1. Lambert E. Murray, Ph.D. Professor of Physics
An Introduction to Astronomy Part II: Historical Development of Astronomy
Lambert E. Murray, Ph.D.
Professor of Physics

2. 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?

3. 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?

4. 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.
Use Starry Night to show the motions of the Sun:
Westward motion during a day
Daily variation in Sun’s position at the same time of day
Sun rising in East at different times of the year

5. 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?

6. 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.

7. Phases of the Moon

8. 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?

9. Facts About the Stars I
There 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.

10. 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.

11. Star Tracks

12. Constellations

13. 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 well-defined 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.

14. The Winter Triangle – An Asterism

15. 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.

17. 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.

18. A More Complex Model

19. 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.

20. The Celestial Sphere

21. The Celestial Sphere with Constellations

23. 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.

24. Aristarchus’ Method for Determining the Relative Distance to the Sun and Moon
Note: This is a schematic.
It is not totally accurate for
the Sun and Moon.

25. Eratosthenes’ Method for Determining the Earth’s Radius
Light from the Sun
Eratosthenes’ Method for Determining the Earth’s Radius

26. 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

28. Lunar Eclipse
This sequence of photographs shows the shadow of the Earth projected across the path of the Moons orbit.

29. 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.
Meteor showers indicated that objects from out in space even fall to the earth.
The failure to observe parallax was the most obvious proof that the earth did not move through space.
Ptolemy’s model.

30. 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.

31. The Failure of Parallax

32. 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”.

33. Ptolemy’s Simple Model for Planetary Motion

34. Ptolemy’s Model for Retrograde Motion

35. Ptolemy’s Model for Mercury and Venus

36. Ptolemy’s Complete Geocentric Model

37. Ptolemy’s More Exact Model

38. Timeline of Ancient Astronomy

39. 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.
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...”

40. Timeline of Renaissance Astronomy

41. 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.
Show slide of retrograde motion and the explanation due to Ptolemy and to Copernicus.
Show transparency of Venus’ phases - this lead Brahe to propose a geocentric picture with Venus and Mercury orbiting the sun.

42. Copernicus’
Model

43. 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.

44. Copernicus’ Model for Retrograde Motion

45. Galileo: Father of Modern Astronomy

46. 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).

47. Galileo’s Observations of Jupiter’s Moons
This observation verified that not everything orbited the Earth.

48. 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!

49. Venus’ Phases in Ptolemy’s Model

50. Venus’ Phases in Copernicus’ Model

51. 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.

52. 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.

53. Tycho’s Model

54. 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.
Make an ellipse on the board using two suction cups.
Describe the law of equal areas using this ellipse.
Write out the equation for the third law.

55. 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:
All bound objects orbit the sun in elliptical orbits.
As an object orbits the sun, it sweeps out equal areas in equal times.
The square of the orbital period is proportional to the cube of the semi-major axis.

59. Kepler’s Law of Equal Areas
A highly elliptical orbit such as this is characteristic of comets.

60. 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:

61. 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.

62. Orbital Periods of Visible Planets
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
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
11.86 yrs
Saturn
10.0
9.54
31.62 yrs
29.46

63. 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
For every action, there is an equal and opposite reaction.

64. 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.
Based upon this law and the basic laws of motion, Newton was able to derive all of Kepler’s laws of planetary motion!

65. 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

67. Eclipses and Eclipse Seasons

68. Lunar Eclipse
This sequence of photographs shows the shadow of the Earth projected across the path of the Moons orbit.

69. 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.

70. Eclipse Geometry

71. Types of Lunar Eclipses

72. Solar Eclipse

73. 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.

74. Total and Partial Eclipses
Those located at X observe a total eclipse, while
those located at Y observe only a partial eclipse.

75. Annular Eclipses Those located a A observe an annular ecliplse, while
those located at P only observe only a partial eclipse.

76. 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.

77. Solar Eclipse Paths through 2017

78. 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.

79. Eclipse Seasons

80. Seasons of the Year and Time

81. Seasons are Caused by the Earth’s Tilt

82. Geocentric View

83. Seasonal Heating Effects

84. 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 – days

85. Sidereal Day vs. Solar Day

86. Synodic Month vs. Sidereal Month

87. Time Zones

88. End of Part II