1. The Origin of Modern Astronomy
Chapter 4:
The Origin of Modern Astronomy

2. Ancient Greek Astronomy
Models were generally wrong because they were based on wrong “first principles”, believed to be “obvious” and not questioned:
Geocentric Universe: Earth at the Center of the Universe.
“Perfect Heavens”: Motions of all celestial bodies described by motions involving objects of “perfect” shape, i.e., spheres or circles.

3. Ancient Greek Astronomers
Aristotle
(384 – 322 B.C.), major authority of philosophy until the late middle ages: Universe can be divided in 2 parts:
1. Imperfect, changeable Earth,
2. Perfect Heavens (described by spheres)

4. Issues of Ancient Astronomy (I)
Fundamental argument for the geocentric universe:
Motion of Earth should result in an observable parallax, which was not seen.

5. Issues of Ancient Astronomy (II)
The retrograde (westward) motion of the planets
The “solution”: Epicycles!

6. Introduced by Ptolemy (ca. A.D. 140)
The Ptolemaic system was considered the “standard model” of the universe until the Copernican Revolution.

7. The Copernican Revolution
Nicolaus Copernicus (1473 – 1543):
heliocentric universe (sun in the center)

8. Copernicus’ new (and correct) explanation for retrograde motion of the planets:
Retrograde (westward) motion of a planet occurs when Earth passes the planet.
This made Ptolemy’s epicycles unnecessary.

9. Later used by Kepler to develop laws of planetary motion.
Tycho Brahe (1546 – 1601)
Use of high-precision instruments for precise astronomical observations, meticulously reported in tables.
Later used by Kepler to develop laws of planetary motion.

10. A Quadrant designed by Brahe
Used to precisely measure an object’s angular distance above the horizon

11. Found a consistent description by abandoning both:
Johannes Kepler (1571 – 1630)
Used the precise observational tables of Tycho Brahe to study planetary motion mathematically.
Found a consistent description by abandoning both:
Circular motion and
Uniform motion.
Planets move around the sun on elliptical paths, with non-uniform velocities.

12. Kepler’s Laws of Planetary Motion
The orbits of the planets are ellipses with the sun at one focus. c
Eccentricity e = c/a

13. Eccentricities of Ellipses1) 2) 3)
e = 0.02
e = 0.1
e = 0.2 5) 4)
e = 0.4
e = 0.6

14. Eccentricities of planetary orbits
Orbits of planets are virtually indistinguishable from circles:
Most extreme example: Pluto: e = 0.248
Earth: e =
But their centers may be significantly offset from the sun.

15. A line from a planet to the sun sweeps over equal areas in equal intervals of time.
A planet’s orbital period (P) squared is proportional to its average distance from the sun (a) cubed:
Py2 = aAU3
(Py = period in years; aAU = distance in AU)

16. Galileo Galilei (1594 – 1642)
Introduced the modern view of science: Transition from a faith-based “science” to an observation-based science.
Greatly improved on the newly invented telescope technology. (But Galileo did NOT invent the telescope!)
Was the first to meticulously report telescope observations of the sky to support the Copernican model of the universe.

17. Major discoveries of Galileo (I):
Moons of Jupiter
(4 Galilean moons)
Rings of Saturn
(What he really saw)

18. Major discoveries of Galileo (II):
sunspots (proving that the sun is not perfect!)

19. Major discoveries of Galileo (III):
Phases of Venus (including “full Venus”), proving that Venus orbits the sun, not Earth!

20. Historical Overview

21. Isaac Newton (1643 - 1727) Major achievements:
Building on the results of Galileo and Kepler
Adding physics interpretations to the mathematical descriptions of astronomy by Copernicus, Galileo and Kepler
Major achievements:
Invented calculus as a necessary tool to solve mathematical problems related to motion
Discovered the three laws of motion
Discovered the universal law of mutual gravitation

22. The Universal Law of Gravity
Any two bodies are attracting each other through gravitation, with a force proportional to the product of their masses and inversely proportional to the square of their distance: Mm
F = - G r2
(G is the gravitational constant.)

23. Understanding Orbital Motion
The universal law of gravity allows us to understand orbital motion of planets and moons:
Example: Dv
Earth and moon attract each other through gravitation. v v’
Since Earth is much more massive than the moon , the moon’s effect on Earth is small.
Moon F
Earth’s gravitational force constantly accelerates the moon towards Earth.
Earth
This acceleration is constantly changing the moon’s direction of motion, holding it on its almost circular orbit.

24. Orbital Motion (II)
In order to stay on a closed orbit, an object has to be within a certain range of velocities:
Too slow : Object falls back down to Earth
Too fast : Object escapes the Earth’s gravity

25. Orbital Motion (III): Geosynchronous Orbits
Orbital Motion (III): Geosynchronous Orbits

26. The Tides  2 tidal maxima  12-hour cycle
Caused by the difference of the moon’s gravitational attraction on the water on Earth
The Tides
Forces are balanced at the center of Earth
Excess gravity pulls water towards the moon on the near side
Excess centrifugal force pushes water away from the moon on the far side
 2 tidal maxima
 12-hour cycle

27. Near full and new moon, those two effects add up to cause spring tides
Spring and Neap Tides
The sun is also producing tidal effects, about half as strong as the moon.
Near full and new moon, those two effects add up to cause spring tides
Near first and third quarter, the two effects work at a right angle, causing neap tides.
Spring tides
Neap tides

28. Acceleration of the Moon’s Orbital Motion
Earth’s tidal bulges are slightly tilted in the direction of Earth’s rotation
Gravitational force pulls the moon slightly forward along its orbit