Tuesday, September 18 , 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
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 17 th
century and electronic clocks in the 20 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. 2
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