1. Astronomy 04 The Solar System
Chapter 4
“The Origin of Modern Astronomy”
Review for Test 1 this Fri 1/31
Test 1 on Chaps 1,2,3,4,5.1 next
Mon 2/3

2. Chapter 4: “The Origin of Modern Astronomy”
Archaeoastronomy is the study of the astronomy of ancient peoples.

3. Stonehenge

4. Astronomical alignments
at Stonehenge

5. Chapter 4: “The Origin of Modern Astronomy”
Cosmology
is our concept of the cosmos, its basic structure and origin.
The ancient civilizations had a geocentric cosmology.
It was during the time of ancient Greece that astronomy and astrology separated.

6. Chapter 4: “The Origin of Modern Astronomy”
Long before Columbus, Aristotle argued Earth was round.
Aristotle observed the curved edge of the shadow of Earth during a lunar eclipse.
He also noticed as one moved south stars would disappear in north while new ones appeared in south.
This could only happen if earth was a sphere.

7. Chapter 4: “The Origin of Modern Astronomy”
Aristarchus of Samos argued that Earth orbited the Sun.
But other Greeks argued that if it did we would observe parallax in the positions of nearby stars.

8. Chapter 4: “The Origin of Modern Astronomy”
Parallax is the apparent change in the position of an object due to a change in the location of the observer.
If Earth orbits the Sun then nearby stars should shift their positions in the sky with respect to more distant stars.

12. Chapter 4: “The Origin of Modern Astronomy”
Parallax wasn’t observed but that was because they underestimated the distances of stars.
The distances to the stars was so great that stellar parallax can only be observed with a telescope.

13. Chapter 4: “The Origin of Modern Astronomy”
Eratosthenes measured the size of Earth.
He noticed that on the first day of summer at Syene, Egypt, sunlight struck the bottom of a vertical well at noon.
This indicated the Sun was overhead.

14. Chapter 4: “The Origin of Modern Astronomy”
At the same time and date, Eratosthenes observed that the Sun was not directly overhead in Alexandria, a city a short distance to the south of Syene.
The Sun did cast a shadow which made an angle with the vertical of about 1/50th (7 degrees) of a circle.

15. Chapter 4: “The Origin of Modern Astronomy”
Now if the Sun’s rays striking the two cities were parallel, then the only way the Sun was not overhead in Alexandria was because overhead was not the same in both cities. ( i.e. the Earth surface was curved).
Then the Earth must be 50 X the distance between the two cities.
This is pretty close to accurate - within about 1% of actual 25,000 mile circumference !

16. Astronomy 04 The Solar System
Chapter 4
“The Origin of Modern Astronomy”
Review for Test 1 this Fri 5/2
Test 1 on Chaps 1,2,3,4,5.1 next
Mon 5/5

17. Chapter 4: “The Origin of Modern Astronomy”
Claudius Ptolemy who lived in Roman Alexandria about 140 A.D was the last great astronomer of antiquity.
He wrote the Almagest, a compilation of the astronomical knowledge of the Greeks.

18. Chapter 4: “The Origin of Modern Astronomy”
Ptolemy formalized the geocentric model of the universe by putting forth a geometrical representation of the solar system that predicted the positions of the planets with reasonable accuracy.
Ptolemy solved the problem of the planet’s motion by having each planet travel on a small circular orbit called an epicycle.

20. Chapter 4: “The Origin of Modern Astronomy”
Ptolemy solved the problem of the planet’s motion by having each planet travel on a small circular orbit called an epicycle.

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Nicolaus Copernicus ( ), a polish cleric challenged the thinking of Ptolemy.
He proposed a Heliocentric Model of the Solar System.

25. Chapter 4: “The Origin of Modern Astronomy”
He wrote “On the Orbits of Celestial Bodies”, published in 1543, the year of his death.
Copernicus argued that the apparent motions of the Sun, Moon, planets and stars about the Earth each day could be represented just as well and more simply by having the Earth and other planets orbit the Sun.

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His theory placed the planets in their correct order in distance from the Sun.

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Copernicus recognized that if the planets orbit around the Sun, rather than around the Earth, he could easily explain the observed retrograde motion of planets like Mars.

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Among the points he proposed in his Heliocentic Theory were:
The Sun is the center of the Solar System; the Earth and planets orbit the Sun in circular orbits.
Day and night are the result of the rotation of the Earth on its axis.
Mercury and Venus are closer to the Sun than the Earth is.

31. Chapter 4: “The Origin of Modern Astronomy”
Only 3 motions of the Earth are needed:
1. daily rotation on its axis
2. yearly revolution around the Sun
3. wobble of the Earth on its axis, accounting for precession of the equinoxes

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Galileo Galilei ( )
In 1609 Galileo turned a Dutch invention called an
optic tube skyward and began recording what he saw.

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The Sun had sunspots.

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The Milky Way is made of individual stars.

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Jupiter had at least four satellites.

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Venus exhibited phases like the Moon.

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His discovery of moons circling Jupiter indicated that not everything had to revolve about Earth. This observation directly supported the Heliocentric Model of Copernicus. After Galileo it became very difficult to deny the Heliocentic Model.
The U.S. honored this great man - known as the Father of Modern Astronomy – by naming the first spacecraft to orbit Jupiter – the Galileo spacecraft.

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Tycho Brahe ( )
Established Observatory of North Sea Island of Hven.
Compiled 20 years of astronomical observation.

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Invited Johann Kepler to join him in analyzing his astronomical data just before his death.

43. Tycho Brahe’s Observatory at Uraniborg

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Johan Kepler ( )
He developed the Three Laws of Planetary Motion

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Law 1: Law of Ellipses
Planets orbit Sun in ellipses not circles with Sun at one focus of the ellipse.

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Law 2: Law of Equal Areas
The straight line joining a planet and the Sun sweeps out equal areas of space in equal intervals of time.

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Law 3: The Law of Periods
A planet’s distance from the Sun is related to its period of revolution. The farther a planet is from the Sun the longer it takes to go around the Sun.

53. Chapter 5.1: “Isaac Newton”
Isaac Newton was born 11 months after Galileo died.

55. Chapter 5.1: “Isaac Newton”
Among his achievements are:
His work entitled Principia formulated the laws of motion that are the basis of mechanics.
It is probably the greatest scientific work ever written. He therefore explained why the planets follow Kepler’s Laws.

56. Chapter 5.1: “Isaac Newton”
He showed that gravity not only makes an apple fall to the ground but also governs the motions of planets and their satellites, and should apply to any bodies (for example, binary stars).

57. Chapter 5.1: “Isaac Newton”
He expressed the universal law of gravitation in mathematical form, showing that the force of gravity falls off inversely with the square of the distance between two bodies.