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Solar system: medieval to modern views

Nicolas Copernicus: the first modern astronomer

Nicolaus Copernicus was a Polish astronomer who put forth the theory that the Sun is at rest, near the center of the Universe, and that the Earth, spinning on its axis once daily, revolves annually around the Sun.

This is the heliocentric, Sun-centered, system.

Born in 1473, in Poland. Nicolaus Copernicus is the Latin version of his name, which he chose later in life – as was the custom among scientists of the day. His original name may have been Mikolaj Kopernik.

After his father died when Copernicus was only ten, his uncle took him under his care, and ensured Copernicus received a good education prior to entering the priesthood. From 1491 to 1495, Copernicus attended the Cracow Academy, where he first learned astronomy.

For more than a thousand years, astronomy had been based on the Geocentric Model of the Universe, which stated that the Earth was the center of all creation, with the Sun, planets, and stars all orbiting it.

Copernicus studied law and medicine at university, but returned to Poland after witnessing a lunar eclipse in Rome in 1500. In 1501 he went back to Italy for further studies at the Universities of Padua and Ferrara… It was in this period that he probably read ancient Greek theories on the movement of the Earth through the heavens, including some writings that espoused a heliocentric view that all of the planets, including the Earth, orbited the Sun. This was in direct contradiction of the teachings of the Catholic Church, which espoused the Ptolemaic view of the Universe.

In 1504, Copernicus began the research that culminated in his heliocentric theory. He had already returned to Poland, taking a position at the Collegiate Church of the Holy Cross in Breslau, Silesia (now Wroclaw, Poland)… Until just before his death, Copernicus conducted most of his astronomical observations and calculations there…His observations were made with the “naked eye,” as the invention of the telescope would not occur for decades after his death.

In 1514, he distributed a hand-written, unpublished book which concluded:

1) There is no one center in the Universe. [correct, as far as we know]

2) The Earth’s center is not the center of the Universe. [correct]

3) The center of the universe is near the Sun. [not correct]

4) The distance from the Earth to the Sun is imperceptible [too small too notice] compared with the distance to the stars. [correct]

5) The rotation of the Earth accounts for the apparent daily rotation of the stars. [correct]

6) The apparent annual cycle of movements of the Sun is caused by the Earth revolving around it, and, [correct]

7) the apparent retrograde motion of the planets is caused by the motion of the Earth, from which we observe the other planets. [correct]

8) The orbits of the planets around the sun are perfect circles.  [not correct, they’re really ellipses.]

Retrograde motion

People look into the sky year-round, and observe the motion of the stars and the planets. While stars follow a pattern, planets seemed to break the rules.

Look at the path of Mars over several months.

What causes Mars to (apparently) move forward…stop…
move backwards… and then move forwards again?

What we’re seeing is an optical illusion.

Study the Retrograde motion app to see what is really going on.

purple line = direct line of sight, from Earth, up into the sky, to Mars.

red tab on gray line = apparent position of Mars in the sky, as seen from Earth

Planets moving on green, and blue, orbits. Actual motion of the planets.

Here is another way to visualize retrograde motion.
Same motion as in the above app, drawn by a different artist.

Censorship of Copernicus

In 1616 the Vatican’s Sacred Congregation of the Index added Nicolaus Copernicus’s “De revolutionibus orbium coelestium” (“On the Revolutions of the Heavenly Spheres”) to its list of banned books.


Tycho Brahe and Galileo Galilei

In the early 1600’s, Italy, Galileo Galilei created the first telescope, and used it study the planets.


Galileo made an amazing number of discoveries. One of them was that Venus has phases, just like the Moon has phases.
What does this mean? Let’s recall why the Moon has phases.

Phases of the moon APP!

These are actually evidence that the moon is orbiting the Earth.

We’re seeing the moon lit up from different angles as it revolves around us.

At the time, it was assumed that Earth was the center of the solar system.
But if that was the case, then Venus should not have phases.
Yet Galileo clearly proved that Venus does have phases.

Look at the comparison below:
What should Venus look like, throughout the year, in a Ptolemaic system?
What would it look like in Copernican (heliocentric) system?

Here are actual photos of Venus (superimposed on an artistic image of it’s orbit)

These photos look more like the Copernican system: therefore,what is the logical conclusion?

Phases of Venus APP!

Ptolemy’s model
Venus in Ptolemy’s model of the Solar System

Amazing phases of Venus app
Galileo and Einstein Phases of Venus app – Grab the solar system


Tycho Brahe (1546 – 1601)

A Danish nobleman known for his accurate and comprehensive astronomical and planetary observations. His observations were some five times more accurate than the best available observations at the time.

As an astronomer, Tycho worked to combine what he saw as the geometrical benefits of the Copernican system with the philosophical benefits of the Ptolemaic system into his own model of the universe, the Tychonic system.

His system correctly saw the Moon as orbiting Earth, and the planets as orbiting the Sun, but erroneously considered the Sun to be orbiting the Earth.

He was the last of the major naked eye astronomers, working without telescopes for his observations.

Tycho proposed a compromise model of the solar system to explain Galileo’s observation that Venus has phases – without making it necessary for Earth to be moving.

In his fascinating compromise system:
all other planets orbit around the Sun, yet the Sun & planets orbit around the Earth.
This model satisfied all the observations then available to astronomers (although it violated the principle of Occam’s razor.)

Is this system correct? No. But given what he knew at the time, it might be considered reasonable. Daniel J. Berger writes:

Copernicus’ model was hotly disputed, notably by the best living astronomer, Tycho Brahe. While Tycho acknowledged that Copernicus had succeeded in removing equants, he proposed a system which kept Copernicus’ best results while avoiding the serious difficulty of finding an explanation for a moving Earth: the Sun moves around the Earth, dragging the rest of the planets with it.

This is exactly equivalent, on the basis of ground-based observation, to a heliocentric model.

The primary reason Tycho insisted that the Earth could not move is because his precise measurements of stellar size placed the stars too close for the Earth to move.
If the Earth moved, we would see stellar parallax.
It was not known until the 18th Century that the apparent size of stars is an illusion caused by atmospheric distortion.

It cannot be over-emphasized that an explanation of a moving Earth was not scientifically possible, given the dominant four-elements theory and the associated, loosely observation-based idea that anything made of the four elements always fell toward the center of the universe.

There was simply no way, without invoking supernatural help, to explain how it was that the Earth would not fall to the center of a heliocentric universe. And if the planets (including the Sun) were made of something else — the quintessence — why couldn’t they naturally move in circles, just as Earthly matter naturally fell to the center?


Johannes Kepler

Kepler was originally an assistant of Tycho Brahe – but as Tycho grew older, Kepler became more significant. After Tycho’s death, Kepler became the preeminent astronomer of his day.

Kepler built upon Copernicus’s model. He discovered that instead of traveling in perfect circles, planets move in ellipses, with the sun at one focus. Kepler discovered three laws of planetary motion.

Math interlude: You already know some geometry: It has to do with shapes and angles on a flat surface:

Geometry also has to do with other shapes:

Geometry can also be three dimensional.

Here are the five Platonic solids.

They are regular, convex polyhedrons, with the same number of faces meeting at each vertex.

Only five solids meet those criteria, discovered by the ancient Greek philosopher, Plato (400 BCE.)

Many people believed that these shapes somehow tied into laws of nature.

Kepler used these shapes in making one of his models of the solar system.

Prof. Michael Fowler, University of Virgnia, writes:
http://galileoandeinstein.physics.virginia.edu/lectures/lecturelist.html }

Johannes Kepler (1571-1630) believed in Copernicus’ picture.  Having been raised in the Greek geometric tradition, Kepler believed God must have had some geometric reason for placing the six planets at the particular distances from the sun that they occupied.

Kepler thought of their orbits as being on spheres, one inside the other.

One day, he remembered that there were just five perfect Platonic solids.

Only 5 planets (other than earth) were known at the time.

So he speculated that perhaps the orbits of one planet, fit inside the next, like one Plantonic solid nested within another one.

Here is Kepler’s Platonic, geometric model.

keplers Platonic solid model solar system

Given the uncertainties of observation at the time, this model might be the right one.

Kepler realized that Tycho’s astronomical observations could settle the question one way or the other.

Kepler went to work with Tycho in 1600.  Tycho died the next year.

Kepler “stole” the data, and worked with it for nine years.

Later, Kepler reluctantly concluded that his geometric scheme was wrong.

The mark of a good scientist is being able to reject a favorite theory, if the science shows that s/he is incorrect.

Kepler’s rejection of his own beautiful geometric theory – no matter how much he loved it – is an inspiring act of courage.

In its place, Kepler later discovered his three laws of planetary motion:

I    The planets move in elliptical orbits with the sun at a focus.
Ellipse and circle app

II   In their orbits around the sun, the planets sweep out equal areas in equal times.

III  The squares of the times to complete one orbit are proportional to the cubes of the average distances from the sun.

Later, Isaac Newton used these laws to discover the law of gravity.

Next learn about modern views of the solar system.


Isaac Newton (1642-1726)

English physicist and mathematician,  one of the most influential scientists of all time.

Isaac Newton

Newton’s canon – how to get objects into orbit

Dr. William Romanishin, emeritus professor of astronomy at the U. of Oklahoma, writes:

Imagine shooting a cannonball from a cannon parallel to the surface of the Earth from a high mountain. The faster the cannonball moves, the further it would travel before hitting the Earth. At a certain speed, the cannonball would fall around the curving Earth, and would come back and hit the cannon. Such a path is called an *orbit*. If the cannonball were above the Earths atmosphere, so that there was no friction with the air, it would just go around and around forever. The cannonball is in “free fall”- it is falling around the Earth- but it never gets any closer to the surface! The cannonball (or Space Shuttle or whatever) does not need any further “push” once it gets into an orbit.

A simple way to think about what is going on is to think about tying a string around a rock and swinging it around in a circle. The string exerts a force on the rock which causes it to keep moving in the circle, rather than in a straight line. (If the string breaks, the rock flies off in a stright line.)

In an orbit, *gravity* provides the “string” that holds the orbiting body in place. So, a body in an orbit is constantly being pulled by gravity into a curved path. By Newton’s first law, a curved path requires a force- if there were no force then the object would move in a straight line.


Newton's canon

Next learn about modern views of the solar system.


Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework

Understandings about the Nature of Science (Crosscutting Concepts):  Science knowledge has a history that includes the refinement of, and changes to, theories, ideas, and beliefs over time.

Science Is a Human Endeavor:  Scientific knowledge is a result of human endeavor, imagination, and creativity. Individuals and teams from many nations and cultures have contributed to science and to advances in engineering.

Massachusetts History and Social Science Curriculum Framework

World History I Learning Standards: Scientific Revolution and The Enlightenment in Europe
WHI.33 Summarize how the Scientific Revolution and the scientific method led to new theories of the universe and describe the accomplishments of leading figures of the Scientific Revolution, including Bacon, Copernicus, Descartes, Galileo, Kepler, and Newton.

Next Generation Science Standards
Connections to Nature of Science: Science Models, Laws, Mechanisms, and Theories Explain Natural Phenomena.
A scientific theory is a substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment, and the science community validates each theory before it is accepted. If new evidence is discovered that the theory does not accommodate, then the theory is generally modified in light of this new evidence. (HS-ESS1-2),(HS-ESS1-6)

Massachusetts Curriculum Framework for Mathematics
Expressing Geometric Properties with Equations G-GPE
Translate between the geometric description and the equation for a conic section.
MA.3.a. (+) Use equations and graphs of conic sections to model real-world problems.
Derive the equations of ellipses and hyperbolas given the foci, using the fact that the sum or difference of distances from the foci is constant.

1 Comment

  1. […] Solar system: medieval to modern views […]


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