July 4 , Lecture 2, AST201H1
How did astronomy start?
Observational science. Astronomy was the science of farmers. Observational.
France: cave paintings from 18,000 BC may suggest knowledge of lunar
phases (29 dots).
What is science?
Scientific thinking, study of physical world, branch of science, systematic
body of knowledge, something studied or performed methodically, knowledge,
technical terms and concepts.
Scientific thinking is a fundamental part of our nature.
Scientific thinking is a way of learning about nature through careful
observation and trial-and-error experiments.
China: Earliest known records of supernova explosions.
Ancient astronomy was geocentric or Earth-centered. Provided basis of
modern science. Ancient Greeks thought that the world (universes around us). Stars
so far away don’t actually move that much or at all. Motions of the stars. Live
through thousand of years to see the constellations. Greek geocentric model. (c. 400
B.C.) Harmonic and circles. We know that not everything revolves around the Earth.
Basing their conclusions on observations. Based on believes, not really using critical
For thousands of years, astronomers insisted on The Pythagorean ideal of
circular motion. To say that Mars don’t just go around the Earth but in a smaller
circle that goes around the Earth. As they observed more and more planets, they just
keep adding more circles to explain the motions of celestial objects. Established a
model to predict. “Saving the appearances” perfection and harmony.
Occam’s Razor. The simpler solution is most likely to be correct. We have
geocentric models with additional circles that could predict our observations. But
the model became more and more complicated. Explain the motion of the object of
the sky by adding infinite amount of circles.
Nicholas Copernicus, 1473-1543. Elegant solution. Sun is the center and Earth
revolves around the sun and everything else revolves around the sun as well.
Church didn’t approve.
Galileo Galilei, 1564-1642. Heliocentric Model. Sun is at the center.
The Scientific Method
Theory or model Hypothesis Experiment or observation Theory or model
“The most beautiful and deepest experience a man can have is the sense of the
mysterious. It is the underlying principle of religion as well as all serious endeavour
in art and science. He who never had this experience seems to me, if not dead, then
at least blind.” – Albert Einstein Tycho Brahe: Danish nobleman who revolutionized astronomy by making the first-
ever accurate and complete charts of the motion of the planets. He made lots and
lots of observations, complete and accurate. Observations were taken by Johannes
Kepler (Imperial mathematicus of the Holy Roman Empire). Inherited Tycho’s data
and used it to determine the Laws of planetary motion.
If I drop a ball, it falls to Earth.
The Moon also has mass and is therefore attracted by Earth’s gravity. Why doesn’t it
crash into Earth, it goes around. Faster, trajectory is more curved. Velocity is high
enough, it will miss the earth and will go onto the other side and it will be in orbit.
An object moving under the influence of gravity alone is said to be in free-fall.
If velocity at which the ball moves around is such that makes the orbit circular,
circular velocity. Vcircular. Higher circular velocity is needed to make orbit velocity.
Escape velocity doesn’t depend on the direction in which the objec