The Science of Astronomy - Chapter 3 (Sept 18th).docx

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Tuesday, September 18th, 2012
AST101H1 Astronomy
The scientific method; falsifiability; fundamental forces and
gravity; Kepler's Laws
Textbook (pg. 55)
Chapter 3 The Science of Astronomy
The entire cosmos is in constant motion.
3.1 The Ancient Roots of Science
Astronomy is the oldest of the sciences, sprung from the scientific thinking that is a fundamental part of human
behaviour, as roots of science can be traced back to nearly all people and cultures.
In what ways do all humans use scientific thinking?
Scientific thinking comes naturally to all humans. In essence, science is a way of learning about nature through
careful observation and trial-and-error experiments.
The development of science has been a gradual process for humanity, as it requires painstaking attention to
detail, relentless testing to ensure reliability, and a willingness to give up old beliefs that are not consistent with
new discoveries.
How did astronomical observations benefit ancient societies?
In central Africa, people long ago learned to predict accurate weather patterns through observation of the Moon
and the Sun along the ecliptic, which varies at times throughout the year. Astronomical observations were made
mainly to satisfy inherent curiosity, but also to keep track of time and the seasons, especially important for
people who depended on agriculture.
Modern measures of time come directly from ancient observations of motion in the sky, including the length of
a day (the time it takes the Sun to make one circuit in the sky), a month (the Moon), and a year (the seasons).
What did ancient civilizations achieve in astronomy?
Ancient peoples told time by observing the Sun’s path throughout the day via simple clocks, like Egyptian
obelisks. The positions of the stars also gave a sense of time, as long as the approximate date was known.
The origins of the modern clock can be traced to ancient Egypt about 4000 years ago, as they divided the
daylight into 12 equal parts, much as we do today. However, their hours also varied in length because the
amount of daylight varies throughout the year. Egyptians also used star clocks to determine the time of night, as
they also divided it into 12 equal parts.
- They eventually abandoned star clocks in favour of water clocks, which became mechanical clocks in the 17th
century and electronic clocks in the 20th century.
Often many ancient cultures built large structures to help keep track of the seasons, like the Stonehenge and the
Templo Mayor in the Aztec city of Tenochtitlan. Other structures were built to mark the Sun’s position on special
dates like the summer or winter solstice (e.g. sun daggers)
Ancient civilizations also tracked the lunar phases and some used the lunar cycle as the basis for their calendar.
A basic lunar calendar has 12 months, with some 29 days and others 30 days (which are chosen to agree with
the approximate 29 ½ day lunar cycle), thus a lunar calendar has only 354 or 355 days, 11 days fewer than a
calendar based on the Sun. This is still used in the Muslim religion.
- The 19-year cycle on which the dates of lunar phases repeat is called the Metonic cycle.
The study of ancient structures in search of astronomical connections is called archaeoastronomy. Such
scientists start by evaluating if an ancient structure has any particular astronomical alignments.
Before structures such as the Stonehenge or Templo Mayor could be built, careful observations had to be made
and repeated over and over again to ensure their accuracy.
The path that led to modern science came from the Mediterranean and Middle East, specifically Ancient Greece.
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3.2 Ancient Greek Science
Much of what we call science came from Ancient Greece, which arose as a Middle-Eastern power around 500 BC
Why does modern science trace it roots to the Greeks?
Greek philosophers developed at least three major innovations that helped pave the way for modern science:
- A tradition of trying to understand nature without relying on supernatural beliefs
- Use of mathematics to verify scientific explanations and models of nature
- Use of observations to aid in reasoning
Scientific Model = A representation of some aspect of nature that can be used to explain and predict real
phenomena without invoking myth, magic, or the supernatural. It does not fully explain all observations of
nature and a failed model is useful in pointing outs ways to work toward building a better model.
- This practice is central to modern science
How did the Greeks explain planetary motion?
Geocentric Model = Any of the ancient Greek models that were used to predict planetary positions under the
assumption that the Earth lay in the centre of the universe.
The geocentric model was developed and fine-tuned by many scientists until Aristotle came along and first
suggested the idea of gravity, which pulled heavy things (such as stars and other planets) towards the centre of
the universe, thereby allowing water, dirt, and rock, to collect together and form Earth. Aristotle was wrong
about gravity and the Earth’s location, but the geocentric view still dominated.
Ptolemaic Model = The geocentric model of the universe developed by Ptolemy in A.D., in which he stated that
each planet moves around Earth on a small circle that turns upon a larger cycle.
How was Greek knowledge preserved throughout history?
Alexander the Great, who had a deep respect for science and knowledge, commissioned the building of the
Library of Alexandria in Egypt which remained the world’s preeminent centre of research for some 700 years
until its destruction.
Much of Greek knowledge was thus lost and what was preserved was largely due to the scholars of the Muslim
Empire, who established a “House of Wisdom” in Baghdad. This knowledge later spread throughout the
Byzantine Empire and helped to ignite the European Renaissance.
3.3 The Copernican Revolution
Copernican Revolution = The dramatic change, initiated by Copernicus, that occurred when we learned that
Earth is a planet orbiting the Sun rather than the centre of the universe.
How did Copernicus, Tycho, and Kepler challenge the Earth-centred model?
Nicholas Copernicus read Aristarchus’s notes on the Sun-centred idea and was able to discover simple geometric
relationships that allowed him to calculate each planet’s orbital period around the Sun and its relative distance
from the Sun in terms of Earth-Sun distance.
- The Copernican model wasn’t too popular as he believed that planets must move in perfect circles, forcing
him to add in numerous complexities into his model, making it no more accurate than Ptolemy’s model.
Tycho Brahe was renowned for the best set of naked-eye observations ever made and was the first to observe a
supernova. He was convinced that planets must orbit the Sun, but his inability to detect stellar parallax led him
to conclude that Earth must remain stationary. He then advocated a model in which the Sun orbits the Earth,
but other planets orbit the Sun, which few people took seriously.
Johannes Kepler’s key discovery was that planetary orbits are not circles, but instead ellipses
- Ellipses = A type of oval that happens to be the shape of bound orbits. An ellipse can be drawn by moving a
pencil along a string whose ends are tied to two tacks; the locations of the tacks are the foci of the ellipse.
- Focus (plural: foci) = One of two special points within ellipse that lie along the major axis; these are the
points around which we could stretch a pencil and string to draw an ellipse. When one object orbits a
second object, the second object lies at one focus of the orbit.
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- Semimajor Axis = Half the distance across the long axis of an ellipse (the short axis is called the minor axis);
in this text, it is usually referred to as the average distance of an orbiting object, abbreviated a in the
formula for Kepler’s third law.
Eccentricity = A measure of how much an ellipse deviates from a perfect circle; defined as the center-to-focus
distance divided by the length of the semimajor axis.
- A circle has zero eccentricity and greater eccentricity means a more elongated ellipse
What are Kepler’s three laws of planetary motion?
Kepler’s Laws of Planetary Motion = Three laws discovered by Kepler that describe the motion of the planets
around the Sun.
1. The orbit of each planet about the Sun is an ellipse with the Sun at one focus (nothing is at the other focus).
o This law tells us that a planet’s distance from the Sun varies during its orbit. It is closet at the point
called perihelion and farthest at the point called aphelion. The average of a planet’s perihelion and
aphelion distances is the length of its semimajor axis.
2. As a planet moves around its orbit, an imaginary line connecting it to the Sun sweeps out equal areas in
equal times.
o The planet moves a greater distance when it is near perihelion than it does in the same amount of time
near aphelion. Thus planet travels faster when it is neared to the Sun and slower when it is farther from
the Sun. (In the image, the two triangles have equal orbits)
3. More distance planets orbit the Sun at much slower average speeds, obeying a precise mathematical
relationship known as p2 = a3. (Wider orbits take longer to orbit)
o p = is the planet’s orbital period in years and a is its average distance from the Sun in astronomical units
o We can use this law to calculate a planet’s average orbital speed