The History of the Sextant

I used to think that having a calendar integrated into your watch was kind of basic, nothing much to say beyond that. I hadn’t thought it through. From the most intricate mechanical twists to a crocodile-proof watch, that reflection took me far. Try to keep up! I met Jean-Marc Lefèvre, a master horologist in Geneva, who placed a Patek Philippe Ref. 3940 in front of me like it was a miniature cosmos.   "You see this cam?" he said, pointing near the center. "It rotates once every four years. That’s how the watch knows if it’s a leap year." Building a perpetual calendar into a mechanical watch is no small feat. The Gregorian calendar, with its irregular months and leap years, doesn’t run like clockwork. To manage it mechanically, Patek Philippe designed a gear that rotates once every four years, linked to a set of cams and levers that automatically adjust for short months and leap years. Introduced in 1925, it allowed watches to track civil time accurately for decades, no manual correction needed. A Different Kind of Calendar While staring at Jean-Marc’s watch, I recalled something I’d seen months earlier, on a different continent. In East Africa, I had come across a calendar of another kind, ancient, instinctive, and alive. Not visible from the ground, but from space, the Great Migration appears as a living loop stretching across Tanzania and Kenya. Over 1.5 million blue wildebeest, along with zebras and gazelles, move through the Serengeti and Maasai Mara in one of the most predictable natural events on Earth. A Migration That Tells Time I met Dr. Nyasha Mbeke, a wildlife ecologist in Arusha, who’s tracked this movement for over a decade.   "The wildebeest don’t read calendars," she told me outside her field station near Ngorongoro. "But their timing is so regular, we can tell the month just by where they are." In January, the herds gather in the south to calve, around 500,000 births in a matter of weeks. By April, they shift northwest as the plains dry. In June, they face deadly river crossings. July and August take them north. By November, rains pull them south again. Natural Mechanics This isn’t random wandering. Wildebeest are biologically tuned to East Africa’s cycles. They can sense distant rain through changes in humidity and pressure. Their eyes detect subtle shifts in grass color. Internally, they follow circannual rhythms, biological clocks regulated by light and hormones like melatonin. Their bodies are optimized for long-distance travel: strong hearts, efficient kidneys, and high red blood cell counts. A Watch with No Gears and Crocodile Proof Like the cams and levers in a mechanical watch, the wildebeest follow a system driven by nature: rainfall, grass growth, instinct, and the unseen tracks of generations. Watching their movement across satellite maps, dark clusters sweeping across green plains, I was struck by the elegance of it. No batteries. No calibration. Just biology in sync with Earth. This natural watch is even crocodile-proof. Literally. When the herds cross the Grumeti River, hundreds of hungry reptiles are waiting, less impressed by the poetic timing of migration, and more focused on the buffet arriving by the thousands. Fortunately for the wildebeest (and the calendar), there are so many of them that a few losses to crocodile jaws don’t stop the march. The feast goes on, and so does the movement of time across the savannah. In our world, it’s easy to stay on schedule. But if you ever feel unmoored, watch the wildebeest. You might not learn the date, but you’ll remember what time really means.  

Setting the Horizon: Why the Sextant Still Matters The first time I held a brass sextant, it wasn’t in a museum. It was aboard a training vessel, the Hawthorne, off the coast of Nova Scotia. Salt in the air, no visible shoreline, and just the waning sun hovering low on the horizon. Our instructor, an old Royal Navy man named Captain Ellis, handed me the instrument and said, “Shoot the sun. It’ll tell you where you are.” That moment, where math, light, and motion all clicked into a clean, gliding arc, sparked a fascination that’s never left me. The sextant is more than a tool. It’s a portal to a time when knowing your place in the world required craftsmanship, patience, and a glance skyward. So in this post, I trace the history of the sextant, from ancient roots to digital obsolescence, with stories from those who built, wielded, and preserved it. Star-Bound Beginnings: From Astrolabes to Shadow Devices Long before the sextant came into existence, ancient mariners were already looking to the sky for guidance. I visited Dr. Leyla Arabi at the University of Coimbra’s Institute for Historical Navigation. She showed me a 15th-century mariner’s astrolabe, a brass disc dense with degree markings. “This was cutting-edge at the time,” she said, turning it in her hands. “But using it at sea? Nearly impossible in rough weather.” Instruments like the astrolabe, the quadrant, and the cross-staff laid the groundwork, but they all struggled with one key issue: the sea moves. Looking at a star and the horizon simultaneously was unreliable on a pitching ship. By the late 1500s, John Davis’s backstaff allowed navigators to measure the sun’s altitude with their backs to it, relying on shadows instead of direct sight. It was clever and safer, but not precise enough for the evolving demands of global navigation. The Sextant Is Born: Mirrors, Math, and a Moment in 1731 The breakthrough came with double reflection. The principle was simple but revolutionary: two mirrors let a small instrument measure large angles, and with better accuracy. This concept had been sketched by none other than Isaac Newton, but he never built a working device. That job fell to John Hadley in England and Thomas Godfrey in Pennsylvania. In 1731, both independently developed what became known as the octant, technically a 45° arc, but capable of measuring 90° thanks to the mirror system. At the Royal Society archives, I met curator Malcolm Price, who showed me a replica of Hadley’s early instrument. “What’s incredible is how quickly this became indispensable,” he said. “By mid-century, no serious navigator would leave port without one.” To tackle longitude (a far more difficult problem than latitude), a broader arc was needed. Enter Captain John Campbell, who in 1757 proposed enlarging the octant to a 60° arc, creating the first true sextant. Instrument maker John Bird delivered a 20-inch brass beauty, now preserved at the National Maritime Museum. Refining the Tool: Ramsden’s Machine and the Rise of Precision Even as the core optics of the sextant remained consistent, 18th and 19th-century engineers turned it into a masterclass in precision engineering. Jesse Ramsden, a name often whispered with reverence in horological circles, created a mechanical dividing engine in the 1760s. This allowed for ultra-fine scale graduations, making sextants readable to fractions of a minute. I spoke with Lucien Carter, a private collector in Portsmouth, who let me handle a Ramsden-era sextant. “It reads to ten arc seconds,” he said proudly. “And the arc hasn’t warped a bit in 200 years.” Wood frames gave way to brass and bronze. Peep sights became telescopes, and vernier scales were replaced with micrometer drums by the early 1900s. Colored filters helped users view the sun safely, and quick-release clamps made angle adjustments smoother. By the 1930s, a well-made sextant was a miracle of mechanical precision. And yet, it still relied on the same principle Hadley had revealed two centuries earlier. The Ocean’s Compass: How the Sextant Transformed Navigation If you had to choose one instrument that made global maritime exploration possible, it would be the sextant. By the late 18th century, it became standard kit on ships of exploration and commerce. Captain James Cook, for instance, took several sextants on his Pacific voyages. One, crafted by Ramsden himself, still sits in London’s Maritime Museum, worn from use but still readable. I visited the museum's conservation lab where maritime historian Dr. Isabel Grant explained its importance: “Cook didn’t just use these for latitude. He was measuring lunar distances, working out longitude in the field, this was revolutionary.” The sextant’s reliability was legendary. After the Mutiny on the Bounty, Captain Bligh navigated an open boat 3,600 miles with just a sextant, quadrant, compass, and watch. Over a century later, in 1916, Frank Worsley used an 8-inch Heath & Co. sextant to guide Shackleton’s lifeboat, James Caird, across the frigid Southern Ocean. That very sextant, its frame battered by salt and time, is now preserved at the Scott Polar Research Institute in Cambridge. Skyward Navigation: The Sextant Takes Flight By the 20th century, aviation pioneers faced the same challenge as mariners: how to navigate without landmarks. The solution? Adapt the sextant. In 1922, Portuguese navigator Admiral Gago Coutinho invented a bubble sextant, using spirit levels to simulate the horizon in flight. He and his pilot flew from Lisbon to Rio using star fixes alone. Dr. Helena Vargas, an aerospace historian I met at the Lisbon Air Museum, emphasized the leap: “Coutinho’s flight proved celestial navigation worked in the air. It opened the door for long-range aviation.” Aircraft sextants evolved rapidly. The Weems & Plath models and U.S. Navy Mark series instruments were used in bombers during World War II. By the 1950s, even commercial jets like the Boeing 707 had sextant domes in the cockpit ceiling. Sunset of a Legacy: GPS and the Sextant’s Decline The story shifts dramatically in the 1990s. GPS, with its near-perfect accuracy, arrived like a lightning strike. Suddenly, navigating by stars felt archaic. Sextant sales plummeted. I spoke to an equipment dealer in Rotterdam who’d sold marine sextants for 40 years. “After GPS? It was over,” he said. “We went from selling hundreds a year to five. Maybe.” But not everyone gave up. In 2015, due to concerns about GPS vulnerability, the U.S. Naval Academy reinstated celestial navigation training. “If the system goes down, a sextant still works,” said Lt. Commander Jordan Hill, a navigation instructor I met in Annapolis. “It’s a fail-safe you can’t hack.” The Instruments That Endured: Preserving Sextants and Their Stories Today, historic sextants are kept not just in glass cases, but in the memory of exploration itself. I’ve seen Cook’s brass sextant in Greenwich, Bligh’s navigator in Sydney, and Worsley’s battered instrument in Cambridge. Each tells a story not just of where someone was, but how they got there, and why it mattered. Aviation museums hold Coutinho’s prototype, while World War II relics like the RAF Mk IX bubble sextant still show up at collectors’ auctions, often with flight logs tucked in their velvet cases. In sailing communities, a small but dedicated group still practices the art. I joined a sight-taking session off Cape Cod last summer. No electronics, no maps. Just arc, shadow, and sky. As one old-timer named Rob put it, “With a sextant, you don’t just know where you are. You understand why you’re there.” Final Bearings: The Sextant’s Enduring Compass The history of the sextant is a history of human orientation, not just geographically, but intellectually. It embodies our desire to measure, to explore, to know. Even in the GPS era, it still teaches us something timeless: how to look outward with purpose, and inward with precision. And sometimes, on a quiet ocean evening, that’s all you need. At least, that’s what I choose to believe.

The First Time I Used a Sextant (and Why You Should Try It Too) I remember my first successful “sight.” I was standing on the deck of the Hawthorne, a sturdy cutter used by a navigation school off the coast of Maine. The sun was just dipping toward the sea. My hands were clumsy, the brass arc cold. Captain Ellis, the instructor, looked over my shoulder and muttered, “You’re about to find yourself.” That was the moment I realized a sextant isn’t just a tool. It’s a bridge between mathematics and the horizon? a way to know your position on Earth using nothing but light, angles, and time. In an era where GPS does it all silently, learning how to use a sextant connects you with centuries of seafarers, explorers, and aviators who relied on this elegant instrument to navigate vast oceans and uncharted skies. If you've ever been curious about celestial navigation, this guide is for you. I’ll walk you through how to use a sextant, in plain English, no jargon. We'll keep it simple and hands-on, with a summary tutorial at the end. But first, a bit of context. What Is a Sextant, Really? A sextant is a precision instrument used to measure the angle between a celestial object (like the sun, moon, or a star) and the horizon. This angle, known as the altitude, allows you to determine your latitude, and with additional information and a bit more calculation, even your longitude. It works using two mirrors. When you align the reflected image of a celestial body with the visible horizon, you can read the angle off a graduated scale. That angle is your key to knowing your position on the planet. The beauty of the sextant is its independence: no batteries, no electronics, just physics and optics. Tools You’ll Need Before You Begin To take a sight with a sextant, you’ll need: A sextant (obviously)? preferably one with a micrometer drum and adjustable index mirror. A marine chronometer or accurate watch? to note the exact time of your sight. A nautical almanac? which gives celestial positions for specific times and dates. Sight reduction tables or a navigation calculator? to turn your angle and time into a position. A clear view of the horizon? sea or artificial horizon if on land or in flight. Now let’s walk through the basics of how to use a sextant. Step-by-Step: How to Use a Sextant I spoke with marine navigator Elena Rivas, who trains cadets in traditional celestial navigation. Here’s the method she teaches: 1. Set the Sextant to Zero “First thing, always check your index error,” Elena told me. To do this, align the horizon using the horizon mirror and reflected view. If the line isn’t level, note the difference? this is your index error, which you’ll need to subtract or add from your final angle. 2. Point at the Horizon Hold the sextant vertically with the telescope aimed toward the horizon. Keep your eye on the horizon line. 3. Bring the Celestial Object Down Using the index arm, move the mirror so that the image of the sun, star, or moon drops down to the horizon line. You should see both the horizon and the reflected celestial object in the same field of view? lined up perfectly. 4. Rock the Sextant Rock the instrument slightly side to side. The lowest point of the arc (the dip) is the correct reading. This ensures you’re measuring the true altitude. 5. Read the Angle Once aligned, lock the arm. Read the angle on the arc and micrometer drum. This is your observed altitude. 6. Record the Time Note the exact UTC time of the sight using your watch. Accuracy is crucial, especially for longitude. 7. Correct and Calculate Apply corrections: Index error Dip (based on height of your eye above sea level) Refraction (from standard tables) Sun's semi-diameter (if measuring the sun’s lower limb) Now you have the corrected altitude. Use the nautical almanac and sight reduction tables to plot your line of position. A Simple Summary Tutorial: “Sun Sight at Sea” in 60 Seconds If you're short on time, here’s the ultra-simplified version of how to use a sextant to find latitude with the sun at local noon: At noon, take a sight of the sun at its highest point (when it stops rising). Record the angle and exact time. Look up the sun’s declination for that date in the almanac. Latitude = 90° – observed altitude + or – declination Add if the sun is in the same hemisphere Subtract if it's in the opposite hemisphere That’s it? you’ve got your latitude. It won’t be perfect, but it will be close enough to get you home. Final Thoughts: Why Learn This Now? I asked Captain Ellis recently why he still teaches sextant use when every phone has GPS. He said, “Because knowing how to use a sextant means you’ll never be lost, even when everything else fails. And maybe more importantly, it teaches you how to see.” He’s right. Learning how to use a sextant isn’t just about survival. It’s about learning to observe, measure, and understand your place in the world through your own hands and eyes. So go ahead? try it. Aim for the sun, line up the horizon, and find yourself the old-fashioned way.