Senior Secondary Science Programme
Astronomy through history
Teachers’ Notes
Subject
Physics
Level
S4-S6
Duration:
15 minutes
Key Points
1. Compare the heliocentric model with the geocentric model in explaining the motion of planets on
the celestial sphere
2. Describe Galileo’s astronomical discoveries and discuss their implications.
The programme consists four segemts:
1. Astronomy of the ancient Greeks
The ancient Greek used to discuss rationally. Unlike other civilizations, they didn’t have
superstitious thought about the sky. They believed that human beings can understand the universe
through mathematical geometric model together with rational thinking.
Thales was one the mathematician and philosopher who first thought that human could
understand the universe by rational thinking. He inherited the basic idea of Egyptians that the
fundamental substance that composing universe is water. The land is a flat plate or a cylinder
floated on the water.
At that time many ancient Greek astronomers believed that the Earth was the centre of the
universe. Among those Greek philosopher Plato advocated a concentric spherical spheres
structure model with Earth at the centre. Such model is called geocentric model. He
believed that the Earth is the centre of the universe, and other planets are situated on
different spheres. Inner to outer form the Earth, namely the Moon, the Sun, Venus, Mars,
Jupiter and Saturn. However, there are drawbacks to use this model to explain the
celestial phenomenon. While the motions of the Sun and stars are more or less regular,
but the motions of the planets are much more complicated. Using Mars as an example, if
you observe the path of Mars for a long period of time, you’ll discover that it moves
slowly across the background stars from west to east. It is known as prograde motion. But
after some time, it will cease moving and then moves westwards for a few months before
returning to eastward motion. The westward motion is called retrograde motion.
2. Ptolemy’s geocentric model
The ancient Greeks tried to explain the change in positions between the celestial bodies
and the Earth and the retrograde motion of a planet by using the models of epicycle and
deferent. Deferent is the larger circle with the Earth as its centre, while epicycle is the
smaller circle centered on the deferent. Each planet moves in a circular motion along the
epicycle from west to east, and the center of the epicycle rides on the deferent around the
Earth from west to east. Observing from the Earth, the apparent motion of the planets in
the sky is comprised of two or more circular motions to form the motion of prograde,
retrograde and stopping. Around A.D 140, astronomer Claudius Ptolemy of Alexandria
concluded and developed the work of his predecessors and established the theory of
geocentric and it was recorded in his treatise “Almagest”. In this masterpiece, Ptolemy
used the epicycle and the deferent models and making the path of planets slightly
off-centre, or eccentric. He then established the model of nested spheres called Ptolemaic
geocentric model. In his model, Ptolemy placed the Sun, the Moon and five known
planets on different eccentric spheres and they are moving along the spheres. The Moon is
on the first sphere nearest to the Earth, called lunar sphere. On the second sphere is
Mercury, called Mercury sphere. Venus is on the third sphere and called Venus sphere.
And then in the order are the Sun, Mars, Jupiter and Saturn, and they are called solar
sphere, Mars sphere, Jupiter sphere and Saturn sphere respectively. Of these seven
spheres, only the Sun and the Moon moved directly around the Earth, whereas Mercury,
Venus, Mars, Jupiter and Saturn moved along their epicycle. The centres of the five
epicycles moved around the Earth on their deferents. The observations of celestial
phenomenon over generations were concluded in Ptolemy’s geocentric model. It tried to
set up a unified model to explain the complicated motion of the celestial bodies. However
as times went by, the discrepancies in this model were gradually found. It was because the
model didn’t reflect the fundamentals of the planetary motion.. Until Copernicus
proposed his heliocentric theory in mid-16th century, geocentric theory was gradually
abandoned.
3. Copernicus’s heliocentric model
In early 16th Century, Polish astronomer Copernicus discovered that past astronomers
had proposed that the Earth was not motionless. Instead it was rotating like a wheel about
a centre. Copernicus was inspired and tried to pursue a better model of the universe which
is more prefect than the Ptolemy’s model. In his book, “On the Revolutions of the
Heavenly Spheres”, published in 1543, Copernicus clearly and firmly stated: “the Sun is
at the centre of the universe. The Earth, together with other planets, was made to revolve
around the Sun”. In the heliocentric model of Copernicus, the Sun is placed at the centre
of the Universe. The stellar celestial sphere is at the most outer level. Each planet
revolves in different concentric orbit around the Sun at uniform speed. From inner to
outer are Mercury, Venus, Earth and the Moon, Mars and Saturn. Copernicus emphasized
the symmetric and harmonious of the motion of celestial bodies. The main differences
between the models of Copernican and Ptolemy are the position and motion of the Earth.
Copernicus’ heliocentric model is superior as it employed simple geometry to express the
complicated motion of celestial bodies in a simple and concise way. As the Earth and
planets revolve around the Sun, retrograde motion of planets occurs naturally when the
Earth and planets are at different positions. Using the retrograde motion of Mars as an
example, Mars is on the fourth orbit from the Sun, just beyond the Earth. As the Earth is
closer to the Sun and moves faster than Mars. At a particular moment, the Earth come
closer to the Sun and overtakes Mars. As seen from the Earth, Mars appears to move
backwards resulting in the retrograde motion.
4. Galileo’s discoveries
Galileo was an Italian physicist, mathematician and astronomer. He played an important
role in the development of astronomy as he was the first astronomer to use a telescope for
astronomical observation.
With a hand-made telescope, Galileo discovered that:
Firstly, there are many craters and mountain terrains on the moon.
There are black spots, called sunspots on the surface of the Sun.
The Milky Way is made up of numerous stars.
There are four satellites revolving around Jupiter.
Venus undergoes a complete cycle of phases like that of the Moon.
According to Ptolemy’s geocentric model, Venus would always appear as a crescent as
observed from the Earth. But if Venus is orbiting around the Sun, Venus undergoes a
complete cycle of phases from an arc to a full phase as observed from the Earth.
The discoveries of Galileo’s observation elucidated that all planets orbit around the Sun
instead of the Earth. This is the strongest evidence to support Copernicus’ model.
Suggested Learning Activities
Preparation before viewing the programme
1.
2.
Teacher may discuss with students to what extent they know about ancient astronomy
Teacher may ask student the ancient astronomers they know about
Activities after viewing the programme
1.
2.
Student may, under the guidance of teacher, review the physical concepts presented in the
programme.
The teacher may discuss with students the differences between different models in explaining
the motions of the Sun and planets. .