Tag Archives: astrolabe

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

Back to the beginning of The Western Civ User’s Guide to Time & Space

It’s been a long trip so far

I want to take a moment to thank everyone who’s been hanging with me throughout this little history. I feel like I’ve been telling a year-and-a-half-long joke and I’m just about to get to the punchline.

Since January last year, we discovered how people in the past measured space and measured time. We followed along as travelers ventured further and further from home—traders and explorers became bold enough to make long voyages across the oceans without knowing exactly where they were. To navigate, you need to measure both time and space. Most of the inventions that measure time and space didn’t appear until around the 1500s.

Here’s what you need if you plan to cross an ocean and would like to know where you are going:
1. an accurate map, to know what you’re looking for
2. a compass, to orient the map
3. an astrolabe, to find latitude (how far north or south you are)
4. universal time, to find longitude (how far east or west you are)—Galileo proposed that if you can see the positions of Jupiter’s moons, and you know what time it is locally, you can figure out your longitude. Jupiter’s moons would be a universal clock.
5. an accurate clock, to know local time

The Exploration Age sailors set sail without an accurate map or clock, because those things didn’t exist yet (a pendulum clock doesn’t work on a rocking ship).
Astrolabes or Jupiter’s moons are only useful when the sky is clear. So, even in the 1500s sailors didn’t have all the tools they needed for navigating.

Back to the beginning of The Western Civ User’s Guide to Time & Space

East is east and west is west

Navigators still faced the problem of not knowing how far east or west they were. Latitude is how far north or south you are. You can tell that with an astrolabe. Longitude is how far east or west.
It was a problem Amerigo Vespucci tried to solve. In 1502, he wrote: “…I learned [my longitude] … by the eclipses and conjunctions of the Moon with the planets…” He was trying to find longitude by observing the Moon’s and Mars’ positions in relation to the Earth. Not only was this an overly-complicated method, it had several drawbacks—mainly it only worked during a specific astronomical event.


I’d be a clod not to link Rudyard Kipling’s poem, which I quoted in the title above—http://www.kiplingsociety.co.uk/poems_eastwest.htm

While we’re at it, here’s Bob Hope and Jane Russell in The Paleface, singing Buttons & Bows.

Hypatia, the lady philosopher/mathematician/astronomer

Sometimes writing about history is hard. At the beginning of this project, I’d envisioned a fun and slightly wacky tour through Western Civilization presented by a fun and slightly wacky uncle. The story of human beings is often violent and cruel, though. Human beings are flawed creatures. There are parts of civilization’s history that make me heartsick.

I want to tell you about a glorious and brilliant woman who lived in Alexandria, Egypt in the 4th century ad. Her name is Hypatia. She was the daughter of a philosopher and mathematician, and she became a philosopher and mathematician herself. Hypatia was lovely to look at, but her intellect was what made her irresistible. I became interested in her when I read that if she hadn’t outright invented the astrolabe, at the least she played an big part in developing it.

Hypatia helped her dad expand Ptolemy’s work on astronomy. She eventually did her own scholarly work on the stars and the geocentric model of the universe. It might have been she who came up with the idea of ‘flattening’ the spheres to create the star-map that is central to the astrolabe’s design (this is my own conjecture).

Hypatia was also a philosopher. Her philosophy was Neo-Platonism, which adapts Plato’s ideas about what makes the world tick. Hypatia was a gifted teacher. She often gave lectures in the agora (Greek for public square) about philosophy, astronomy, and her many intellectual interests. At that time Alexandria was boiling with religious trouble. Christians and Jews battled with each other and with philosophers who rejected a belief in God. The Roman Empire was in its last days, about to split into East and West. Politics absolutely played a part in the violent clashes between religion and philosophy.

I’m sorry and ashamed to tell you that a mob of Christians ambushed Hypatia, and murdered her. It’s nearly impossible for me to believe that these were followers of Jesus. But it happened. They behaved as a mob—the very opposite of civilization—and murdered a woman for promoting a philosophy that rejects Christianity.

You can read more about Hypatia here and here. There’s a movie about her, too.  Here’s a review—caution: some violence.  (I have a quibble with History Buff’s conclusion that, because of Christianity, scholarship was discouraged in the Dark Ages. Beginning with Charlemagne, the Holy Roman Empire fostered a culture of learning that was encouraged in the monasteries. Monks copied books by hand—the books from the Greek philosophers that had been translated into Arabic, which the monks then translated into Latin. See How The Irish Saved Civilization by Thomas Cahill.)