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The Science of Astronomy
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Ancient Civilizations
Ancient Greek
European Renaissance
Modern Science
Ancient Astronomical Knowledge
Many of the surviving ancient structures have obvious astronomical purpose. These
ancient structures clearly demonstrated that all ancient civilizations developed
extensive knowledge of the celestial objects…most likely because of the need to
predict the seasons due to the development of agriculture. Astronomical knowledge
are also very useful tool for navigation. Usually knowledge of mathematics and
geometry were usually developed at the same time.
Marking the Seasons
• In Hawaii, the first rise of the star cluster Pleiades is used to mark the
beginning of the year.
Sun rises over the heel stone at
summer solstice
Sun Dagger at summer solstics at Chaco Canyon, New Mexico
Navigating the World
• If you are sailing in the open sea from Tahiti to Hawaii in one of those
voyaging canoe, how can you tell where you are at on Earth?
– Longitude?
– Latitude?
A Polynesian navigational instrument
Ancient Greek Science
The Ptolemaic Model of the
Universe
• Earth is at the center of the
universe
• All the objects move in perfect
circular orbit
• Planets moves in small circles
upon larger circles to explain the
retrograde motion.
Minority Opinion
• Pythagoras (582–500 BC)
The astronomy of the Pythagoreans
marked an important advance in
ancient scientific thought, for they
were the first to consider the earth
as a globe revolving with the other
planets around a central fire.
• Aristarchus (310-230BC)
Aristachus sought to explain the
apparent retrograde motion of
planet with a Sun-centered model.
Missed Opportunity
Although Pythagoras and Aristarchus
proposed heliocentric model of the cosmos,
their ideas were not widely accepted by
their contemporaries, probably because:
• Aristarchus model could not predict
the retrograde motion any better than
Ptolemaic model.
• If the Earth is revolving around the
Sun, the stellar parallax must exist,
but the ancient Greeks were not able to
detect any stellar parallax.
• The ancient Greeks believed that the
heavens must be geometrically perfect:
heavenly objects must move in perfect
circles and must reside on huge,
perfect sphere encircling Earth.
What would you believe if you lived
around 200 B.C. in Greek?
Although the majority of ancient Greek philosophers
arrived at the wrong conclusion about the model of the
universe, they did so based on sound logical reasoning
processes, good (albeit crude in today’s standard)
observational data, (no stellar parallax, apparent
retrograde motion of planets), and good modeling
efforts (Ptolemaic geocentric model and Aristarchus’s
heliocentric model). They followed a very rigorous
scientific method, and their failure was not the failure
of the scientific method. It was due to the limited
technology. They couldn’t have done better!
The ancient Greeks were the first to rely on logical
thinking to explain the natural phenomena. This is the
same principle that was followed by the scientist of the
15th and 16th century to proof the validity of the
heliocentric model of the solar system, and is the
foundation of modern science.
The Dark Ages
• During the dark ages of
Europe, the rest of the world
continue to develop. But the
knowledge of the Greeks were
preserved in the city of
Alexandria, in Egypt.
The Copernican Revolution
1. Copernicus (1473-1543) revived the idea of a Sun-centered
solar system model:
However, like Aristarchus, Copernicus’s model was not accurate enough to
convince many people.
2. Tycho Brahe (1546-1601) made accurate (arc minutes) nakedeye measurement of planet motion
Tycho believed that planets must circle the Sun, but his failure to detect stellar
parallax forced him to put the Earth at the center of the system, with the Sun
orbiting the Earth, and the planets orbit the Sun.
3. Johannes Kepler (1571-1630) were able to make accurate
prediction with his heliocentric model of planetary orbits,
agreeing with Tyco’s observation
Kepler’s initial failure (using prefect circular orbits) to match Tyco’s observation
led him to adopt a model with elliptical planetary orbit.
4. Galileo Galilei’s (1564-1642) telescopic observations helped
solidify the heliocentric view of the solar system.
Kepler’s Reformation
In attempting to explain Tycho’s observation of the planetary motion,
Kepler concluded that planets do not orbit in perfect circles. Instead,
the planets travel around the Sun in elliptical orbit.
The ellipse The distance from one focus to a point on the ellipse to
another focus is a constant
Kepler’s First Law
The orbit of each planet about the Sun is an ellipse with the Sun
at one focus.
Kepler’s Second Law
As a planet moves around its orbit, it sweeps out equal areas in equal
time.
Interesting Properties of Elliptical
Orbits
Kepler’s second law
states that as a planet
moves around its orbit, it
sweeps out equal areas in
equal time.
– It also means that the
orbital speed is not
constant like in a circular
orbit. It is depends on its
distance from the Sun.
– It is slower when it is
further away for the Sun
– It is faster when it is closer
to the Sun.
Click image to start animation
Kepler’s Third Law
More distant planets move more slowly in their orbit:
• The planets orbital period is related to the average distance to the Sun
(Orbital period in years ) 2 = (average distance in AU) 3
or
p2=a3
where p is the orbital period measured in year, and a is the average from the Sun
to the planet in AU.
A Sun-centered solar system model with the
planets moving in elliptical orbits allowed
Kepler to make accurate predictions of the
planet’s positions in the sky. So, now the
heliocentric view has a better model than
geocentric view. But there were other
questions/objections to the heliocentric model
that need to be answered…
The Challenges
Major Objections to the Sun-centered solar system model:
1. If Earth is moving, then objects such as birds, falling stones,
and clouds would be left behind as Earth moved along its
path.
2. The heavens must be perfect and unchanging.
3. If the Earth is orbiting the Sun, then stellar parallax must be
detectable.
These objections must be addressed before the Sun-centered
model can be accepted.
Galileo’s observation with the new telescope helped to answer
these questions…
Galileo’s Telescopic Observations
1. Venus goes through the phases like the Moon
 Venus must be orbiting the Sun, not the Earth!
 This implies that not everything orbits Earth!
2. The Four Moons of Jupiter
 Satellites can follow a moving planets
 This proofs that not everything orbits Earth!
3. Sun has sunspots, and Moon has mountains and valley
 The heaven is not perfect!
Answering the Critics
1.
If Earth is moving, then objects such as birds, falling stones, and clouds would
be left behind as Earth moved along its path.
• Galileo showed that a moving object remains in motion unless a force acts to
stop it.
 Newton’s first law of motion
• Galileo saw through his telescope that there are four Moons orbiting Jupiter,
not Earth.
 Objects can orbit a planet, thus the Moon can orbit the Earth without
been left behind.
2. The heavens must be perfect and unchanging
• Tycho’s observation of supernova and comets
 Heaven can be changing.
• Galileo’s telescope showed that the Sun has sunspots, and the Moon has
mountain and valleys
 Heaven can be imperfect.
3. If Earth is orbiting the Sun, then why couldn’t we observe any stellar
parallax? Stars are too far away. We do measure it today!
In Hawaii, the linear speed of Earth’s rotation is about 1,566 km/hr = 0.435 km/sec, or
435 m/sec. If I drop a stone from a height of 1.25 meter above the ground, it is going to
take approximately 0.5 second to reach the ground. The ground moves 217 m during the
time it takes the stone to fall to the ground. How comes the stone does not get left
behind?
While we hold the ball before
releasing it, the ball is also
traveling with 1,566 km/hr. It is
traveling with the same speed as
the ground does while it is falling
to the ground, because no force was
applied to it to stop its motion in
this direction. So, it does not got
left behind!
 Newton’s First Law of
Motion! Chapter 4
Summary
• The ancient Greeks were the first to use logical scientific method to try to
explain the nature.
• The same scientific method was used by the scientists of the 15th and 16th
century to finally establish the heliocentric model of the solar system.
• Tyco obtained very precise observations of planetary motion.
• Kepler was the first to device an accurate planetary model capable of
predicting the position of the planets with great accuracy.
• Galileo’s telescopic observation helped to disprove many of the ancient
believes, and firmly established the sun-centered model of the solar system
.
 It is interesting to note that up to this point, there were still no discussions
on why the planets should move in elliptical orbits, or on what is keeping
the planets from running away from the Sun?
Measures of Angles
• One complete circle can be divided into 360
degrees.
• One degree is divided into 60 ‘arc minutes’.
• One arc minutes is further divided into 60 ‘arc
seconds’.
 One complete circle has 1,296,000 arc seconds.
 Back