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by Kary Pardy

When Antique Technology Still Works:

Finding our Way with a Sextant

Do You Know Where You’re Going To? The octant was refined to a sextant, named because it enlarged the
In 1714, the British Parliament established the Longitude Act, and octant (1/8 of a circle) to 1/6th of a circle, and now measurements
with it, the Longitude Rewards. Equivalent to just over a million dollars of up to 120 degrees were possible. While the octant could reliably
today, these prizes offered rewards to any who could come up with a calculate your position on a nautical chart by calculating the sun or the
reliable way to measure longitude at sea. Latitude was simple enough North Star’s angle to the horizon at a given time, the sextant’s larger
and could be measured relative to the altitude of the sun at noon, but angle could accurately measure lunar distance between the moon
longitude proved much trickier. The old method, dead reckoning, and another celestial object to determine Greenwich Mean Time, an
was based on the speed the vessel was traveling and the vessel’s invaluable component when determining longitude.
direction, and was dangerously inaccurate when sailors got too far from
land. Add in the problem of magnetic north vs true north (which can Enter: The Sextant
vary by a critical 10 degrees in the Indian and Atlantic Oceans), and The sextant was a game-changer – not only could it plot a location
you get several deadly shipwrecks, including the destruction of a British with accurate latitude and longitude, but it also addressed the
naval squadron off the coast of Sicily in 1707. It was time to find, and challenges of past technologies. A sextant measures celestial objects
fund, a better way. relative to the horizon, which makes it more precise than a purely
Enter English mathematician John Hadley and Philadelphia glazier instrumental measurement. Even when bouncing around on a rolling
Thomas Godfrey, who both developed similar solutions around 1731. sea, a sextant will read accurately because both the horizon and the
They developed an instrument, the octant, that uses two mirrors to celestial object will move in the field of view and the angle (the most
measure celestial bodies above the horizon via angles up to 90 degrees. important part of the measurement) will stay the same. It also has the
advantage of working at night, as you can measure off the stars.

How to use a sextant? Wikihow.com offers a step by step guide
with visuals, Davis instruments has a downloadable manual
on their website, and there are several other online resources
Photo: WikiHow.com
The sextant caught on quick for its reliability and precise readings,
and saved lives in the process. A famous tale of sextant use comes
from Ernest Shakelton’s Trans-Antarctic Expedition (1914-1916).
When Shackleton’s ship Endurance got caught in pack ice and
crushed, the crew took lifeboats to Elephant Island where they were
Sextants can be sensitive: Severe changes in temperature warp the arc and
damage the accuracy of the angle, so many are fitted with weatherproof stranded upon arrival. Captain Frank Worsley used a Heath & Co.
cases. The handle is also separated from the frame so that body heat sextant to navigate himself, Shackleton, and four crewmen 800 nautical
cannot impact the frame over time. Sailors in tropical climates miles through dangerously rough seas to find help on South Georgia
combat this weakness by painting their sextant's white to reflect Island, ultimately saving the crew. The same “Hezzanith” sextant is
sunlight and keep cool. Sextant circa 1865. now in the collection of the Scott Polar Research Institute at the
photo: Cooper Hewitt Collection, on loan from the Smithsonian National Museum of American History University of Cambridge.

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