.
•     Looking at the horizon is the key to focusing the telescope. Now, try to create a mark on the stem’s circumference.
•     Ensure the sextant is held in a steady position while you use it. Stand with your feet planted slightly apart while holding the sextant to achieve this.
•     Swing the sextant to the other side as you observe the celestial body’s altitude. This way, the body will appear to project a movement along the arc. Now, the altitude measurement at the lowest point can be carried out effectively on this arc.
•     Stand as close as possible to the ship’s middle line or the centre line.
•     Suitable dark shades must be used as you observe the sun.
•     Always remember to rotate the micrometre if a backlash error is detected. This rotation must be in one direction only.
•     Checking the altitudes of celestial bodies must take place during twilight.
•     As far as possible, avoid making observations at night. This is because the moonlight creates an illusion of a good horizon, which is not true in most cases.

Conclusion

A sextant is an instrument used to measure a celestial body’s altitude. Calculation of a position line on an aeronautical chart by utilising sextants. This is based on two factors, the angle and the measurement time . People use sextants to measure Vertical Sextant Angles (VSAs), Horizontal Sextant Angles (HSAs), and altitudes. The production of the first sextant is attributed to John Bird, who made it in 1759.