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The history of astronomy, and the progress of science
The history of astronomy, and the progress of science
• Bring clickers to class on Thursday! And register them at www1.iclicker.com
• Read section 3.3 for Thursday
What is science?
• Science is often considered to be the study of physical phenomena, i.e. of nature or the natural world.
• This is true, but science is more of a process than a subject. You can apply scientific reasoning to subjects that don’t seem “scientific.”
What is science
• A highly idealized model of how science is done. In practice it is much more non-linear and blurry
What is science?
The hallmarks of science:• Seek explanations for observed phenomena that rely
solely on natural causes• The creation and testing of models of nature that
explain phenomena as simply as possible• A scientific model must make testable predictions
about natural phenomena that would force us to revise or abandon the model if the predictions do not agree with observations
What is science?
First and foremost, anything you think of as “the truth” is open to revision
• it may be flat-out wrong…• or it may need to be modified…• or maybe you just don’t understand it well enough…• But you can never just accept something as an
infallible truth, and you always have to be searching for ways in which you might be wrong. Being a scientist is all about humility
What is science?
You have to be painstaking and rigorous in your measurements
• Measurements are never perfect: there are always errors. You have to have a very good understanding of all the possible errors in your measurements.
What is science?
And you have to be painstaking and rigorous in your reasoning
• Beware of the use of (possibly unrecognized) hunches and intuition. They can be useful guides, but ultimately you have to be able to prove it.• The progress of science has been largely about learning
not to impose our pre-conceived notions on nature; you go where the evidence takes you
• Besides, does your intuition prepare you to understand any of the crazy stuff that we’re learning about in this class?
• Astronomy is the study of heavens
• Astrology is the belief that events on Earth are influenced by the positions of the Sun, the Moon, and the planets
Historically, the desire to make more precise astrological predictions was a major driver for the development of astronomy. Although today we can definitively say that astrology is not a science, in the past these were often considered to be parts of the same field of knowledge.
Hipparchus (c. 190-120 B.C.)
The most important of the ancient astronomers. He compiled a catalog of roughly 1000 stars, and classified their brightness according to the magnitude system
• The brightest stars are first magnitude, the next brightest are second magnitude, and so on until the sixth magnitude
The magnitude system
• We still use this system today! We now define it such that each step in magnitude corresponds to a factor of 2.5 in brightness, e.g. a magnitude 2 star is 2.5x fainter than a magnitude 1 star. A magnitude 6 stars is 2.55=100x fainter.
• The brightest star in the sky, Sirius, has magnitude -1.46. The Sun has a magnitude -27, and the full moon is -13. The faintest objects observable with current telescopes are approximately magnitude 30.
The ancient Greek understanding of the universe
• Many ancient thinkers, such as Pythagorus and Aristotle, believed that the heavens must be perfect and unchanging.
• Since the most perfect shape is a circle (obviously), stars and planets must move on circular orbits and must also have perfectly spherical shapes.
• Geocentric, and the planets are embedded in spheres made of a transparent “fifth element”
The ancient Greek understanding of the universe
Review
• Science is all about using experiments or observations to learn lessons from the natural world, creating a model that seems to successfully describe what’s going on, and then relentless trying to tear the model apart.
• This requires a healthy dose of humility — your pre-conceived notions are probably wrong, and your model is probably not a very good one.
Review
• But science isn’t black magic. Most of what scientists do is just common sense and everyday problem-solving, performed in a systematic way.
Review
• But science isn’t black magic. Most of what scientists do is just common sense and everyday problem-solving, performed in a systematic way.
“When you have eliminated the impossible, whatever remains, however improbable, must be the truth” — Sherlock Holmes in The Sign of
Four
Review
• Hipparchus (c. 190-120 B.C.) compiled a catalog of about 1000 stars, and classified their brightnesses according to the magnitude system.
• The ancient Greeks believed in a geocentric universe, with the Earth surrounded by a series of celestial spheres
Clicker question
Are you present in class today?A: yesB: no
Clicker question
In the magnitude system, how does the brightness of a magnitude 3 star compare to a magnitude 2 star?
A. it is 50% fainterB. it is 50% brighterC. it is 2.5x fainterD. it is 2.5x brighter
Clicker question
In the magnitude system, how does the brightness of a magnitude 3 star compare to a magnitude 2 star?
A. it is 50% fainterB. it is 50% brighterC. it is 2.5x fainterD. it is 2.5x brighter
Each step upwards in magnitude is a factor of 2.5 fainter in brightness. So a magnitude 3 star is a factor of 2.5 fainter than a magnitude 2 star. And a magnitude 4 star is 2.5x2.5 fainter.
The ancient Greek understanding of the universe
Although the notion of a heliocentric universe was occasionally considered, the idea never got far. Two principle reasons:
• If the Earth were moving, why don’t we feel it?• If the Earth were moving, then we should observe
shifts in the positions of stars: this is called parallax
Parallax
The ancient Greek understanding of the universe
• But if the Earth is stationary, and if the stars move in perfect, unchanging circular orbits, then how can you explain retrograde motion?
• The answer is that the planets were thought to move in epicycles, or circles-on-circles
The ancient Greek understanding of the universe
http://media.pearsoncmg.com/aw/aw_0media_astro/if/if.html?ptolemic_model
• Ancient Green astronomy culminated in the Ptolemaic model• This model required the planets to move in epicycles,
and the main circular orbits had to be slightly off-center from the Earth. So this model got mathematically complicated and was difficult to use.
• Nonetheless, it could make pretty accurate predictions for planetary positions, and was used for the next 1500 years.
Claudius Ptolemy (c. 100-170 A.D.)
Nicolaus Copernicus (1473-1543)
Polish astronomer, astrologer, and medical doctor. He was aware the the Ptolemaic model was not perfectly accurate, so he set out to improve it
Nicolaus Copernicus (1473-1543)
• He was drawn to a heliocentric model. But his ultimate model was still pretty complicated (required epicycles) and not completely accurate, in part because he still required that all orbits be circular.
• This marked the beginning of the Copernican Revolution
• It was fairly well-received by many scholars — in part because of its natural expiation of retrograde motion.
The idea of a heliocentric universe was deemed to go against the teachings of the Catholic Church, but there wasn’t too much controversy, in part because Copernicus died on the day his book was published, and in part because his publisher included a preface saying that the ideas in the book weren’t necessarily meant to be taken literally — they are just a way of doing calculations.
Nicolaus Copernicus (1473-1543)
Tycho Brahe (1546-1601)
Danish nobleman. He had the king of Denmark as a patron, then later the king of Bohemia
Tycho Brahe (1546-1601)
He wanted to perform new measurements that were more accurate than Hipparchus’, which were still being used! He constructed large (and expensive) instruments to perform ~1’ accuracy observations
Tycho Brahe (1546-1601)
He created a system where the planets orbited the Sun, but the Sun orbited the Earth. His system was not very influential, but his high-precision star catalogs were extremely important
Johannes Kepler, 1571-1630
• German mathematician, astronomer, and astrologer
• Worked briefly as Tycho’s assistant before his death; afterwards he took Tycho’s measurements back to Germany, and analyzed in terms of a heliocentric model
• Using the very accurate measurements, he realized that the planets must move in elliptical orbits
Johannes Kepler, 1571-1630
He concluded that the planets obey three “laws”• The orbit of a planet is an ellipse, with the Sun at one
focus• A line segment joining a planet and the Sun sweeps
out equal areas during equal intervals of time• The square of the orbital period of a planet is
proportional to the cube of the semi-major axis of its orbit: p2=a3
Johannes Kepler, 1571-1630
The first law — planetary orbits are ellipses
• An ellipse has two foci, and is defined as the locations where the sum of the distance between the two foci is constant
Johannes Kepler, 1571-1630
• The second law — equal areas in equal times
Johannes Kepler, 1571-1630
• The third law — p2=a3. Objects at large distances take longer to complete their orbits.
Galileo Galilei 1562-1642
• Italian mathematician and scientist
• Constructed the first (useful) telescopes, which he used to make several crucial discoveries
Galileo Galilei 1562-1642
Among other things he observed that:• Jupiter has moons orbiting around it — this proved
that everything does not orbit the Earth
Galileo Galilei 1562-1642
Among other things he observed that:• Jupiter has moons orbiting around it — this proved
that everything does not orbit the Earth
Galileo Galilei 1562-1642
Among other things he observed that:• Venus has phases, just like the Moon — it must be
orbiting the Sun
Galileo Galilei 1562-1642
Among other things he observed that:• The Moon has mountains and craters, and the Sun
has sunspots. Therefore these heavenly bodies aren’t “perfect” in the sense believed since ancient Greece
Galileo Galilei 1562-1642
Among other things he observed that:• The Milky Way is actually just a bunch of stars
Galileo Galilei 1562-1642
He argued, on the basis of his observations, strongly for heliocentrism
• His observations of Jupiter and Venus proved that everything doesn’t orbit the Earth
• Since the heavenly bodies aren’t “perfect,” it’s also possible that their orbits aren’t perfect circles… supporting Kepler’s first law
• The stellar content of the Milky Way supports the idea that stars are very far away, and so it may not be surprising that astronomers hadn’t detected parallax
Galileo Galilei 1562-1642
• He also made some pioneering studies on gravity and motion. He concluded that objects in motion tend to stay in motion, which explains why, for instance, falling objects do not “fall behind” the moving Earth. This made the idea of a moving Earth much more plausible.
Review
• Ptolemy (c. 100-170) — made “the greatest” version of the ancient Greek geocentric model
• Copernicus (1473-1543) — proposed a heliocentric model, which also worked quite well
• Tycho Brahe (1546-1601) — made much more accurate measurements of the positions of stars and planets
• Kepler (1571-1630) — used Tycho’s measurements and a heliocentric model to identify three “laws” of planetary motion
Review
Kepler concluded that the planets obey three “laws”• The orbit of a planet is an ellipse, with the Sun at one
focus• A line segment joining a planet and the Sun sweeps
out equal areas during equal intervals of time• The square of the orbital period of a planet is equal to
the cube of the semi-major axis of its orbit: p2=a3
Review
• Galileo (1562-1642) — built the first useful telescopes• Found that Jupiter has moons orbiting around it• Found that Venus has phases, just like the Moon —
suggesting that Venus orbits the Sun• Found that the Moon isn’t “perfect”• Found that the Milky Way is just a collection of stars• He used these arguments, along with Newton’s first
law, to argue for heliocentrism
Galileo Galilei 1562-1642
• Galileo believed that he was allowed by his friend the pope to publish a book about heliocentrism, but was told that he could only present it as an interesting hypothesis, and not as the actual truth.
• But after publication he was brought in front of the inquisition, and they produced (possibly forged) documents saying that he was actually told not to publish anything at all.
• He was forced to recant his position, and was placed under house arrest for the remainder of his life.
• By 1757 the arguments for heliocentrism became nearly incontrovertible, and the Church began accepting the idea
• Galileo’s book on the subject was removed from the Church’s list of banned books in 1824
• In 1981 the pope convened a panel to investigate the Galileo affair. In 1992 they released a report vindicating him.
Galileo Galilei 1562-1642
Galileo is often credited with making the pioneering arguments for what we now consider to be the scientific method:
Galileo Galilei 1562-1642
“Philosophy is written in the grand book which ever lies before our eyes — I mean the Universe — but we cannot understand it if we do not first learn the language and grasp the symbols in which it is written. This book is written in the mathematical language… without which one wanders in vain through a dark labyrinth”
Isaac Newton, 1643-1727
• Probably the greatest scientist in history
• Developed the three laws of motion and the ”universal” law of gravitation• Using these laws, he was able to
mathematically derive Kepler’s laws
• Made it much harder to doubt heliocentrism
Two major questions in early 20th centuryastronomy
In 1700s and 1800s it became universally accepted that the planets orbit the Sun, and that the Sun is just one of many stars in the Milky Way galaxy. But what about the rest of the Universe?
• Is the Milky Way galaxy all that there is?
• Does the Universe evolve, or is it eternal and unchanging?
Is the Milky Way galaxy all that there is? And, what is the nature of the spiral nebulae?
Astronomers had long known that, in addition to stars (and planets), the universe also has many nebulae (plural for nebula)
There was a special class of nebulae known as “spiral nebulae”
Is the Milky Way galaxy all that there is? And, what is the nature of the spiral nebulae?
• There was a long-running question about what the spiral nebulae were. Immanuel Kant (1724-1804) believed them to be other galaxies, or “island universes,” entirely separate from our own.
• Or perhaps they are part of our own Galaxy. For instance, they could be other solar systems in the process of formation.
Is the Milky Way galaxy all that there is? And, what is the nature of the spiral nebulae?
The Shapley-Curtis debate (the “Great Debate”)
• In 1920 two prominent astronomers, Harlow Shapley and Heber Curtis, participated in a debate the nature of the spiral nebulae and the size of the Universe.
• The basic question was whether the spiral nebulae are part of our own Galaxy, or whether they exist outside of our Galaxy — that would imply that they are very large and far away and that the Universe is way bigger than our own Galaxy
The Shapley-Curtis debate — Is the Milky Way all that there is?
Harlow Shapley argued that the Milky Way is the entirety of the Universe, and that the spiral nebulae are inside the Milky Way
• He used new measurements of star clusters to suggest that the Milky Way is much larger than previously believed, perhaps 300,000 lightyears across. And that we are not very near the center.
• If the MW is really that large, he argued that the spiral nebulae couldn’t possibly lie outside of it. And if they did, they would also have to be impossibly large.
Harlow Shapley argued that the Milky Way is the entirety of the Universe, and that the nebulae were inside the Milky Way
• His killer argument was that another astronomer had recently claimed to measure rotation in the spiral nebulae. If they were really large, then the measured rate of rotation would mean that the material in the nebulae is moving faster than the speed of light… which is not possible.
The Shapley-Curtis debate — Is the Milky Way all that there is?
Heber Curtis argued that the spiral nebulae are other galaxies similar to our own Galaxy
• One of his arguments was that they had been found to be flying away from us at tremendous speeds, which should not be possible if they were part of our own Galaxy
• They also each have a large number of novae (star explosions), perhaps even more than our own Galaxy. So why should each nebula have as many novae as our entire Galaxy? They must be galaxies in their own right.
The Shapley-Curtis debate — Is the Milky Way all that there is?
Heber Curtis argued that the spiral nebulae were other galaxies similar to our own Galaxy
• He measured a much smaller value for the size of the MW, and believed us to be near the center. The physical sizes that he estimate for the spiral nebulae were similar to his size for the MW.
• He admitted that, if the spiral nebulae were indeed found to be rotating, then his argument would fall apart. But he believed the rotation measurements to be incorrect.
The Shapley-Curtis debate — Is the Milky Way all that there is?
• So who was right? Well both astronomers made some good points and some bad points… and they made some good points using incorrect reasoning, and some bad points using good reasoning. Life is complicated!
• Debates don’t actually settle anything in science; in fact the word “debate” is often used somewhat ironically. You can’t actually conclude anything until you have firm arguments that will convince even very skeptical people.
The Shapley-Curtis debate — Is the Milky Way all that there is?
Is the Universe eternal and unchanging, or does it evolve?
• The ancient Greeks believed that the Universe was unchanging. This view proved remarkably durable.
• Tycho Brahe observed a supernova, which showed that the Universe couldn’t be completely unchanging. He also provided evidence that comets are astronomical phenomena, rather than being within the Earth’s atmosphere (as had been believed by the Greeks).
Does the Universe evolve?
After developing his theory of gravity, Newton could have easily proven that the Universe cannot be static: a static Universe would begin to contract due to gravity. But either he didn’t do the calculation, or he didn’t put much thought into it.
Does the Universe evolve?
After developing his theory of gravity, Newton could have easily proven that the Universe cannot be static: a static Universe would begin to contract due to gravity. But either he didn’t do the calculation, or he didn’t put much thought into it.
Newton thought that maybe the gravity from distant stars would counteract the contraction… but the math doesn’t actually work out
Does the Universe evolve?
Einstein’s new theory of gravity, the General Theory of Relativity, had the same issue: the Universe can’t be static. Einstein assumed that his theory was not quite correct, so he just added an extra term in his equations that represents a kind of anti-gravity.
Does the Universe evolve?
• The first step in resolving this issue was made by Henrietta Leavitt, who discovered in 1912 that a certain kind of star — called a Cepheid variable — follows a period-luminosity relation.
• Why is this important? Using this relation, you can get a very good estimate of the distance to the star.
(1868-1921)
Does the Universe evolve?
• Edwin Hubble then used these Cepheid variable stars to estimate the distances to the spiral nebulae — finding that they are very far away indeed, and hence that they are actually separate galaxies!
So the Milky Way is not the entire Universe. It is just one galaxy among billions… and the Universe is huge
(1889-1953)
Does the Universe evolve? — YES!
• By the time that Hubble realized that the spiral nebulae were actually other galaxies, it was already known that the spiral nebulae are receding from us at great velocities. This means that the Universe is expanding! It is not static at all! And actually, it all started in a big bang!
Does the Universe evolve? — YES!
• By the time that Hubble realized that the spiral nebulae were actually other galaxies, it was already known that the spiral nebulae are receding from us at great velocities. This means that the Universe is expanding! It is not static at all! And actually, it all started in a big bang!
Although Hubble always gets credit for discovering that the spiral nebulae are actually separate galaxies, and are receding from us, actually Vesto Slipher and Georges Lemaître made these discoveries first
Does the Universe evolve? — YES! But wait…
• Enter the model for a “steady-state” Universe. In this model the Universe is always expanding, but never changing. This is because new matter is continually being created in empty space, filling up the voids left by the expansion. Thus the Universe can be expanding and unchanging… and there was never a big bang.
Does the Universe evolve? — Yes, it really does!
• The issue was finally resolved in 1964, when it was Penzias and Wilson discovered the cosmic microwave background radiation — the heat left over from the big bang.
The development of our modern cosmology
We’ve gone from a picture where the Earth is the center of a perfect and unchanging Universe, surrounded by concentric spheres containing the planets and the stars to a picture where we are orbiting the Sun, which is just one of billions of stars in the Milky Way, which is just one of billions of galaxies in an expanding Universe, which originated in a big bang 14 billion years ago.
The development of our modern cosmology
• It has been a long, slow, messy process. It has been a process of letting go of our pre-conceived notions (things that seem like they should be true) and recognizing our blindspots, and of learning directly from nature using experiments and observations
The development of our modern cosmology
• It has been a long, slow, messy process. It has been a process of letting go of our pre-conceived notions (things that seem like they should be true) and recognizing our blindspots, and of learning directly from nature using experiments and observations
• Some key examples:• Everything moves in circles
The development of our modern cosmology
• It has been a long, slow, messy process. It has been a process of letting go of our pre-conceived notions (things that seem like they should be true) and recognizing our blindspots, and of learning directly from nature using experiments and observations
• Some key examples:• Everything moves in circles• The Earth is not moving
The development of our modern cosmology
• It has been a long, slow, messy process. It has been a process of letting go of our pre-conceived notions (things that seem like they should be true) and recognizing our blindspots, and of learning directly from nature using experiments and observations
• Some key examples:• Everything moves in circles• The Earth is not moving• The Universe is unchanging
But we also see ways in which the development of modern cosmology follows “good science.” People created models that seemed to describe the Universe, tweaked and updated the models in order to provide a better match, and abandoned the models when a better one came along — and when new evidence showed that the previous models were insufficient.
The development of our modern cosmology
What is scientific certainty?
One of the key points in the scientific worldview is that all knowledge is open to revision.
• There are many things that we accept as true now, but who knows how different our understanding might be hundreds of years in the future?
What is scientific certainty?
• Scientists don’t often describe things as “scientific fact.” When they do use the word “fact,” it usually refers to something that is easily seen or measurable.
What is scientific certainty?
• Scientists don’t often describe things as “scientific fact.” When they do use the word “fact,” it usually refers to something that is easily seen or measurable.
• In this sense, an idea cannot be a fact. Ideas, or bodies of ideas, are usually referred to as theories.• Thus the word “theory” does not necessarily mean
that the ideas are very uncertain. Some theories (e.g. string theory) are very uncertain, and other (e.g. the special theory of relativity) are held with a high degree of confidence
What is scientific certainty?
There are many areas of scientific research (e.g. medicine/health, climate science, and some areas of astronomy) where it can be extremely difficult to prove that some things are true or not true. In these cases, often the best we can hope for is a general consensus among the scientific experts, and hope that time will bring further advances.
What is scientific certainty?
• But science is not a democracy. Just because most experts think something is true does not make it true.
• The scientific community is generally pretty good at remaining skeptical of even widely-held beliefs, and at considering alternative viewpoints. This is how science is self-correcting.
Clicker question
Darwin's theory of evolution meets all the criteria of a good scientific theory. This means:A. Scientific opinion is about evenly split as to whether
evolution really happened.B. Scientific opinion runs about 90% in favor of the theory of
evolution and about 10% opposed. C. After more than 100 years of testing, the theory has
successfully met every scientific challenge to its validity.D. There is no longer any doubt that the theory of evolution is
absolutely true.
Clicker question
Darwin's theory of evolution meets all the criteria of a good scientific theory. This means:A. Scientific opinion is about evenly split as to whether
evolution really happened.B. Scientific opinion runs about 90% in favor of the theory of
evolution and about 10% opposed. C. After more than 100 years of testing, the theory has
successfully met every scientific challenge to its validity.D. There is no longer any doubt that the theory of evolution is
absolutely true.
Occam’s razor
• Given two competing — and equally successful — explanations for an observed phenomenon, the simpler one is preferred
• This idea has really been taken to heart by scientists. The development of physics has largely been driven by the search for a small number of principles or equations that describe a huge range of phenomena
William of Occam1285-1347
