What Is A Sextant? How Does It Work?

Until the sextant was created, ocean navigation had been
inaccurate and very dangerous. The determination of latitude was in use from around 120AD, yet the calculation of longitude proved elusive, making sea travel dangerous and expensive.

After four English warships were wrecked on the Scilly Isles in 1714 due to navigational error, the British parliament issued the Longitude Act, with a large cash prize for whoever could discover a useable method of determining longitude.

Two technologies solved this problem. One was the precise
measure of time using accurate and transportable marine
chronometers and the other was the sextant.
The first sextant was produced by the instrument maker
John Bird in 1759. Using two double-reflecting mirrors,
the angular distance of the sun, a star, or other
heavenly body, could be measured from the
horizon. The angle and time measured could then
be used to calculate a position line on a nautical chart.

The impact of the device was huge. It directly led to the
great voyages of discovery by Captain James Cook and
others, and helped the expansion of the British Empire.

The sextant was used exclusively until the First World
War and merchant vessels are still required to have
one today, in the event of electronic failure. The skilled
use of a sextant is still considered to be the ultimate
test of seamanship

The Arabs, in early days, they used one or two fingers width, a thumb and little finger on an outstretched arm or an arrow held at arms length to sight the horizon at the lower end and Polaris at the upper.

Before leaving homeport, the navigator would tie a knot in the cord so that, by holding it in his teeth, he could sight Polaris along the top of the transom and the horizon along the bottom. To return to homeport, he would sail north or south as needed to bring Polaris to the altitude he'd observed when he left home, then sail down the latitude. Over time, Arab navigators started tying knots in the string at intervals of one Issabah. The word Issabah is Arabic for finger, and it denotes one degree 36 minutes, which was considered to be the width of a finger. They even developed a journal of different ports that recorded which knot on the Kamal corresponded to the altitude of Polaris for each port they frequently visited.



In later years, they used a simple device called a Kamal to make the observation. Notice the knots in the cord attached to the carved mahogany transom.

Throughout antiquity, the Greeks and Arabs steadily advanced the science of astronomy and the art of astrology. About a thousand years ago, in the 10th century, Arabs introduced Europe to two important astronomical instruments-the quadrant and the astrolabe.



Astronomers Astrolabe. Arabic astronomer's astrolabe made by Hajji Ali of Kerbala around 1790. It's about 3 and one-half inches in diameter. It was used to find the time of rising and setting of the sun and the altitude of the sun and selected stars. Importantly, it was used to find the direction of Mecca for the devout Moslem's morning and evening prayers.

In the word "astrolabe" - "astro" means 'star' in greek language and "labe" roughly translates as 'to take' or 'to find' in greek language.

The astronomer's beautiful, intricate and expensive astrolabe was the grandfather of the much simpler, easy to use mariner's quadrant and astrolabe. The mariner's quadrant-a quarter of a circle made of wood or brass--came into widespread use for navigation around 1450, though its use can be traced back at least to the 1200s.

Mariner's brass quadrant.
The scale spans 90 degrees and is divided into whole degrees. A plumb bob
establishes a vertical line of reference. The quadrant shown here is a
replica of the type Columbus might have used on his voyages to the New
World. This one is marked off at the latitudes of Lisbon, Capo Verde
and Sierra Leone, down near the Equator where Columbus is known to have
visited.

The quadrant was a popular instrument with Portuguese explorers.
Columbus would have marked the observed altitude of Polaris on his
quadrant at selected ports of call just as the Arab seaman would tie a
knot in the string of his Kamal.