Geocentrism

Discussion

astronomy

Think of your life now and think of how it used to be ten years ago. What did you do that was different? How was the world different? The internet? What's that? Why would I ever want a computer in my home? [I originally wrote this in the late 90s and it sounds dated now.] Go back even further and imagine yourself living one hundred years ago. No television, no radio, no automatic teller machines, no satellite communications, maybe a few phonograph records. What did people do? How did they keep themselves occupied? Go back even further. What was life like one thousand years ago? Ten thousand years ago? Yes, humans lived back then, in fact, they thrived. Humans had spread across the globe by this time. The occupied almost every piece of dry land except Antarctica and a few exceptional places like Iceland or Bermuda. What did they do? How did they amuse themselves? How did they fill up the empty hours of their days?

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Why they amused themselves, of course. With no external source of entertainment, they had no choice. They sang and danced and ate and had sex and told stories and pursued specialized side interests (like weaving or brewing or religion). In other words, their lives were much like ours, except they did it all themselves. Ten thousand years ago, your life was your own (or at least, it only belonged to the small collection of people around you).

The only thing larger than your clan was the universe itself. And so it still is. The only difference is that we in the present have a better understanding of our place in the cosmos than those who lived ten thousand years ago. It is nonetheless still fascinating but you will appreciate it all the more if we begin this epic with the original stories told when humans first called themselves "the people".

One of the things you can do to occupy yourselves is to gaze up at the night sky. We don't see much in the cities today, but in the days before electric light, the Sun, moon, planets, and stars were visible on any clear night.

Keep Going. Get these paragraphs in logical order …

Astronomy is the oldest science. (Astrophysics was invented sometime in the early 20th century.) Astronomy is all light (electromagnetic radiation) and gravity (geometry).

The most obvious thing in the sky is the Sun, which makes a complete journey around the Earth (I am using the language of appearance here) on average every 24 hours. One meaning of the word "day" is the length of time the Sun is visible in the sky. For those of us living on the equator this time period is constant, but for everyone else it varies. In the tropics the day is longest in the rainy season and shortest in the dry season. For those of us living in temperate regions the day is longest is the summer and shortest in the winter. The farther one gets from the equator, the more extreme this kind of day changes in length. It's so extreme at the poles that a day of this kind will technically last half a year.

There is, of course, another definition of the word "day" and this is the one we'll be using from now on. The sun rises in the east, travels across the sky rising highest above the horizon at a time English speakers call noon, disappears below the horizon to the west, travels below the Earth as it were reaching its lowest point beneath our feet at a time. The time when this occurs is called midnight and this event has become the dividing point separating one day from the next for most of the world's people. The period of time separating one midnight from the next consecutive midnight is normally what physicists and astronomers mean when they say the word day.

To a creative mind, the pinpoints of light in the sky called stars can be grouped and arranged. The human mind is an excellent pattern recognition device and can find apparent groups in the random splattering of the stars across the sky. These patterns may be associated with mythological characters and events may arise to fit a supposed pattern after it has been seen. In either case, the patterns and the stories associated with them are wholly the creation of the human mind. To the ancients, however, these associations were real (or at least more than just a cute little story). A cluster of stars named after a mythological character or event is called a constellation.

When one looks at the sky seriously and studies it as ancient people did, the first thing one notices is that the constellations remain essentially fixed relative to one another. The big dipper of today is essentially the same as the big dipper of 8000 years ago. The second thing one would notice is that the entire night sky seems to be rotating so that after 24 hours the stars make a nearly complete circuit, returning almost exactly to the place in the sky. If one believes that the Earth is a sphere (something that educated people have known for thousands of years) then the sky is an even bigger sphere, colored black as night, and studded with glistening bright points of light. The terrestrial sphere of the Earth is like a small pit suspended in the center of a much larger celestial sphere.

Note how I said almost exactly the same place. If we divide our average day as defined above into twenty four evenly sized parts called hours. And if we count twelve hours from midnight to noon and then start over again with another twelve hours from noon to midnight. And if we decide to go outside and check the stars every night at the same numbered hour and see what we can see, then we would notice that the pattern of the fixed stars had made a complete revolution across the sky plus a little bit more. These little bits add up over the course of year so that in this time the fixed stars make one more revolution about the Earth than the Sun does — 366 revolutions for the stars versus 365 for the Sun.

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The names of the days of the week, in fact the whole notion that there should be seven days and not some other number, can be traced back to the wandering stars.

Aristotle

Aristotle of Stagira (384–322 BCE) Greece

Natural vs. violent motion

Four elements: earth, water, air, fire.

Quintessence — the fifth element. The stuff in the sky is different, better, than the crummy corruptible junk on the Earth, therefore it must follow the perfect path — a circle.

For the perfect is naturally prior to the imperfect, and the circle is a perfect thing. This cannot be said of any straight line:-not of an infinite line; for, if it were perfect, it would have a limit and an end: nor of any finite line; for in every case there is something beyond it, since any finite line can be extended. And so, since the prior movement belongs to the body which naturally prior, and circular movement is prior to straight, and movement in a straight line belongs to simple bodies-fire moving straight upward and earthy bodies straight downward towards the center - since this is so, it follows that circular movement also must be the movement of some simple body.

Aristotle, ca 350 BCE

Ptolemy

Claudius Ptolemy (85–165) Alexandria. Almagest. ca. 150

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• History
  • Originally called Μαθηματικὴ Σύνταξις (Mathematike Syntaxis) in Greek, which translates as "Mathematical Compilation".
  • Later called Μεγαλε Συνταξης (Megale Syntaxis or the "Greater Collection" to distinguish it from the earlier, so-called "Lesser Collection" ofAristarchus of Samos. It then acquired the title of Η Μέγιστη Βιβλίο (Ε Megiste Biblio) or "The Greatest Book".
  • In the 9th century, Arabic scholars faithfully translated the word "book" in the title from βιβλίο (biblio) to الكتاب (al kitab) but only transliterated "the greatest" from η μέγιστη (e megiste) to المجسطي (al mijisti). Altogether now — الكتاب المجسطي (al kitab al mijisti).
  • When the work returned to Europe in the 14th century (after being largely forgotten there for 1200 years), al mijisti (or al mejisti or al majesti since vowels can shift over time) became Almagestum in Latin. Most languages now use variations like Almagest in English or Almagesto in Spanish. Even the Greeks say Αλμαγέστη (Almagesti) more often than Μαθηματικὴ Σύνταξις (Mathematike Syntaxis).
• Contents
  • data
    • sine table 0°–90° in half degree intervals
    • astronomical tables: position of sun, moon, and planets relative to the fixed stars
  • geocentric model
  • complicated description (geocentric, more or less)
    • deferent: big circle
    • epicycle: small circle
    • eccentric: center of deferent displaced away from Earth
    • equant: planet moves at uniform rate about a second off-center point
  • Very successful for predicting the positions of the planets in the sky.
    • Whether Ptolemy believed his own system was anything more than a way of knowing where to look is open to debate.