An Introduction to Astronomy
Part II: Historical Development of
Astronomy
Lambert E. Murray, Ph.D.
Professor of Physics
The Gift of the Greeks
 The
Greek philosophers were the first to realize
that the universe was “comprehensible”
– By careful observation of the motions of the stars
and planets they developed a “model” for the
universe that satisfactorily explained the known
universe for nearly 1500 years.
 What
facts can we learn about our “universe” by
careful observation of the different objects in
the day and night sky?
What do you know about the Sun’s
Motion?
Where and when does the Sun rise and set?
 Are the days always the same length? Why?
 Why is it hotter in the summer and colder in the
winter?
 How high does the Sun get in the daytime sky?
Does this change during the year?
 Why don’t you see the stars during the day?
 What causes solar eclipses?

Facts about the Sun
 It
rises in the East and sets in the West.
 It reaches different maximum heights in the
summer and winter.
 It rises north of East in the summer and
South of East in the winter.
 The length of day and night changes with
the seasons.
 Sometimes the sun is blotted out – a solar
eclipse.
What do you know about the Moon’s
Motion?
Where and when does the Moon rise and set?
Does it rise and set at different times each night?
 What direction is the Moon moving relative to the
stars?
 What causes the phases of the Moon? Is it the
Earth’s shadow? Where would you expect to see a
full moon?
 Can you see the Moon during the day?
 What causes lunar eclipses?

Facts About the Moon I
It has the same basic daily pattern as the Sun –
moving from East to West during the day/night.
 The moon changes its position relative to the stars
(and Sun) each night – moving slowly in an Eastward
direction relative to the constellations.
 The moon passes through phases, completing one
cycle about every 28 days.
 The moon can sometimes be observed during the
daytime.
 The Full Moon is seen when it is opposite the Sun in
the sky, while a New Moon is seen near the Sun.
 Sometimes the moon is blotted out – a lunar eclipse.

Phases of the Moon
What do you know about the motion
of the Stars?







What is a constellation?
Do you always see the same constellations at night? How
do they change during the year?
How do the Sun and Moon move relative to the stars?
How do the stars appear to move during the night?
Why can’t you see the stars during the day? What if we
had no atmosphere?
What happens to constellations as you move north?
Is everything that looks like a star a star? How can you tell
which are stars?
Facts About the Stars I
are a very large number of stars –
many are invisible to the naked eye.
 Most stars appear to move in fixed groups
(called constellations) with the same basic
daily motion as the Sun and Moon, moving
from East to West.
 Stars are seen only at night (although the
brightest ones are seen just before sunset
and are still visible just after sunrise).
 The North Star is approximately fixed in the
night sky.
 There
Facts About the Stars II
 Different
constellations are visible at
different times of the year, and these
constellations appear to move Westward
during the year.
 As one moves northward, the North Star
appears to move upward in the night sky,
while the stars in the south drop below the
horizon.
 Some stars (the wandering stars) appear to
move among the other stars. These stars
sometimes move in a bizarre manner.
Star Tracks
Constellations
Constellations and Asterisms
Usually we think of a constellation as a particular
grouping of stars that may “look” like some stick
figure man, lion, etc. Many of these grouping of
stars have been identified by various names in
various nations over past history. To make things
more uniform, the International Astronomical
Union in 1928 divided the night sky into 88 welldefined regions (named constellations) associated
with these well know star groupings.
 An asterism is a group of easily identifiable stars
which may be a part of one or more constellations.

The
Winter
Triangle –
An
Asterism
Early Models of the Solar System
A simple model to describe the motion of the stars
in a 24 hour period might be to picture the stars on
a spherical shell which rotate around the earth.
 An alternate model, which works just as well, is
for the earth to rotate inside the shell of stars.
 In order to explain the motion of all the other
celestial bodies, more spherical shells must be
added to this model.

Aristotle’s Model of the Universe
Aristotle’s Model of the Solar System was based
upon celestial observations and upon terrestrial
observations (fire and air always rise).
 This diagram of his model indicated very little
detail in the actual way the planets moved, but the
position of the sun and various planets could be
modeled by having the different shells move at
different rates and at slightly different angles to
one another.

A More Complex Model
The Celestial Sphere
The celestial sphere is a model of the night sky
where we assume that all the stars in the heavens
are attached to a sphere surrounding the Earth.
 Positions on the celestial sphere are designated in
one of two ways:

– Local Altitude and Azimuth angles
– Declination and Right Ascension angles
Declination is like the latitude angle on the Earth, but
measured from the Celestial Equator. This angle is measure in
degrees.
 Right Ascension is like the longitude angle on Earth, but
measured from the Vernal Equinox. This angle is measured in
hours, minutes, and seconds.

The
Celestial
Sphere
The Celestial
Sphere with
Constellations
Additional Facts Known to Early Greek
Astronomers
Aristotle had argued that the earth, moon, and sun
were spherical objects based partly upon
observations of eclipses (about 350 B.C.).
 Using geometric techniques, the early Greek
astronomers had determined approximate values
for:

– the diameter of the earth
– the relative distances from the earth to the moon and to
the sun.
– the relative diameter of the moon and the sun.

They could accurately predict the occurrence of
eclipses.
90°
REM
Aristarchus’ Method for
Determining the
Relative Distance to the
Sun and Moon

RES
90°
Note: This is a schematic.
It is not totally accurate for
the Sun and Moon.
Light from the Sun
7°
Obelisk in
Alexandria
Eratosthenes’
Method for
Determining
the Earth’s
Radius
Well in
Syene
RE
Aristarchus’ Proposal to
Determine the Moon’s Distance
If we assume the Earth’s shadow is approximately the same
diameter as the Earth, we can approximate the diameter of
the Moon (by seeing how far the Moon moves through the
Earth’s shadow). Thus:
Distance to Moon = Diameter of Moon/Angular size of Moon
Lunar Eclipse
This sequence of photographs shows the shadow of the Earth
projected across the path of the Moons orbit.
Scientific Evidence for the Geocentric
Model in 200 B.C.
 All
things fall to the earth - even objects
from “space”.
 The motion of the sun, moon, stars, and
planets could be well explained using
Ptolemy’s geocentric model.
– The model was based upon “perfect circles”.
– This model worked well for over a thousand
years.
 We
can’t “feel” the earth move.
Arguments for a Heliocentric
Model in 200 B.C.
Aristarchus proposed an alternate, heliocentric
(sun-centered) model which could also explain the
observed motions of the celestial bodies.
 His major reason for proposing this model was the
enormous size of the sun.
 However, one observation decided against this
model – there was no observed parallax of the
stars.

The Failure of Parallax


Ptolemy Refines the Model
 Ptolemy’s
principle contribution to
astronomy was his efforts in fine-tuning the
geocentric model so that this model could
accurately describe and predict the motions
of the celestial bodies.
 His model was based upon the concept of
“perfect circles”.
Epicycle
Deferent
Ptolemy’s
Simple Model
for Planetary
Motion
Planet
Earth
Ptolemy’s Model for Retrograde Motion
Ptolemy’s Model for Mercury and Venus
Ptolemy’s Complete Geocentric
Model
Ptolemy’s More Exact Model
Timeline of Ancient Astronomy
The Marriage of Aristotle and Christianity
•
•
In the 13th century St. Thomas Aquinas blended the
natural philosophy of Aristotle, which included the
Ptolemaic model, with Christian beliefs.
A central, unmoving Earth fit perfectly with prevalent
Christian thinking, and various scriptures where found,
whose literal interpretation, seemed to agree with this
model.
o
o
o
o
1 Chronicles 16:30: “He has fixed the earth firm, immovable.”
Psalm 96:10: “He has fixed the earth firm, immovable ...”
Psalm 104:5: “Thou didst fix the earth on its foundation so that it
never can be shaken.”
Isaiah 45:18: “...who made the earth and fashioned it, and himself
fixed it fast...”
Timeline of Renaissance Astronomy
Copernicus Proposes a “New” Model
A rebirth of astronomy occurred in the 14th
century. As observations improved, continuous
refinements to Ptolemy’s model were required.
 Finally, by the 16th century the “corrected”
Ptolemaic model had become very complex.
Copernicus suggested the heliocentric model as a
“simpler” geometrical model which would
produce the same observed results, but fewer
circles were required.

Copernicus’
Model
Requirements of the Model
To be a correct model of the Solar system,
Copernicus’ model had to agree with observations.
 His model could explain retrograde motion as long
as the inner planets had shorter periods than the
outer planets (see next slide).
 However, there was still the problem with the lack
of observable parallax.

Copernicus’ Model for Retrograde Motion
Galileo: Father of Modern Astronomy
Galileo’s Careful Observations Put
an End to the Geocentric Model
 Galileo
was the first person to direct a
telescope toward the heavens. His
observations had a profound impact on
astronomy (and religion).
– He observed the Moons of Jupiter
– He observed the phases of Venus
– He observed Sunspots on the Suns surface (and
later went blind).
Galileo’s Observations of Jupiter’s Moons
This observation verified that not everything
orbited the Earth.
Galileo’s Observations of Venus
Like Ptolemy’s model Venus appears larger (thus closer)
when we view its dark side. However, notice how much
of Venus’ surface is illuminated when it is far from us!
Venus’ Phases in Ptolemy’s Model
Venus’ Phases in Copernicus’ Model
Galileo’s observations of the phases of
Venus indicated the Venus must orbit the
Sun – a major modification of Ptolemy’s
model – and the end of the geocentric
model of the solar system.
Tyco Brahe Faults Copernicus Model
Copernicus originally utilized circular motion for
the planets. But he found he could not reproduce
the more accurate observations with such a model.
 Tycho Brahe, rejected Copernicus’ model because
of the lack of parallax. He proposed a slightly
different geocentric model in which the Sun and
Moon orbit the Earth, but all the other planets
orbit the Sun.

Tycho’s Model
But What about the Scriptural Evidence
for the Geocentric Model?
As more and more evidence began to build which
indicated the correctness of Copernicus’ model,
faithful Christians had to ask some fundamental
questions about their interpretation of scripture.
 By the end of the 17th century, most Christians
had come to accept the heliocentric model.
 These Christians had to make adjustments to their
interpretation of certain scriptures: the Earth
being “fixed” must be interpreted differently.

Kepler’s Laws of Planetary Motion

Based upon 50 years of careful observations by
Tycho Brahe, Kepler, a mathematician, derived
three laws of planetary motion:
1. All bound objects orbit the sun in elliptical
orbits.
2. As an object orbits the sun, it sweeps out
equal areas in equal times.
3. The square of the orbital period is
proportional to the cube of the semi-major
axis.
Kepler’s Law of Equal Areas
A highly elliptical orbit such as this is characteristic of comets.
Kepler’s 3rd Law – Orbital Periods
Using Kepler’s 3rd Law, we can relate the orbital
period of other planets to that of the Earth:

2
P
T
3
P
R
2
E
T

3
E
R
1
[
earth

year
]

2
3
1[ A.U .]
TP [earth  years ]  RP [ A.U .]
2
3
1
Bode’s Law
Bode’s Law is a simple relationship which
can be used to remember the approximate
distance of each planet from the Sun.
[0,3,6,12,24,48,96, ]  4
R[ A.U .] 
10
Orbital Periods of Visible Planets
Planet
Approx Dist
(Bode)
[A.U.]
Actual Dist
[A.U.]
Approx
Period
True Period
Mercury
.4
.387
92 days
88
Venus
.7
.723
214 days
225
Earth
1.0
1.0
365 days
365
Mars
1.6
1.52
739 days
687
Asteroids
2.8
(Ceres) 2.77
4.7 yrs
Ceres 4.6 yrs
Jupiter
5.2
5.2
11.86 yrs
11.86 yrs
Saturn
10.0
9.54
31.62 yrs
29.46
Newton’s Laws of Motion
[Law of Inertia] All objects remain at rest, or
move with constant speed along a straight line,
unless acted upon by some outside force.
 The acceleration of a body is proportional to the
force applied and the mass of the body

F  ma

For every action, there is an equal and opposite
reaction.
Newton’s Law of Gravity

Any two objects in the universe experience a force
of mutual attraction. This force is proportional to
the product of the two masses and inversely
proportional to the square of the distance between
them.
GMm
F
R2

Based upon this law and the basic laws of motion,
Newton was able to derive all of Kepler’s laws of
planetary motion!
Demonstration of Orbital Motion in
Gravitational Fields





Simple Orbital Motion (Kepler’s three laws)
– Elliptical motion
– Equal areas in Equal times
– Circularizing Orbits
– Unbound Motion
Multiple Planets Orbiting a Single Sun and Orbital
Stability
Gravitational Boosts
Comets and Meteor Showers
Multiple Sun Systems
Eclipses and Eclipse Seasons
Lunar Eclipse
This sequence of photographs shows the shadow of the Earth
projected across the path of the Moons orbit.
Lunar Eclipses

Lunar eclipses occur when the moon passes
through the shadow of the Earth
The location of the moon relative to the Earth’s
shadow determines the type of eclipse that occurs.
 Recall that the size of the Earth’s Shadow is
roughly three times the size of the Moon.

Eclipse Geometry
Types of Lunar Eclipses
Solar Eclipse
Solar Eclipses
 A solar
eclipse occurs when the moon
passes between the Earth and the Sun
 A movie of the motion of the Moons
shadow across the Earth
– The area of total shadow is relative small
– The Earth rotates as the Moon passes by
producing a curved path for the shadow.
Total and Partial Eclipses
Those located at X observe a total eclipse, while
those located at Y observe only a partial eclipse.
Annular Eclipses
Those located a A observe an annular ecliplse, while
those located at P only observe only a partial eclipse.
Length of Eclipses
The maximum duration of a total lunar eclipse is
about 1 hour and 47 minutes, the time it takes for
the Moon to pass through the Earth’s Umbra.
 The length to time for a solar eclipse can be
anywhere from a few seconds, up to a maximum of
7 and a half minutes, depending upon the size of
the Moon’s Umbra at the Earth’s surface.

Solar Eclipse Paths through 2017
Eclipse Seasons
Why don’t a solar and a lunar eclipse occur every
month?
 The Moon’s orbit around the Earth is tilted relative
to the orbit of the Earth around the Sun.
 This means that there are “eclipse seasons” that
occur about every 6 months. But even then
eclipses do not always occur, because of the
relative position of the Sun, Earth, and Moon.

Eclipse Seasons
Seasons of the Year and Time
Seasons are Caused by the Earth’s Tilt
Geocentric
View
Seasonal Heating Effects
Time is Measured by the Earth’s
Rotation and Revolution
 The
Solar Day and Time Zones
 The Sidereal Day (measured relative to the
stars) – 23 hrs 56 min
 Sidereal Month (measured relative to the
stars) – 27.5 days
 Synodic Month (Lunar Month) – 29.5 days
 Solar Year – 365.25 days
Sidereal Day vs. Solar Day
Synodic
Month
vs.
Sidereal
Month
Time Zones
End of Part II