Tag Archives: astronomy

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.”

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


Egyptian sundials


An Egyptian lady catching some rays from Ra.

Let’s travel west from Sumer, away from the MidEast, along the northern coast of Africa to Egypt. About 1,000 years after civilization was up and running in the Tigris-Euphrates valley, the Egyptians got started on their civilization which thrived from 3100 bc to 332 bc. Like the Sumerians, Egyptians depended on a river—the Nile—and a system of irrigation to water their crops to keep the economy going. Their writing system was hieroglyphics—symbols that represented sounds, or ideas, or things. Their government was monarchical—they had a single ruler, called a Pharaoh. The Egyptians worshiped a pantheon—which means a bunch of gods and demi-gods. The Pharaoh was worshiped as a god, too.

The Sumerian culture must have influenced the Egyptians somewhat. The Egyptians divided the day into two halves, each having 12 hours—twelve is an easy Base Sixty number. The Egyptians are thought to have invented the sundial. The earliest example of a sundial has 12 hours marked using lines on a semi-circle, 15° apart.



A fragment of a limestone sundial. The gnomon goes into the hole at top.


This sundial is a half-bowl cut out of a block of stone.

A sundial is a simple way to measure the passage of the Sun. There’s a post (called a gnomon, pronouced NOM-ON) sticking up from a flat, horizontal surface. Lines are drawn on the flat surface, radiating out from the gnomon. When the Sun is shining, the gnomon casts a shadow on the lines. Each line represents the passage of an hour.

The Egyptians built huge obelisks—big stone monuments. These were sundials, too. The obelisk cast a shadow on the ground, which was marked for every hour. As the Sun moved across the sky, the shadow would move along the dial, showing the time. Of course, sundials only work when there’s daylight. How did they tell time at night?




Base Sixty

Today we count using Base Ten (1-2-3-4-5-6-7-8-9-10, then 11-12-13-14-15-16-17-18-19-20, 21 to 30, 31 to 40, 41 to 50 and so on). Here’s something really interesting about the Sumerians. They counted numbers using Base Sixty! Fractions hadn’t been invented yet, so 60 was actually a handy base for counting. Sixty can be divided by 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, and 60. Try it!

Base Sixty is natural, it occurs in the world around us. By watching the sky, the Sumerians saw the Moon make a complete cycle 12 times a year. Each cycle (month) takes about 30 days (one day = the time it takes Earth to completely revolve around her axis). That’s about 360 days per year (one year = the time it takes Earth to travel around the Sun). These numbers fit neatly into the Base Sixty way of counting.

Do we still use Base Sixty? You betcha! A ruler is 12 inches long—12 x 5 = 60. We buy eggs and doughnuts by the dozen. Can you think of any other examples?

How about a clock? There are 12 hours on the face of an analogue clock—12 x 5 = 60. Sixty minutes in an hour; sixty seconds in a minute. What about 24 hours in the day—does that work? Nope, sixty doesn’t evenly divide by 24. But a protractor uses Base Sixty—a circle is divided into 360 degrees. If you were to mark 24 hours around a protractor, each hour would use 15 degrees (hang onto that thought—it will become important later on in the show).

I bought this one at Protractor Supply.

I can hear you saying, “Hold on, Manders. Years aren’t 360 days long. They’re 365 days and six hours!” Okay, I have to admit, that’s a good point. Sumerian astronomers were fantastic at math and did mostly everything right—but they got one really important piece of information wrong. They thought the Earth was the center of the universe with the Sun, Moon and planets revolving around her. It’s impossible to make the lunar year (12 cycles of the Moon) agree exactly with the solar year (Earth’s trip around the Sun), so we had to add 5 extra days, plus one day every fourth year, to make it work on a calendar. That’s not really the best solution, as we will see.