1. Origins of Modern Astronomy
PSCI 131

2. Astronomical Terms Rotation Motion around an axis Produces day/night
From: physicalgeography.net

3. Astronomical Terms Revolution Motion around another object; orbiting
Produces seasons
From: science-class.net

4. Astronomical Terms Orbital period Time required for one revolution
Length of one year for a given planet
school-for-champions.com

5. Astronomical Terms Orbital distance Average distance from the Sun
Solar system shown to size and distance scale
From: commons.wikimedia.org

6. Astronomical Terms Astronomical unit (AU) Distance from Earth to Sun
Used as a “measuring stick” for solar system distances
From: wchs-astronomy.wikispaces.com

7. Astronomical Terms Parallax
Apparent shift of objects due to motion of observer
Stellar parallax:
apparent shift of stars due to Earth’s movement around Sun

8. Astronomical Terms Geocentric: Earth-centered
Heliocentric: Sun-centered

9. ORIGINS OF ASTRONOMY Ancient Greece (600 BCE – 150 CE)
European Renaissance (1400s-1700s)

10. Ancient Greece (600 BCE – 150 CE)
PSCI 131: Origins of Astronomy –Ancient Greece
Ancient Greece
(600 BCE – 150 CE)

11. Assumptions of the Ancient Greeks
PSCI 131: Origins of Astronomy – Ancient Greece
Assumptions of the Ancient Greeks
Universe
Earth
Seven “planetai” (wanderers in Greek)
Stars
Geocentrism
From redorbit.com

12. PSCI 131: Origins of Astronomy – Ancient Greece
Question
Which planets are visible in the night sky without a telescope?

13. Important Discoveries
PSCI 131: Origins of Astronomy – Ancient Greece
Important Discoveries
Earth is spherical
Aristotle ( BCE)
Observation of lunar eclipses
Since the Earth always casts a curved shadow on the moon it must be spherical
From 8planets.co.uk

14. What would Earth’s shadow look like if Earth were a flat disk?
PSCI 131: Origins of Astronomy – Ancient Greece
What would Earth’s shadow look like if Earth were a flat disk? A B C

15. Important Discoveries
PSCI 131: Origins of Astronomy – Ancient Greece
Important Discoveries
Earth’s circumference
Eratosthenes
Measuring angle of shadows
Basic geometry
Eratosthenes, BCE
From 3villagecsd.k12.ny.us

16. Important Discoveries
PSCI 131: Origins of Astronomy – Ancient Greece
Important Discoveries
Earth’s circumference = 360 degrees = ____ stadia
Need distance between two points in degrees and stadia
Distance from Syene to Alexandria was known to be 5000 stadia (about 480 miles)
How did Eratosthenes find distance in degrees?
Angle of shadows in both cities on same day
At Syene: no shadow
At Alexandria: shadow made 7 degree angle

17. 6 km = 1 stadia

18. Important Discoveries
PSCI 131: Origins of Astronomy – Ancient Greece
Important Discoveries
Heliocentric (sun-centered) solar system
Aristarchus ( BCE)
Sun much larger than Earth, further away than Moon
Basic geometry
First to attempt to measure the relative distance between the Earth-Moon and the Earth-Sun without the aid of trigonometry.
89 degrees 50 minutes instead of 87 degrees

19. Important Discoveries
PSCI 131: Origins of Astronomy – Ancient Greece
Important Discoveries
Rejection of heliocentric model
Predicted stellar parallax, but none was observed
Need a telescope to see it
Aristotle championed geocentric model
Stronger influence so geocentric model persisted for nearly 2000 years

20. Important Discoveries
PSCI 131: Origins of Astronomy – Ancient Greece
Important Discoveries
Star catalog
Hipparchus ( BCE)
First to realize that ideas must be proven with empirical evidence (observation rather than logic)
Also realized that more data meant more certainty in the idea or model
Created highly accurate star atlases in an attempt to measure the length of the year more accurately
He measured the length of year to within 6.5 minutes of actual time
He made more detailed corrections to locations and distances measured by Eratosthenes

21. Important Discoveries
PSCI 131: Origins of Astronomy – Ancient Greece
Important Discoveries
Ptolemaic System
Claudius Ptolemy ( AD)
Geocentric model
First organized explanation of celestial motion

22. Important Discoveries: Ptolemaic System
PSCI 131: Origins of Astronomy – Ancient Greece
Important Discoveries: Ptolemaic System
Deferent: orbital path
Epicycles: smaller circles
Used to explain retrograde motion

23. Important Discoveries: Ptolemaic System
PSCI 131: Origins of Astronomy – Ancient Greece
Important Discoveries: Ptolemaic System
Retrograde motion: “backward” motion of a planet along its orbital path
The Ptolemaic System used epicycles to explain retrograde motion

24. Important Discoveries: Ptolemaic System
PSCI 131: Origins of Astronomy – Ancient Greece
Important Discoveries: Ptolemaic System
Retrograde motion is really an illusion
Earth passes Mars in its orbit; Mars appears to move backward as seen from Earth

25. European Renaissance (1400s – 1700s)
PSCI 131: Origins of Astronomy –European Renaissance
European Renaissance
(1400s – 1700s)

26. Timeline: Nicolaus Copernicus
PSCI 131: Origins of Astronomy – European Renaissance
Timeline: Nicolaus Copernicus
Copernicus
1473
1543
1400
1776
1492
1450
First printing press

27. Nicolaus Copernicus “Copernican Revolution”
PSCI 131: Origins of Astronomy – European Renaissance
Nicolaus Copernicus
“Copernican Revolution”
After discovering Aristarchus’ writings he became convinced of the Heliocentric model
His model still contained the epicycles to explain retrograde motion
Published his book for the Sun-centered solar system model as he lay on his deathbed.
Set stage for later discoveries

28. PSCI 131: Origins of Astronomy – European Renaissance
Timeline: Tycho Brahe
Copernicus
1473
1543
1400
1776
1492
1450
1546
1601
First printing press
Brahe

29. Tycho Brahe Very precise astronomical observations Geocentrist
PSCI 131: Origins of Astronomy – European Renaissance
Tycho Brahe
Very precise astronomical observations
Without a telescope
His observations of Mars were far more precise than any made previously.
Geocentrist
He couldn’t observe the apparent shift (parallax) in the position of the stars that should result if the Earth traveled around the Sun.

30. Timeline: Johannes Kepler
PSCI 131: Origins of Astronomy – European Renaissance
Timeline: Johannes Kepler
Copernicus
Kepler
1473
1543
1571
1630
1400
1776
1492
1450
1546
1601
First printing press
Brahe

31. Johannes Kepler Brahe’s assistant Used Brahe’s observations
PSCI 131: Origins of Astronomy – European Renaissance
Johannes Kepler
Brahe’s assistant
Used Brahe’s observations
To prove heliocentrism
To develop laws of planetary motion still in use today

32. First Law of Planetary Motion
PSCI 131: Origins of Astronomy – European Renaissance - Kepler
First Law of Planetary Motion
Orbits are ellipses, not circles
Sun located at one focus of the ellipse
Nothing is at the other focus

33. Second Law of Planetary Motion
PSCI 131: Origins of Astronomy – European Renaissance - Kepler
Second Law of Planetary Motion
Law of equal areas
During a given amount of time, a planet will always “sweep out” the same area in its orbit over that time
Planet speeds up as it approaches the sun and slows down far away

34. Second Law of Planetary Motion
PSCI 131: Origins of Astronomy – European Renaissance - Kepler
Second Law of Planetary Motion
Orbital speed varies inversely with orbital distance
Slower orbital speed
From:

35. Third Law of Planetary Motion
PSCI 131: Origins of Astronomy – European Renaissance - Kepler
Third Law of Planetary Motion
Orbital period proportional to orbital distance
p2 = a3
p: orbital period in Earth years
a: orbital distance in astronomical units (AUs)
Example: if p = 8 yrs, a = 4 AUs
(8)2 = 64 then 64 = a3
3√(64) = 4

36. Laws of Planetary Motion
PSCI 131: Origins of Astronomy – European Renaissance - Kepler
Laws of Planetary Motion
Significance
First mathematical model of celestial motion
Not based on philosophical assumptions

37. Timeline: Galileo Galilei
PSCI 131: Origins of Astronomy – European Renaissance
Timeline: Galileo Galilei
Kepler
1571
1630
Copernicus
1564
Galileo
1642
1473
1543
1400
1776
1492
1450
1546
1601
First printing press
Brahe

38. Galileo Galilei “Father of modern observational science”
PSCI 131: Origins of Astronomy – European Renaissance
Galileo Galilei
“Father of modern observational science”
Improvements to telescopes
Did not invent them
First astronomer to use telescope

39. The Galilean Moons Discovered four of Jupiter’s moons
PSCI 131: Origins of Astronomy – European Renaissance - Galileo
The Galilean Moons
Discovered four of Jupiter’s moons
Io, Ganymede, Callisto, Europa
First observation of moons other than Earth’s
Showed that Earth isn’t unique in having a natural satellite
Sizes of the Galilean moons shown relative to Jupiter
From: en.wikipedia.org

40. PSCI 131: Origins of Astronomy – European Renaissance - Galileo
The Galilean Moons
Galileo’s notebook showing changing positions of Galilean moons over two weeks
Jupiter and the Galilean moons through a small telescope, as Galileo saw them
From: splung.com
From: physics.wisc.edu

41. PSCI 131: Origins of Astronomy – European Renaissance - Galileo
The Phases of Venus
Observed that Venus goes through phases like Earth’s moon
Proof of heliocentrism
From: oneminuteastronomer.com

42. PSCI 131: Origins of Astronomy – European Renaissance - Galileo
The Moon’s Surface
Observed the Moon’s surface wasn’t smooth as the ancients proclaimed
Galileo saw mountains, craters and plains
Moon similar to Earth and not a perfect celestial sphere
Thought plains were bodies of water
Ex. Sea of Tranquility

43. Sunspots Dark regions caused by slightly lower temperatures
PSCI 131: Origins of Astronomy – European Renaissance - Galileo
Sunspots
Dark regions caused by slightly lower temperatures
Refuted celestial immutability
– 2 hours of sunspot motion
Series of Galileo’s 1612 sketches
Sunspots through a modern telescope

44. Timeline: Isaac Newton
PSCI 131: Origins of Astronomy – European Renaissance
Timeline: Isaac Newton
Kepler
1571
1630
Copernicus
1564
Galileo
1642
1473
1543
1400
1776
1492
1450
1546
1601
1642
1727
First printing press
Newton
Brahe

45. Sir Isaac Newton Law of Universal Gravitation Law of Inertia
PSCI 131: Origins of Astronomy – European Renaissance
Sir Isaac Newton
Law of Universal Gravitation
Law of Inertia
Explained why planets move in orbits

46. Law of Inertia (Newton’s First Law)
PSCI 131: Origins of Astronomy – European Renaissance - Newton
Law of Inertia (Newton’s First Law)
An object in motion continues in motion with the same speed and in the same direction unless acted upon by an external force
From: drcruzan.com

47. Universal Gravitation
PSCI 131: Origins of Astronomy – European Renaissance - Newton
Universal Gravitation
All objects in the universe exert an attractional force on each other
Force increases with mass and decreases with distance
More massive objects exert a greater gravitational attraction than do less massive objects.

48. Gravity + Inertia = Orbit
PSCI 131: Origins of Astronomy – European Renaissance - Newton
Gravity + Inertia = Orbit
An orbit is a balance between gravity and inertia

50. Summary of Newton’s Laws
PSCI 131: Origins of Astronomy – European Renaissance - Newton
Summary of Newton’s Laws
1st Law
A body at rest, or in uniform motion, will remain so unless acted upon by an unbalanced force.
2nd Law
The change in motion (acceleration) is proportional to the unbalanced force
3rd Law
For every action there is an equal and opposite reaction

51. Gravity Gravity is the force that
PSCI 131: Origins of Astronomy – European Renaissance - Newton
Gravity
Gravity is the force that
holds us to the Earth
causes a rock to fall towards the ground
causes the Earth to go around the Sun
causes the Sun to be pulled towards the center of the Milky Way galaxy
Gravity acts between any two objects even if they are far apart.
“action at a distance”