Antique of the Month - An early 19th Century Octant- A Maritime Navigation Instrument
The featured antique this month is a ship navigator’s instrument known as an octant. First constructed in the mid-1700s, octants were an upgrade to a similar previous instrument known as a quadrant. The octant was itself succeeded later by an upgrade known as a sextant. The octant, like the quadrant and sextant, was used to determine a ship’s position at sea when no land was in sight. However, it could only determine Latitude (north and south of the equator), not Longitude (east and west of the Prime Meridian). The octant was more exact than the quadrant, and the sextant more exact than the octant.
Latitude is determined based upon how high or how low the sun is in the sky at a particular time of year. The octant measures the angle the sun is on the horizon. As an example, if you are directly over the equator at noon, the sun will be directly overhead. But if you go north or south of the equator, the sun will deviate from the horizon by a certain angle. This octant will tell you that angle, and then, consulting a set of tables that include the time of year, you will be able to tell how far above or below the earth’s equator you are. This is Latitude.
However, the octant cannot determine Longitude. That is determined using a chronometer, a maritime navigation timepiece (a clock that still works when the ship is rolling at sea) that is set at Greenwich Mean Time at the Prime Meridian, which is London, England. Given the earth rotates every 24 hours, the earth can then be divided up into vertical lines of Longitude. So, when you are at sea, whenever the sun is at its apex, that is noon your time. You then look at the chronometer, set to Greenwich Mean Time (or, London, England, time). The difference in time is then calculated to the earth’s rotation and thus you can determine exactly how far east or west you are. When you have both Latitude (using the octant) and Longitude (using the chronometer), you can determine your exact position on the earth’s surface. See elsewhere on this blog under the antiques listing to see my antique chronometer, which is Russian.
This particular octant is English. It was made by a famous nautical instrument maker in Liverpool, England, named Thomas Jones in c. 1820, right about the time ship chronometers began to be also manufactured in numbers. I do not know the man associated with the initials “TWT” painted on the cover of the octant’s case, however, he was undoubtedly a ship’s officer, likely the ship’s navigator, possibly even the ship’s captain, undoubtedly serving on a sailing ship, from about that time. It’s possible he was American, per the Boston information shown inside the case.
Because this octant was made in England and the inside information indicates a Boston seller with the words, “Second-hand Instruments Bought, Sold, and Exchanged,” I believe this particular antique octant is not in its original case, hence the discrepancy between the octant made in England and the case made in Boston. While the octant itself is c. 1820, the case I think is later, possibly c. 1830-50.
Whoever “TWT” was that owned this case, he must have bought this replacement case in Boston. Given that the octant was from England, it’s likely this owner was a transatlantic sailor, buying the octant first in England and then years later replacing the case it was originally in with this one which he bought in Boston.
Here are links to two videos that show how the Octant was used at sea to determine a ship’s Latitude (location above or below the equator):
