Rearranging the geocentric system

Last time I explained Aristotle's geocentric system, where the Earth is supposed to be fixed immovable in the center of the universe and the celestial bodies are placed on concentric spheres, all plunged in the Ether which made them rotate.

But the observations didn't match the predictions of the proposed geocentric system. The first signs of the system being non-consistent were the movements of the Sun's nearest planets : Mercury and Venus. From our view they will always be close to the sun, we will never see them in an opposition position (when the Earth is exactly between the planet and the sun). They not only seem to travel next to the sun, but also moving back and forth around it. 

Heraclides(-388 to -310),Greek philosopher and astronomer, proposed that Venus and Mercury revolve around the sun, and so he changed the geocentric system. He would also have been one of the first to support the thesis that the Earth revolves es around an axis of rotation, unlike Aristotle, who assumed that the celestial sphere revolved around the Earth,the Earth itself remaining immutable.

Adapted geocentric system by Heraclides

An other problem is that sometimes planets happen to be in an apparent retrograding motion, which does not fit to their supposedly circular motion. This happens during opposition position. As the Earth is rotating faster than the outer planets, during opposition the Earth is "overtaking" the outer planet (for example Mars), and the planet appears to move backwards in front of our starfield.

Self-made animation to show the apparent trajectory of an outer planet during opposition position

Aristarchus of Samos (-310 / -230), Greek astronomer and mathematician, was the first known man who placed the sun in the center of the universe. He assumed that the Earth rotates on its axis and around the Sun, but his ideas were considered unclean and were therefore rejected.

There is a hint that Heraclides preceded him, through an indirect source : according to Simplicia of Sicilia (490 - 560 AD), Heraclides proposed that the irregular movements of the planets could be explained if the earth moves while the sun stays still.

As I mentioned in the previous post, Ptolemy (90 - 168 AD), a greco-roman mathematician, astronomer, geographer, astrologer and poet of Alexandria, adopted Aristotle's view. He tried to explain the strange behavoir of the planets by assuming that they are moving on epicylces, revolving along a deferent. He observed also that even then the movements of the planets were not regular, they apparently did not turn steadily and circularly around the Earth. So he did a quite complicated model where he placed the Earth away from the center of the system, the center being in the middle between the Earth and an imaginary point named the equant. Note that the Earth and the equant did not move in this system. Now, he supposed that viewed from the equant the planets, respectively the center of their epicycle would appear to move at a steady speed. Which means that they did not move uniformly on the deferent.

Ptolemy's view of the movements of the planets

In order to keep his system coherent with the celestial orbs, Ptolemy considered the orbs to be thick spherical slices rather than a thin sphere. In each slice would be another one in which the planets were located. His geocentric system was more detailed and more accurate than other modes before. His model was almost universally accepted until the 16th century.

A perfect geocentric universe

One of the oldest known model of the universe, after realizing that the earth has to be spherical, has been produced by the ancient Greeks around the 6th century BC.

When you look up in the sky you will notice that every body up there seems to turn around the Earth. So it is easy to think that we are located in the center of this giant "Carousel". As for example the moon or the sun aren't falling down on Earth, it was supposed that they had to be somehow suspended or fixed on something. So the Greeks first imagined giant circular tubes, which later became transparent spheres, called celestial orbs, on which everything was attached. Therefore each known body had to be fixed on one of those spheres, the stars were "fixed" in the background on the last biggest sphere.

That kind of system, where you place the Earth in the center of the universe and assume that everything turns around it, is called a geocentric system.

Geocentric system (self-made diagram - not at all to scale!)

Aristotle (4th century BC) adopted the geocentric model. He supposed that the universe had to be perfect, existing since eternity and lasting forever. He wasn't convinced by evolution as he deduced that every human will make birth to another human, every plant to another plant, and nothing would ever change. He divided the universe in a sublunary and a supralunary world. The sublunary world includes everything between the Earth and the moon, it's where things change, the imperfect world. From the moon on to the stars was the perfect world, where only geometric shapes would exist. He thought that all the stars and planets were illuminated by the sun. He imagined that the planets would have to be perfect spheres, circling in a steady and unchanging movement. This supralunary world would be plunged in a substance called Ether, an unchanging, strong and light substance with the circular motion being it's natural state. This substance would be responsible for the movement of the planets around the Earth. By cons, in the sublunary world elements were heavy, they need to be pushed to get moving and their natural state would be to rest.

Side note : Many of ancient Greece thought that nature was based on geometry, so pure geometric forms were considered to be perfect and somewhat divine. But there were some problems, for example the number Pi can not be exactly calculated as it is not a rational number.

In fact, Aristotle believed in a natural state of things. For example it would be in the nature of water to flow, or of fire to burn. Aristotle wasn't really a good scientist, even though he deduced his ideas on observation, he mostly imagined by himself how the universe could work. He did not test the universe, he had no proof of the Ether or anything he described. And yet, his view was endorsed by many people for a long time.

Of course there were some anomalies in his model, like the comets. As their orbit regularly cross orbits of other planets, they were incompatible with the celestial spheres model. But Aristotle had his own explanation: they were merely atmospheric events in the sublunary world.

Note that even if Aristotle was well known, he got famous in Europe only in the 12th century, when his works have been translated into Latin.

The geocentric system was picked up by Ptolemy in the first century AD to explain the movements of the known stars in the sky. While keeping the earth at the center, he also assumed that the stellar objects were orbiting on specific crystal orbs. Its diagram illustrates this idea : The Earth is placed in the center, on the first orbit comes the Moon, then Mercury, Venus, the Sun, Mars, Jupiter, Saturn, three layers of stars and finally "the empire of heaven, abode of God and of all the elect".

Geocentric system according to Ptolemy

Soon some people started to see flaws in the system, as some planets would not circle steadily around the Earth, and doing sometimes some funny movements instead.

About the Flat Earth

I grew up with the idea that until Galileo Europeans believed the Earth was flat. I was quite surprised that this is a big misconception, I didn't even realize that the Earth had been circumnavigated almost 100 years earlier.

Many ancient cultures believed in a flat Earth, but the ancient Greeks (Pythagorean philosophers, around the 6th century BC) already imagined a spherical Earth. Later, Eratosthenes was the first known person to have calculated the circumference of the Earth, with a remarkable accuracy for that time (I will describe in an other post how he did this without even leaving Egypt).

The spherical Earth theory was maintained by Aristotle, Ptolemy, and finally Johanes de Sacrobosco. He was a scholar, a monk and astronomer in Paris and wrote a short astronomy textbook in the year 1224, Tractatus de Sphaera, which was widely publicized. That book gave ample evidence of the spherical shape of the Earth. It was the reference book for astronomy until the 16th century AD.

A page from Tractatus de Shpaera, about the spherical shape of the Earth

But nowadays, many people assume that until the 15th or 17th century people in Europe believed the Earth was flat. In 1996 there was an advertisement on German TV, about a perfume called "Galileo":

A voice is saying "Die Erde ist eine Scheibe, Galileo!" (The earth is a disc, Galileo!), and he answers "Nein!" (No!). This is so absolutely wrong, as no one at that time thought the Earth should be flat! The problem Galileo had was that he placed the sun in the center of the universe instead of the Earth. Because until then it was largely assumed (since Aristotle) that the Earth was located in the center of the universe, endorsed by the church, fixed and immovable (Psalm 93(92)) and that everything in the heaven was turning around it.

Galileo publicized his first observations in 1610, that was even way after Christopher Columbus's first voyage in 1492 (and stumbled on America while he wanted to go to Japan). If people really believed the Earth was flat, he would never have done that trip. And what about Magellan? His crew circumnavigated the Earth (departed in the year 1519 and came back in 1522), he has unfortunately been killed in the Philippines. So, in Europe it was common knowledge that the Earth wasn't flat during the time of Galileo.

Besides, the oldest still existing terrestrial globe is from 1492. As America wasn't discovered yet (Christoph Columbus did return in 1493 and believed he were in India - that's why he called the people "Indios"), there is no such continent on it.

Martin Behaim's Erdapfel, 1492

Well, I forgot to mention the Flat Earth Society, descending directly of the Universal Zetetic Society. They still exist today, but should I even mention them?

Well, it's only a theory, no?

What do we know about our universe? Why do we believe there was a Big Bang? How can we know the age of our universe? What will the future bring?

I'll try to explain all this in an easy way. I thought about doing this for quite a long time as I'm very much interested in science, and especially astronomy and astrophysics (but less in solving equations). The best way to understand something is by figuring it out, with examples or comparisons. I jumped right into the subject and realized that maybe I would have to start from the beginning, to see the evolution of our vision of the universe.

In my opinion, many are misusing the word "Theory". "Every theory is true until proven wrong." Who did not hear that sentence? Or sometimes you will hear people saying "It's only a theory" as if it's meant to be unproven."

So let's begin with what a theory really is. What has Wikipedia to say about this?

"Theories may be expressed mathematically, symbolically, or in common language, but are generally expected to follow principles of rational thought or logic.

Theory is constructed of a set of sentences which consist entirely of true statements about the subject matter under consideration."

The Free Dictionary gives us the following definition:

"A set of statements or principles devised to explain a group of facts or phenomena, especially one that has been repeatedly tested or is widely accepted and can be used to make predictions about natural phenomena."

They added or is widely accepted because some theories can not be tested. But if it can explain a group of facts or phenomena and make correct predictions, then it's also a theory.

So, a theory should not be confused as a hypothesis. If there were any doubt, it could not be called a theory. Someone who says "it's only a theory" simply has no idea what he's talking about.

And what about "Every theory is true until proven wrong"? Well, the problem here is that one may think that every theory could be proven wrong, what is not the case. It is true that if at any time an observation is made that contradicts a theory, that theory may be wrong. But before changing the theory, it is the observation which must be proven true, by inquiring, repeating and testing it.

For example during the OPERA (http://operaweb.lngs.infn.it/?lang=en) experiment, neutrinos, coming from CERN and recorded in Gran Sasso, were observed to be traveling faster than the speed of light. Which, according to general relativity, should be impossible. Some papers already published that Einstein might have been wrong, but in fact the problem was solved a few months later : the neutrinos weren't traveling faster than the speed of light, there were errors in the measurements due to an improper attached fiber optic cable, and a clock oscillator ticking too fast. At the end, those elements did not travel faster than light.

(http://en.wikipedia.org/wiki/Faster-than-light_neutrino_anomaly)

For the hardcore people out there, here is a link to the (revised) publication "Measurement of the neutrino velocity with the OPERA detector in the CNGS beam" :
http://arxiv.org/abs/1109.4897 (the download is on the right on that page)

I will conclude this by Sir Karl Popper's own conclusions what a theory should be about (published in Conjectures and Refutations in 1963) :

"These considerations led me in the winter of 1919-20 to conclusions which I may now reformulate as follows.

1. It is easy to obtain confirmations, or verifications, for nearly every theory — if we look for confirmations.
2. Confirmations should count only if they are the result of risky predictions; that is to say, if, unenlightened by the theory in question, we should have expected an event which was incompatible with the theory — an event which would have refuted the theory.
3. Every "good" scientific theory is a prohibition: it forbids certain things to happen. The more a theory forbids, the better it is.
4. A theory which is not refutable by any conceivable event is non-scientific. Irrefutability is not a virtue of a theory (as people often think) but a vice.
5. Every genuine test of a theory is an attempt to falsify it, or to refute it. Testability is falsifiability; but there are degrees of testability: some theories are more testable, more exposed to refutation, than others; they take, as it were, greater risks.
6. Confirming evidence should not count except when it is the result of a genuine test of the theory; and this means that it can be presented as a serious but unsuccessful attempt to falsify the theory. (I now speak in such cases of "corroborating evidence.")
7. Some genuinely testable theories, when found to be false, are still upheld by their admirers — for example by introducing ad hoc some auxiliary assumption, or by reinterpreting the theory ad hoc in such a way that it escapes refutation. Such a procedure is always possible, but it rescues the theory from refutation only at the price of destroying, or at least lowering, its scientific status. (I later described such a rescuing operation as a "conventionalist twist" or a "conventionalist stratagem.")

One can sum up all this by saying that the criterion of the scientific status of a theory is its falsifiability, or refutability, or testability."

(http://www.stephenjaygould.org/ctrl/popper_falsification.html)

The speed of light - a history book of the universe

Some people think a light-year is a time period. Maybe it's because of the word "year" that it comes to confusion.

It's simple though, it's the distance light travels in one year. The speed of light being 299 792 458 meters per second, roughly 300 000 km/s, the distance of a light-year is in fact enormous.

The light reaching the Earth from the surface of the sun takes about 8 minutes, that is a distance of about 149 600 000 km. That means that we see the sun as it was 8 minutes ago.

The further the observed objects are, the further we travel back in time. Proxima Centauri, the next star, is about 4,243 light years away. That also means that the light we're seeing now from that star left 4,243 years ago.

The nearest galaxy, the Andromeda galaxy, is 2 538 000 light years away, so we know today what it looked like 2 1/2 million years ago.

In December 1995 the Hubble telescope took a picture of a tiny dark place in space (Hubble Deep Field), and the result was astonishing. Around 3000 dots have been discovered, all galaxies, and some are about 12 billion light years away. The light from those galaxies is therefore 12 billion years old, that's way before the existence of our sun.

The pictures taken from the universe can therefore be compiled as a timeline of the universe.

That also implies that we will never see actual images of our universe, the age of the picture depends of the distance of the object.

Welcome - I'm frustrated

I'm not a physicist at all. But I am interested in science and especially astrophysics. I think it is fascinating and frustrating at the same level to see how the universe evolved and how insignificant our planet is, and thus myself. Why frustrating? Just look at the distances:

The nearest star, after the sun of course, Proxima Centauri, is located at a distance of about 4,3 light years (around 40 millions of millions of km). How long will it take to reach it?

Take the good old space shuttle. In orbit it could reach 28 000 km/h, that's pretty fast. Traveling at that speed you could reach Proxima Centauri in around 166 000 years (we won't even discuss how many fuel we would have to take with, and the problems it will generate, etc).

In 1969 the Saturn V (Apollo 10 mission) rocket reached a maximum speed of 39 897 km/h. Traveling at that speed will reduce the journey to around 116 000 years. That's already 50 000 years less.

Saturn V (Apollo 10)

But what about the mars missions? Well, the Rocket that carried Curiosity traveled at about 33 800 km/h, it was slower than the Apollo 10. The problem is always the fuel needed which has to be carried with that will make the rocket heavier. So you will need more fuel...

In April this year Nasa announced the fusion rocket. That rocket could in theory accelerate a craft to around 322 000 km/h. In theory we could then reach our nearest neighbour in 14 500 years. That's a great improvement, but still a very long journey...

And that is only our next star. Imagine traveling our whole galaxy, which is about 100 000 light years in diameter. The next galaxy, the Andromeda galaxy, is about 2 538 000 light years away. And there  are billions of galaxies in our visible universe. So yeah. I think we are stuck on our island unless someone will find a new way to travel through space. Maybe someone will build the Alcubierre drive?