1. Origin of Modern Astronomy
Who is Stan Hatfield and Ken Pinzke

2. Early Astronomy Ancient Greeks
 Astronomy is the science that studies the universe. It includes the observation and interpretation of celestial bodies and phenomena.
 The Greeks used philosophical arguments to explain natural phenomena.
 The Greeks also used some observational data.

3. Early Astronomy Ancient Greeks
 Geocentric Model = Ptolemy Greek Astronomer
• In the ancient Greeks’ geocentric model, the moon, sun, and the known planets—Mercury, Venus, Mars, and Jupiter—orbit Earth.
 Heliocentric Model = Nicolaus Copernicus
• In the heliocentric model, Earth and the other planets orbit the sun.

4. Early Astronomy Ancient Greeks  Ptolemaic System
• Ptolemy created a model of the universe that accounted for the movement of the planets.
• Retrograde motion is the apparent westward motion of the planets with respect to the stars.
Retrograde motion of Mars
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6. Retrograde Motion
Makes no sense without caption in book

7. 99 Years of Astronomy

8. Early Astronomy The Birth of Modern Astronomy  Nicolaus Copernicus
• Copernicus concluded that Earth is a planet. He proposed a model of the solar system with the sun at the center. Heliocentric Model
This model explained the retrograde motion of planets better than the geocentric model.

9. Early Astronomy The Birth of Modern Astronomy  Tycho Brahe
• Tycho Brahe designed and built instruments to measure the locations of the heavenly bodies. Brahe’s observations, especially of Mars, were far more precise than any made previously.
 Johannes Kepler
• Kepler discovered three laws of planetary motion:
1. Orbits of the planets are elliptical.
2. Planets revolve around the sun at varying speed.
3. There is a proportional relationship between a planet’s orbital period and its distance to the sun.

10. Early Astronomy The Birth of Modern Astronomy German astronomer
Johannes Kepler
( ) helped establish the era of modern astronomy by deriving three laws of planetary motion.

11. Johannes Kepler 1599 – Kepler hired by Tycho Brahe
Work on the orbit of Mars
1609 – Kepler’s 1st and 2nd Laws
Planets move on ellipses with the Sun at one focus
The radius vector sweeps out equal areas in equal times
1618 – Kepler’s 3rd Law
The square of a planet’s orbital period P is proportional to the cube of its semi-major axis R.

12. Early Astronomy
Johannes Kepler used Tycho Brahe’s data to develop three laws that explained the motions of the planets.
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Sun
Equal areas
Earth’s orbit
KEPLER’S EQUAL AREA LAW states that a line connecting Earth to the sun will pass over equal areas of space in equal times. Because Earth’s orbit is elliptical, Earth moves faster when it is nearer to the sun.

13. Early Astronomy Equal areas law Faster Slower
KEPLER’S EQUAL AREA LAW states that a line connecting Earth to the sun will pass over equal areas of space in equal times. Because Earth’s orbit is elliptical, Earth moves faster when it is nearer to the sun.
Equal areas law
Faster
Slower

14. Early Astronomy  Galileo Galilei Italian scientist
Galileo Galilei (1564—1642) used a new invention, the telescope, to observe the Sun, Moon, and planets in more detail than ever before.

15. Early Astronomy The Birth of Modern Astronomy  Galileo Galilei
• Galileo’s most important contributions were his descriptions of the behavior of moving objects.
• He developed his own telescope and made important discoveries:
1. Four satellites, or moons, orbit Jupiter.
2. Planets are circular disks, not just points of light.
3. Venus has phases just like the moon.
4. The moon’s surface is not smooth.
5. The sun has sunspots, or dark regions.

17. Early Astronomy  Sir Isaac Newton English scientist
Sir Isaac Newton (1642—1727) explained gravity as the force that holds planets in orbit around the Sun.

18. Early Astronomy The Birth of Modern Astronomy  Sir Isaac Newton
• Although others had theorized the existence of gravitational force, Newton was the first to formulate and test the law of universal gravitation. The universal law of gravitation, helped explain the motions of planets in the solar system.
 Universal Gravitation
• Gravitational force decreases with distance.
• The greater the mass of an object, the greater is its gravitational force.

19. Gravity’s Influence on Orbits
Makes no sense without caption in book

20. Newton’s Laws of Motion
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

21. 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”

22. Summary
Kepler’s and Galileo’s Laws provided Newton with important clues that helped him formulate his laws of motion
Newton arrived at 3 laws that govern the motion of objects
The law of inertia
The law of force
The law of action and reaction
Newton also arrived at a law of gravity
But it seemed to require action at a distance!