Tag Archives: astronomy

The sextant

The sextant is an instrument used to determine latitude and longitude. It was invented by Edmond Halley or John Hadley or Thomas Godfrey based on Isaac Newton’s ideas. You use a sextant to measure how far above the horizon the Sun is at noon (or how high Polaris is at night).

I’ve talked about finding your latitude or longitude by measuring the angle between a line from you to the moon and a line from you to a star. How do you do that?

Sailors have been using a sextant for centuries. Isaac Newton dreamed up the idea, then Edmond Halley built one in 1692. Several refinements were made around 1730, until we finally got the good old sextant you see people using in the movies with wooden ships and guys wearing wigs. A sextant is like an astrolabe—you sight something familiar in the sky, like a star, by lining it up along a sighter or pointer or alidade. Then you mark the alidade’s position on the frame and use that information to find your location.

Instead of an alidade the sextant has a telescope you look through. Then you find both the object (usually the Sun) and its reflection in the mirror. There are 2 mirrors facing each other and smoked lenses so you don’t fry your eyeballs. One mirror is half mirror/half glass so you can see both images at the same time. You bring the image of the Sun down to the horizon by moving the arm on the sextant and—oh, who am I kidding? Do you think I know what I’m talking about? What I need is a video to show how it’s done. Luckily, there are a bunch of them on the ol’ internet. Listen to these guys.

Here’s an in-depth 4-part tutorial covering everything you ever wanted to know about how to use a sextant.: https://www.youtube.com/watch?v=00ZEIZsl5xk

And a guy who doesn’t own a comb but knows what he’s talking about: https://www.youtube.com/watch?v=DrAkrgZRb9Y

The thing you have to know is: the sextant will give you a precise angle between 2 objects that you can transfer to a chart or map to get your position on Earth.


Look! Look! Here’s a cardboard sextant you can build yourself! https://www.landfallnavigation.com/cardboard-sextant-kit.html?gclid=CjwKCAjwq832BRA5EiwACvCWsWqv38smCfmUdVE5SqxRVkVJI_R0B9aN9jSb-3oaweER7b4KTm4iJhoCUbMQAvD_BwE

Here’s a plastic sextant—https://www.google.com/shopping/product/15301038968540324177?q=navigation+sextant&prds=epd:1143652539522712674,prmr:3,tpim:CKyp-dn168W22QEQ1N-msv7m0OotGMCbhR4iA1VTRCjg3sL8BTCNyIhA,pdprs:6&utm_medium=tu_image&utm_content=eid-lsjeuxoeqt&utm_campaign=134358029&gclid=CjwKCAjwztL2BRATEiwAvnALcqb-eUHLE4Y_XEM7QpPWFcXOcl2YbghLFCIcLIG1ItYYMc__vQ3_jBoCA14QAvD_BwE

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The clockmaker

In our last post you saw how to find your location by observing the moon and stars to calculate lunar distance. The object is to know both your local time and prime meridian time, or Greenwich Mean Time. A navigator needs to be an astronomer and a math whiz to use this method.

You may have asked yourself, “Wouldn’t it be easier to keep 2 clocks aboard the ship—one showing Greenwich Mean Time and the other kept to local time?” That’s an excellent question and I’m glad you asked it. In fact, that’s the question John Harrison asked.

John Harrison, English inventor and horologist, 1767.

John Harrison was a cabinet-maker with a side business building and repairing clocks. To win the Longitude Prize, he went for a straightforward solution: build an accurate clock that always, ALWAYS showed precisely the correct time in Greenwich.

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Lunar distancing

Okay, let’s say you’re in a rowboat at night with some friends—and you haven’t seen land for a while. You’re LOST. Nobody’s getting a signal on their cellphones, so you don’t know where you are. The strange old lady at the boat rental place left nothing but a weird navigational device; a map; and a book of star charts in the boat’s locker. Your friends are getting panicky and start blubbering. What do you do?

Because you’re a devoted reader of The Western Civ User’s Guide to Time and Space, you know exactly what to do. You tell your pals to stop their noise so you can concentrate. It’s a clear moonlit night, so you can see the moon, stars, and the horizon. You pick up the lovely brass sextant and set its sights on the moon—and a star, how about Regulus, just there to the left? You measure the altitude (how high above the horizon) of the moon; the altitude of Regulus; and the distance between them. You figure the angle of the 2 lines from you to the moon and you to Regulus. You do this measuring not in feet or miles but in degrees.

From the moon’s altitude you know what time it is (http://www.astrotulsa.com/page.aspx?pageid=27, scroll down)—and your latitude, too (http://www.lewis-clark.org/article/1268). Knowing the distance from the moon to Regulus, you pick up the book of star charts and find that lunar distance for your local time. Run your finger down the chart to find what time it is in Greenwich, England where it’s zero degrees longitude. The difference in time will tell you your longitude (15° for every hour, 1° for every 4 minutes). Find your latitude and longitude on the map and start rowing home. You don’t even need a compass—you keep Polaris, the North Star, above your right knee as you row.

You get safely back to land! Your friends can’t believe you saved the day with that stupid book. The lady at the boat rental gives you a wink and you all go home to bed.

This is how Nevil Maskelyne proposed finding your position while at sea.

I haven’t read these, but here’s a short list of books about ocean-going girls: https://books.google.com/books/about/From_Cabin_Boys_to_Captains.html?id=wBDWSAAACAAJ

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The astronomer

Gerard van der Puyl’s portrait of Nevil Maskelyne—just gorgeous. What a painter.

Nevil Maskelyne was the fifth Astronomer Royal at Greenwich Observatory. He proposed using the positions of the stars, planets and their moons as a method of calculating your position on Earth, just as Galileo had proposed using the moons of Jupiter as a universal clock. Maskelyne was a hard worker and determined to win that Longitude Prize. He believed that with accurate charts of stars’ positions, you could find longitude anywhere on Earth. At the Observatory, Maskelyne and a team of astronomers ‘worked feverishly through the year 1766, preparing tables for the new Nautical Almanac and Astronomical Ephemeris. Published first with data for the year 1767, it included daily tables of the positions of the Sun, Moon, and planets and other astronomical data, as well as tables of lunar distances giving the distance of the Moon from the Sun and nine stars suitable for lunar observations.’

Here’s Maskelyne in a nutshell:

Oops! Okay here’s Maskelyne in brief:

Sorry! Sorry! Here it is: Maskelyne’s idea was that you have a point zero of longitude—the Prime Meridian—as a reference point for time. Longitude is time measured in degrees. Each hour is 15° of longitude. When you’re at sea you take 2 measurements: the Sun’s position and the moon’s position. The Sun’s position tells you what your local time is; you find the moon’s position (the distance from the moon to one of the 9 suitable stars) in your almanac to tell what time it is at the Prime Meridian. The difference between your time and Prime Meridian time can be converted into degrees, which gives you your longitude.

You’re probably thinking: ‘Okay, Manders, how exactly do you measure the Sun and the moon? Usually when the moon’s out it’s nighttime.’ That’s an excellent point! You measure the Sun during the day and adjust the ship’s clock to the local time, maybe?






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Oh, it’s on

Offering 20,000 pounds back in 1714 was like offering millions of dollars today. Who wouldn’t want to win that longitude prize money? All you had to do was devise a precise method for finding your location on Earth.

The responses didn’t come pouring in overnight. You may have noticed that the big naval disaster on the Scilly Islands happened a whole 7 years before Parliament got around to forming the Longitude Commission. Some serious brainwork—and tinkering—needed to be put in. This was going to take awhile. As mentioned earlier, time/distance/astronomy are interlinked. The people who responded to the Longitude Commission’s offer worked in astronomy and time.

There were 2 main competitors in the race to find longitude: an astronomer and a clockmaker.

The Royal Observatory

Wren tore down the castle standing here and built the observatory out of the recycled stone. What a guy!

By now all 17 of you weirdos who’ve been reading this history have gotten the idea that time/distance/astronomy are interlinked—at least if you want to know where you are.

Eratosthanes, then Ptolemy, then just about everybody who drew a map chose their own prime meridian. That’s fine so far as it goes. But to be useful, a prime meridian needs to have an observatory. An observatory is where astronomers can keep track of the stars and regularly publish charts showing their positions and movements. Navigators can use those charts to check the stars’ positions and tell time from that information.

In ad 1675, King Charles II realized how important astronomy would be to all those ships who were expanding England’s trade around the world. He commissioned an observatory in Greenwich (a section of London) and had it designed by Sir Christopher Wren, the architect who designed pretty much every building in London.

Something I never knew before this morning: Christopher Wren was an astronomer as well as an architect. One of those professions was his side-gig. Makes one’s life achievements seem somewhat inadequate, doesn’t it? You’re welcome.


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Find longitude, win valuable prizes!

In 1714 the British government established the Board of Longitude and held a contest: Who can come up with a system to accurately tell where you are at sea? The Royal Navy disaster on the Isles of Scilly had caused a reaction from the British public—they’d seen their own sailors drowned on the coast of their own country. Whether that disaster was the motive for creating the Board of Longitude may be disputed, but there’s no doubt a better, more accurate system of finding your location at sea needed to be found.

The Longitude Act, ‘An Act for providing a Publick Reward for such Person or Persons as shall discover the Longitude at Sea,’ was passed. The government offered up to £20,000 for a method of finding longitude at sea to within half a degree. That’s a lot of clams!

The Board of Longitude included scientists and astronomers who would judge the quality of any longitude systems that were submitted. One was the Astronomer Royal who worked at the Greenwich (GREN-itch) Observatory. I think if I ever got my career to the point where my job-title was Astronomer Royal I would consider myself to have arrived.

Astronomer Royal John Flamsteed, who also made the first recorded observations of Uranus.


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Eratosthanes and longitude

Old-time tv newsrooms had clocks on the wall set to local times of the big cities.

How did Eratosthanes or Ptolemy determine where the longitude lines should go? I got this from the History Stack Exchange site:

Longitude is calculated by comparing the elevation of an astronomical object to the pre-calculated (or observed) elevation of the same object at a reference location at the precisely simultaneous moment in time. Everything in the sky rotates once around that vast celestial sphere every 24 hours, so the more precisely one can establish simultaneity the more precise one’s measurement of longitude will be.

Whew! In other words: 2 people standing in 2 different places can measure the height in the sky of the moon, or the Sun, or the North Star to figure out how far east or west they are from each other. BUT—the measurement must be taken at exactly the same moment. Eratosthanes figured a way to find longitude without the measuring. Eratosthanes (in Alexandria) and an assistant (in someplace to the west—maybe Benghazi?) watched a lunar eclipse. They agreed to mark the exact local time the eclipse began. The local times won’t be the same, right? When it’s midnight in Benghazi, it will be 12:36 am in Alexandria. The difference in their times told Eratosthanes how many degrees apart from each other they were.

I’ll tell you how in the next post.

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The Tropics

Here’s something interesting: the Tropic of Cancer and the Tropic of Capricorn don’t stay put. They move around a little bit. The Tropic of Cancer marks where the Sun will shine the longest in the northern hemisphere for this year—that is, the summer solstice. It’s at around 23-24 degrees north. On the summer solstice, June 21st-ish, the Sun will shine directly over the Tropic of Cancer.

The Tropic of Capricorn marks where the Sun will shine the longest in the southern hemisphere—that is, the winter solstice. It’s at around 23-24 degrees south. On the winter solstice, December 21st-ish, the Sun will shine directly over the Tropic of Capricorn.


Here’s kind of what they look like on a flat map.

The reason the Sun shines directly on the Tropic of Cancer in summer and the Tropic of Capricorn in winter is that our Earth is tilted as she revolves around the Sun. We learned that way back when we read about Eratosthanes.

It takes one year for Earth to orbit the Sun. When the Northern Hemisphere is tilted toward the Sun it’s Summer there. When the Southern Hemisphere is tilted toward the Sun it’s Summer there.

Latitude and Longitude

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Heliocentric blasphemy!

“We can’t torquemada heliocentrism; we can’t torquemada Copernicism; we can’t torquemada anything!”

We learned about how Aristotle and Ptolemy promoted the idea that the Earth is the center of the universe; she doesn’t move; all the planets and stars revolve around her. This is called geocentrism.

Copernicus had questions: if the planets and stars revolve around the Earth, howcome their orbits aren’t all perfect circles? He proposed that the Sun is the center of the universe and the planets and stars revolve around him. This is called heliocentrism.

When Galileo, with his newly-built telescope, observed moons revolving around Jupiter he could plainly see that not every heavenly body revolved around the Earth. Copernicus was right—at least Jupiter’s moons revolve around Jupiter. If Jupiter’s moons weren’t geocentric, how much else of the universe wasn’t geocentric?

This is the scientific method that is Galileo’s gift to us. He observed and asked questions and looked for proof.

Galileo was a brilliant self-promoter and made friends in high places. Nevertheless, his assertion that Aristotle was wrong got him in trouble with the Catholic Church. It’s not really clear to me what it was exactly that got him in hot water. Yes, there’s a passage in the Bible about the moon and Sun standing still (Joshua 10:13), but so what? The moon and Sun could still appear to stand still in a heliocentric universe.

It may be as simple as: Protestantism was still fresh; Christians were reading and interpreting the Bible for themselves (before moveable type only the priests had copies of the Bible); the Catholic Church’s establishment saw its power being challenged. Then Galileo came along and said everything you believe about God’s creation isn’t so. That may have been enough to cheese off the Church and put Galileo in front of the Inquisition.

Galileo did himself no favors when he published a fictional argument between 3 guys—to explain and prove his thesis—and made the guy with the pope’s point of view the moron. The upshot was heliocentrism was found to be heretical (against biblical belief) and Galileo was told never again to publish his heliocentric blasphemy. He was put under house arrest for the rest of his life.

It’s easy nowadays to paint the Church as the anti-science bad guy. This was indeed an embarrassing day for Christianity. If you look at the entire history of the Church, though, she’s done way more to encourage science and learning than to suppress it. Going back to Charlemagne, monasteries were the place you went to find books by classical thinkers, painstakingly translated into Latin by the monks. Most universities were originally Christian institutions. Anyhoo, a more recent pope finally admitted—after all these centuries—Galileo was right (thanks for linking this, Chuck Dillon!).


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