Category Archives: book promotion

Hey, what time is it?

We’ve talked about telling time with sundials and water clocks and hourglasses. Those things are a headache to carry around. Mechanical clocks, like a pendulum clock, wouldn’t be invented until 1637. What if you’re traveling around in ad 800—how do you know what time it is?

One way to tell time was this fantastic little device called an astrolabe.

This sketch is based on a beautiful antique brass astrolabe.

Wherever you happen to be, if you can see the Sun or the stars, you can tell the time if you’re carrying an astrolabe with you. The main feature of an astrolabe is a flat map of the sky—with the stars and planets on a grid. The grid—called a climate—shows the sky as it appears in your part of the world. It’s circular and fits into a circular frame, called the mater (Latin for ‘mother’). On top of the climate is the rete (Latin for ‘net’), an openwork circular plate with pointers that you can line up to point at the Sun or a specific star on the climate. On top of that is a sighter—a straight arrow kind of piece. All these spin on the same axis. You pick a star, adjust the rete to point at your star on the climate, and hold up the astrolabe and sight the actual star along the sighter. When the sighter lines up with the star, you can read the time with remarkable accuracy. Here’s a video showing how it’s done. This guy even made his own astrolabe. And here’s more.

Here’s a website that explains how to use an astrolabe and even gives you pdfs you can download and print to make your own.

Wait—I thought *I* was the center of the universe

The orb in the center of this contraption is the Earth.

The idea of multiple nesting spheres—each sphere tracking a ‘wandering star’ or planet; the fixed stars; and the motions of the Sun and Moon—is kind of complicated. Astronomers built models of the geocentric universe to try to explain it. These are called armillary spheres.

They were usually made out of brass. Here’s one you can build out of cardboard.

Even though Earth isn’t the center of the universe, the model still works for locating positions of stars as we see them from Earth. Not only was Ptolemy’s data useful for knowing where the stars are and tracking them, but we can also predict where they will be tomorrow or next year. It became possible to know exactly when the Sun will rise and set years in the future. Astronomers could predict eclipses of the Sun or Moon. You can accurately tell the time based on Ptolemy’s data. It was a pain in the neck to carry an armillary sphere around, though. Something more compact was needed.

This is NOT an armillary sphere. It’s an armadillo sphere. Just in case it turns out armadillos are the center of the universe.

https://www.thoughtco.com/armillary-spheres-and-what-they-got-wrong-1991234

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How they got all the planets inside that building I’ll never know

If you’re lucky enough to live near a planetarium, you should go see one of their shows.

As Ptolemy worked out the positions of the stars on a big sphere, he imagined smaller spheres that account for the movement of the Sun and the planets— operating like separate, smaller gears in a giant clock. The amazing thing is, even though Earth isn’t the center of the universe, Ptolemy’s geocentric model is still accurate. Weird, huh? If you ever visit a planetarium, you’ll sit in a round room with a domed ceiling above. At the bottom of the dome, all around the room, is the horizon. The night sky is projected onto the dome with all the fixed stars in their places—they rotate around, just like the real night sky. The planets are also projected onto the dome. The planet projectors operate on a separate gear system exactly like the spheres Ptolemy had proposed.

A rough sketch of a planetarium projector.

Here’s some interesting info about planetarium projectors. 

Here’s a listing of planetariums so you can find one near you.

Every little star in the sky

An azimuth compass shows 360° starting with 0/360°at due North.

Who wants to talk about Ptolemy some more? I told you about how people back then thought Earth was at the center of the universe and all the stars and planets revolve around her. This is called the geocentric model. The geocentric model has the stars on a humongous sphere. Good ol’ Ptolemy calculated the positions of all the stars and planets in the sky. He mapped exactly their azimuth and altitude.

Imagine you’re standing outside on a clear night and can see all the way to the edge of the sky—no trees or buildings in the way. That’s Earth’s horizon (we usually don’t see the horizon unless we’re in a flat desert or a big lake or ocean). You’re standing in the middle of a big circle, the base of a big dome. Directly above your head is the zenith, the center of the dome above. You can use a compass to find the direction of a star’s position—that’s its azimuth.

Using a cross-staff to find a star’s altitude.

You’ll need another device to figure how high that star is from the horizon—you work out the angle with your eyeball being the center point, the horizon at zero and the zenith at 90 degrees. A cross-staff is something Ptolemy may have used. You sight along the staff and point it in the star’s direction. Move the crossbar until it touches the horizon at the bottom and the star at the top. The position of the crossbar on the staff marks the angle of the star’s altitude.

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Charlemagne & Pope Leo III from yesterday

Here’s the painting I did yesterday. Still a bit rough; needs some tightening up. Charles’ right hand doesn’t look like it’s holding the hilt of that sword. I have to figure out how to mount my camera (phone) so it’s not in the way when I paint. I kept bumping into it.

Charlemagne & Pope Leo timelapse

Here’s a little painting—you can watch as it gets painted. https://www.instagram.com/p/B3-HCj-gCVs/

It’s an image for this post.  Enjoy!

Ptolemy—why not take Ptolemy?

Ptolemy showing his geocentric model of the universe. That’s Earth in the middle.

We’re coming up on the Middle Ages, everybody! The Roman Empire has become the Holy Roman Empire—we’re still working on creating individual countries out of the territories of various tribes. The Christian Church has become a stabilizing institution. With Charlemagne’s help, religious centers—like monasteries—are also places of learning. Alcuin of York (Charlemagne’s right-hand man) designed an easier-to-read way of writing using capital letters and small letters instead of ALL CAPS!

Life isn’t easy for regular peasants, which is most people. It’s hard to make a living on a farm. Some people head to the towns to learn a trade. Sanitation isn’t so great and the bubonic plague is always just around the corner. It will be centuries before anyone figures out that plague is carried by fleas.

It usually takes a long time for civilizations to accept new ideas. Even after Eratosthanes showed the Earth to be round, many people thought she was flat. Both educated and uneducated people believed that the Earth was the center of the universe and the Sun, planets and stars revolved around her. They thought of the universe as a giant sphere—a ball—with planets and stars stuck onto it. This sphere would be like clear glass. The word for this idea is geocentricgeo is a Greek word for Earth, centric means smack dab in the middle.

We get this idea from an astronomer named Ptolemy (TOE-leh-mee), who lived in Alexandria, Egypt (100-170 ad, we think). He improved on the basic geocentric idea. Ptolemy used math to place the stars in fixed positions on this big sphere. He observed that the planets didn’t stay fixed on the sphere, though, so they were called wandering stars and got their own smaller spheres that fit inside the big one. The Sun made its own orbit, a circle, called an ecliptic.

Here’s Billie Holiday singing “All of Me.”

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https://www.britannica.com/biography/Ptolemy
https://www.teachastronomy.com/textbook/The-Copernican-Revolution/Ptolemy-and-the-Geocentric-Model/

It’s better than bad, it’s good

There is a mention of an hourglass in a ship’s inventory in the 1300s.

When you need to know where you are at sea, this is what you have to go on.

At sea, it’s difficult to know exactly where you are. No landmarks. No way to tell how far you are from land. Sailors had to be creative. To figure out distance, they used time.

Sailors liked the hourglass because it isn’t as affected by a ship’s bouncing around on the waves as a water-clock would be—it still marks an hour accurately. Smaller sandglasses with less sand mark shorter periods of time. Sailors could judge how fast their ship were traveling by ‘casting the log.’ They had a length of line—heavy cord—with a piece of wood (the log) tied to its end, and knots tied in it at regular intervals. They’d throw the log into the sea and turn the glass at the same time. The log dragged behind and pulled the line with it. When the sand ran out, they’d nip the line and count how many knots had run out. Number of knots = distance traveled in a set space of time.

If you know how fast you’re traveling, you can make a guess how far you traveled in an hour or a day or a week. This is how the sailors used to calculate where they were on the ocean.

Casting the log is said to have been invented in the 1500s. The first evidence of a ship’s log-book is from the 1600s. This is a guess, but it seems to me that the log-book would have been originally a record of the ship’s speeds—that eventually became a record of any important events that happened aboard the ship. With time, ‘log-book’ got shortened to ‘log.’

Today, people who post regularly on the worldwide web refer to their sad ramblings as a ‘web-log’ or ‘blog’ for short. Spare a thought for us poor, attention-starved souls.

Here’s an excellent video about casting the log.

https://www.britannica.com/technology/log-nautical-instrument
https://en.wikipedia.org/wiki/Chip_log
https://www.archives.gov/research/military/logbooks/naval-deck-logs.html

The secret origin of “log in”

More about time and distance.

Read this now! Time is running out!

Hourglasses are good timers that are easy to use, so they’ve been part of people’s lives for centuries. My mom used to have a little one that measured 3 minutes—just right for boiling an egg.

The hourglass became a symbol for time itself, and just how quickly it seems to pass. Go into an old cemetery and you might see one carved into a tombstone—eeek!

On some computers a little icon of an hourglass shows up to tell you that your program is still loading.

By golly, I would have committed murder to own that Wizard of Oz hourglass when I was an art student, How about the lucky prop artist who got to design it!

If you ever saw the old Wizard of Oz movie with Judy Garland, you’ll remember that terrifying big hourglass that belonged to the Wicked Witch of the West. Here’s a website that tells how hourglasses are made. This one’s good, too.

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Hourglasses

An hourglass is a device that measures time. It’s two glass bulbs joined together by a skinny neck. There’s sand inside, so when you set the hourglass with the sand-filled bulb on top, the sand trickles through the neck into the empty bulb below. The hourglass maker put in exactly an hour’s worth of sand, so when the top bulb is empty, one hour has passed. The whole contraption is contained in a frame so it can be stood on either end.

Hourglassses are sometimes called ‘sand clocks,’ but they’re timers. Remember the early Egyptian water clocks? They were timers, too, until Ctesibius figured how to make clocks out of them.

Legend says that a French monk called Liutprand (Lee-UT-prond) invented the hourglass in the 8th century ad. There doesn’t seem to be solid evidence to support or deny that. Charlemagne is supposed to have owned a 12-hour hourglass—Liutprand was French, Charlemagne was French, so maybe there’s a connection? And I guess I’d have to admit that at 12 hours, that hourglass was a clock.

https://www.guinnessworldrecords.com/world-records/first-hourglass

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