Cosmogony A cosmogony is theory about Earth’s place in the universe. A geocentric cosmogony is a theory that proposes Earth to be at the center of the universe. A heliocentric cosmogony is a theory that proposes the Sun to be at the center of the universe.
Which is the geocentric cosmogony and which is the heliocentric cosmogony? geocentric (Earth-centered)heliocentric (Sun-centered)
Planets were often called wandering stars because they seem to move from one constellation to the next.
Planets Known in Ancient Times Mercury –difficult to see; always close to Sun in sky Venus –very bright when visible; morning or evening “star” Mars –noticeably red Jupiter –very bright Saturn –moderately bright
What was once so mysterious about planetary motion in our sky? Planets usually move slightly eastward from night to night relative to the stars. But sometimes they go westward relative to the stars for a few weeks: apparent retrograde motion.
We see apparent retrograde motion when we pass by a planet in its orbit.
Explaining Apparent Retrograde Motion Easy for us to explain: occurs when we “lap” another planet (or when Mercury or Venus laps us). But very difficult to explain if you think that Earth is the center of the universe! In fact, ancients considered but rejected the correct explanation.
Why did the ancient Greeks reject the real explanation for planetary motion? Their inability to observe stellar parallax was a major factor.
Scientists use parallax to measure distances.
The Greeks knew that the lack of observable parallax could mean one of two things: 1.Stars are so far away that stellar parallax is too small to notice with the naked eye. 2.Earth does not orbit the Sun; it is the center of the universe. With rare exceptions such as Aristarchus, the Greeks rejected the correct explanation (1) because they did not think the stars could be that far away. Thus, the stage was set for the long, historical showdown between Earth- centered and Sun-centered systems.
Ancient Science The ancient Greeks were the first to seek to explain nature by means of logic and geometry, and not resort to supernatural explanations. We must not forget that besides the Greeks, there were many cultures that contributed to the advancement of science and astronomy. Egyptian and Mayan measurements of time. Polynesian navigation. Chinese and Islamic astronomers, to mention a few! While astronomy and astrology both grew out of the same source, astrology cannot stand up to the rigors of modern science.
Our mathematical and scientific heritage originated with the civilizations of the Middle East.
The Greeks advanced the concept of Science, and moved away from superstition and mythology. For 700 years, the Greeks postulated such ideas as the Earth is a sphere, that the elements are composed of basic bits, called atoms, and that planets existed and circled in the heavens. Aristotle argued for a Earth-centered universe. Eratosthenes accurately estimated the circumference of the Earth. Around 150 AD, Ptolemy put forth the first published work detailing a geocentric model of the Universe.
Special Topic: Eratosthenes Measures Earth (c. 240 B.C.) Calculate circumference of Earth: 7/360 (circum. Earth) = 5000 stadia circum. Earth = 5000 360/7 stadia ≈ 250,000 stadia Measurements: Syene to Alexandria distance ≈ 5000 stadia angle = 7° Compare to modern value (≈ 40,100 km): Greek stadium ≈ 1/6 km 250,000 stadia ≈ 42,000 km
The most sophisticated geocentric model was that of Ptolemy (A.D ) — the Ptolemaic model: Sufficiently accurate to remain in use for 1,500 years. Arabic translation of Ptolemy’s work named Almagest (“the greatest compilation”) Ptolemy
Copernicus devised the first comprehensive heliocentric cosmology to successfully explain retrograde motion The Copernican Revolution
sidereal period is the time it takes a planet to orbit the Sun once. synodic period is the time that elapses between two successive identical configurations as seen from Earth (e.g., time from opposition to opposition)
Tycho Brahe measured distances using parallax that disproved ancient ideas about the heavens A supernova in 1572 was shown to exist in the distant heavens; this troubled scholars who previously thought the heavens were unchanging. He showed that comets were objects that occurred in the region of the planets, not in Earth’s atmosphere.
Tycho Brahe ( ) Compiled the most accurate (one arcminute) naked eye measurements ever made of planetary positions. Still could not detect stellar parallax, and thus still thought Earth must be at center of solar system (but recognized that other planets go around Sun). Hired Kepler, who used Tycho’s observations to discover the truth about planetary motion.
Kepler first tried to match Tycho’s observations with circular orbits But an 8-arcminute discrepancy led him eventually to ellipses. “If I had believed that we could ignore these eight minutes [of arc], I would have patched up my hypothesis accordingly. But, since it was not permissible to ignore, those eight minutes pointed the road to a complete reformation in astronomy.” Johannes Kepler ( )
Kepler’s First Law: The orbit of a planet about the Sun is an ellipse with the Sun at one focus.
The distance between the two foci impact the eccentricity of the ellipse’s shape.
Kepler’s Second Law: A line joining a planet and the Sun sweeps out equal areas in equal intervals of time.
Kepler’s Third Law: The square of a planet’s sidereal period is proportional to the cube of the length of its orbit’s semimajor axis (p2=a3).
Italian scientist Galileo made discoveries that strongly supported a heliocentric cosmology
Galileo’s telescope revealed phases of Venus which could only occur IF Venus orbits the Sun.
Galileo’s telescope revealed that Jupiter had moons which orbited Jupiter instead of Earth.
The Catholic Church ordered Galileo to recant his claim that Earth orbits the Sun in His book on the subject was removed from the Church’s index of banned books in Galileo was formally vindicated by the Church in 1992.
Isaac Newton formulated three laws that describe fundamental properties of physical reality
Newton’s Three Laws of Motion A body remains at rest or moves in a straight line at a constant speed unless acted upon by an unbalanced outside force. Force = mass x acceleration Whenever one body exerts a force on a second body, the second body exerts an equal and opposite force on the first body.
Newton’s description of gravity accounts for Kepler’s laws Newton’s universal law of gravitation states: Two bodies attract each other with a force that is directly proportional to the square of the distance between them. –This law mathematically proves Kepler’s Third Law (p 2 =a 3 ). –This law describes various orbits objects can take when moving near the Sun. –This law can be used to predict when comets, such as Halley’s comet, will pass near Earth.
Video Early Astronomers
Science vs. Myth An introduction to the scientific method, skepticism, hoaxes & hokum
The Scientific Method Observing the Universe led to the scientific method. As astronomers sought to explain the patterns of the motion of the planets, they developed a methodology that set science apart from pseudo-science. Science: The discovery of natural explanations for what we observe and measure in the natural world, and can experiment on in the laboratory. It is an amazing realization that the Universe is knowable! Our understanding of nature is growing exponentially. There is much we don’t know, but we no longer accept that it can’t be known. In time, we will learn more, and we will fill in the gaps in our knowledge. We no longer need to invent explanations, through the application of the Scientific Method, we know that if it can be explained it will be explained in time.
The Scientific Method The scientific method can be used to predict how physical systems will behave. Scientists first create a scientific theory, based on mathematical models or observation. Then scientists make predictions using this theory. Scientists then perform experiments and make observations that either agree with their theory or disprove it. Scientific theories are based on independent and verifiable testing, not on personal belief systems.
Remember The Scientific Method --- Observation and Experiments are used to Support or Reject a Theory
Not Scientific How can we tell? 1. Some is clearly made up. 2. Images on the moon are presumed to be pyramids, without more than a grainy picture and an elaborate “pyramid scheme” has been concocted. Occam’s Razor says the most likely explanation is the simpler one, as long as it agrees with all the evidence. We don’t choose an explanation that makes us happy or fits our preconceived ideas, if another theory is better. Peer review helps to remove this type of bias.
Pseudoscience Pseudoscience includes astrology, myths and belief systems that cannot be confirmed or disproven by science. There is insufficient evidence to “prove” UFOs or ghosts or demons exist. There is no scientific theory that can be applied and tested. As soon as we have measurable/testable evidence for one of these, we can apply the scientific method.
Video Carl Sagan on Astrology
Science & Common Sense Science explains natural phenomena with formulae that are tested and superior to non-scientific answers. But, scientific answers do not always agree with “Midwestern Common Sense”. Why? We have a lot of experience with the world we live in, but we don’t have experience being the size of an atom, or as massive as a star or as fast as a light beam. Quantum Mechanics is incredibly accurate at making predictions, yet it is hard to reconcile with our common sense. Just because something doesn’t agree with our gut instinct doesn’t mean we should reject it out of hand. Example: A child has a hard time understanding the law of gravity. When told that people live on the other side of the world, and the world is shaped like a beach ball, the small child might wonder why the people don’t just fall off the Earth! Our common sense is biased towards our experiences. 4646
Video Michael Shermer on Cognitive Biases Perception Test
Conclusions Do you understand the scientific method, and how astronomy contributed to its development and human history? Read the handouts and consider examples of pseudoscience and why they are rejected by the scientific method. Think of the arguments made by people to justify their non-scientific beliefs. Can you use Carl Sagan’s “Baloney Detection” to recognize them? [Bring an example to our next class.] Consider your place in the cosmos. Do you have a good perspective of the scale of the universe? The size of atoms, planets, stars, galaxies and the vast distances between them? Consider the 13.7 billion year history of the cosmos. Think of how that history fits onto the Cosmic Calendar, and how short a span of time humans have existed. Do you understand how looking farther out in space allows us to look farther back in time? 4848
References Carl Sagan’s “The Demon Haunted World: Science as a Candle in the Dark”, ISBN