Navigating by the stars: what is a sextant?
Long before GPS, sailors and explorers found their way across oceans and wilderness using the sun, the stars and a clever instrument called a sextant. Here’s how they did it, and how you can try it in your own backyard.
Why the stars?
The stars are like a giant map painted on the sky. As Earth spins, the stars seem to wheel around the sky, but always in the same pattern. If you know which star you’re looking at, how high it is above the horizon, and exactly what time it is, you can work out where you are on Earth. That’s called celestial navigation.
The North Star: nature’s signpost
One star almost never moves: Polaris, the North Star. It sits almost exactly above Earth’s North Pole, so all the other stars seem to circle around it.
- To find it: find the Big Dipper. Follow the two stars at the end of its “cup” upward about five times the distance between them. That fairly bright star on its own is Polaris.
- Face Polaris, and you’re facing true north.
So what is a sextant?
A sextant is a hand-held instrument for measuring the angle between a star (or the sun, moon or a planet) and the horizon, very precisely, even on a rocking ship.
- You look through a small telescope at the horizon.
- Two mirrors bounce the image of the star down into your view. You move an arm until the star appears to sit right on the horizon line.
- Then you read the angle off the curved scale at the bottom.
Why “sextant”? Its curved scale is about one-sixth of a circle (sextans is Latin for “a sixth”). Thanks to the double mirrors, that sixth of a circle can measure angles up to about 120°.
Sailors measured the sun at exactly noon, when it was highest in the sky, to find their latitude every day. Sextants were used for about 250 years, and they’re still carried as a backup on many ships today. Even the Apollo astronauts had a sextant built into their spacecraft to check their position on the way to the Moon!
The hard part: longitude and time
Latitude (how far north or south) is fairly easy from the stars. Longitude (how far east or west) was a puzzle that cost many ships and lives.
The answer was time. Earth turns 360° in 24 hours, which is 15° every hour. If you know the exact time back home, and you can tell from the sun or stars what time it is where you are, the difference tells you how far east or west you’ve gone.
- Lethbridge is about 113° west of Greenwich, England. 113 ÷ 15 = about 7.5 hours. That’s why the sun reaches its highest point here about seven and a half hours after it does in Greenwich.
- In the 1700s, an English clockmaker named John Harrison built a clock accurate enough to keep time on a rolling ship, called a marine chronometer. It changed navigation forever.
The man who gazes at stars: David Thompson
David Thompson arrived at Hudson Bay in 1784, aged just 14, and spent 34 years exploring and fur trading across the West, from the Peace River to the mouth of the Columbia. He crossed the Rockies through what is now Banff National Park.
Everywhere he went, he stopped to measure the stars with a brass sextant and used a telescope to time the moons of Jupiter, which let him work out his longitude. Some Indigenous people called him Koo Koo Sint, “the man who gazes at stars.” His wife, Charlotte Small, a Cree woman, travelled thousands of kilometres with him.
From all those star measurements, he drew a giant map of western Canada so accurate that it was used for many years afterward. Some call him the greatest land geographer who ever lived.
People navigated by the stars long before sextants
Many peoples around the world have travelled by the sky for thousands of years. Polynesian navigators crossed thousands of kilometres of open Pacific Ocean in canoes by memorizing where stars rise and set, and by reading the waves and birds. Indigenous peoples of the plains knew the stars and seasons well, and used the sky to guide travel and mark the time of year.
✏ Try it yourself: make a star-measuring quadrant
You’ll need a plastic protractor, a drinking straw, tape, about 30 cm of string, and a washer or small nut.
- Tape the straw along the straight edge of the protractor.
- Tie the string through the small hole at the centre of the protractor, and tie the washer to the other end so it hangs down across the curved scale.
- On a clear night with an adult, find Polaris. Look at it through the straw.
- Hold still and let the string hang. Pinch it against the scale and read the angle. The string shows 90° minus the star’s height, so subtract your reading from 90.
- Your answer should be close to 50°. You’ve just measured your latitude, just like a sailor or David Thompson!
Find the North Star without the quadrant too: see Compass and orienteering. Learn more about the night sky in Sky & Space.
❓ Quick quiz
Guess first, then tap to see the answer.
What does a sextant measure?
The angle between a star (or the sun) and the horizon.
In Lethbridge, about how high above the horizon is the North Star?
About 49.7 degrees, the same as our latitude.
To find longitude, what did sailors need besides the stars?
A very accurate clock (a chronometer), because Earth turns 15 degrees every hour.
What did some Indigenous people call David Thompson?
Koo Koo Sint, “the man who gazes at stars”.
🏫 For teachers and parents
- Ages: 9 to 15; links to space science and Canadian exploration history.
- Make the quadrant in class (20 minutes), then measure Polaris at home with a parent on a clear night and compare answers the next day. Also measure the angle to the top of the flagpole for a trigonometry link.
- Longitude and time: have students work out how many degrees of longitude separate Lethbridge and Greenwich (about 113), and how many hours that is (about 7.5).
- History: David Thompson and Charlotte Small make a great research project on exploring and mapping the West.
- More ideas in the Teachers’ Corner.
More Earth Explorers
Sources: The Canadian Encyclopedia, David Thompson: the greatest geographer; Smithsonian Air & Space, How things work: celestial navigation.