1. Galileo, Newton and the Birth of Astrophysics
Galileo Galilei
scientist and outspoken supporter of Copernicus’ heliocentric model
lived and worked at the time of Brahe and Kepler
Galileo Galilei ( )

2. Galileo’s Physics Experiments
Performed many experiments on the motion of falling bodies
Showed that objects of different mass dropped from the same height will fall to Earth at the same time (neglecting air resistance).

3. Galileo’s Telescope
He was among the first to use a telescope to observe the sky and publish his observations.
Observed:
Mountains and valley on the Moon (like Earth).
Spots on the Sun and Solar rotation (imperfect Sun).
Phases of Venus
Moons of Jupiter
Galileo’s drawings of the Moon
as seen through his telescope.

4. The Moons of Jupiter
The discovery of these moons proved that at least some things did not go around the Earth.
These 4 moons were later named the Galilean satellites in honor of Galileo.
A copy of Galileo’s original notes
showing Jupiter and four of its moons.

5. Phases of Venus We only can see Venus due to reflected sunlight
We should only see crescent phases of Venus if Venus went around the Earth.
Instead we see almost all of the phases. Venus can’t orbit Earth.
Ptolemy’s
Earth-centered
model. Venus
moving along its
epicycle.
Copernicus’
Sun-centered
model. Venus
in orbit around
the Sun.

6. Photos of Venus’ Phases
What Galileo probably saw may have looked something like this.

7. Galileo and the end of Ptolemy’s model
By the end of Galileo’s life Copernicus’ Sun-centered model of the Solar System and Kepler’s Laws had gained wider acceptance.
Galileo died almost 100 years after Copernicus published his model

8. Isaac Newton (1642 - 1727) Newton was born the year Galileo died.
It had been believed that there was one set of laws that applied to Earth and another set to the stars, Moon and planets.
Newton recognized that there is one set of physical laws that apply everywhere.
The same force that causes an apple to fall also keeps the Moon moving around the Earth.
Newton’s example showing how an object with enough initial velocity could be placed in orbit around Earth

9. Newton’s Achievements
Considered the greatest physicist of all time.
Made significant contributions to astronomy, physics and optics
Invented the reflecting telescope
Developed the branch of mathematics called calculus
His most famous contributions are to the study of motion and gravity.
He recognized that gravity was the only force involved in keeping the planets moving around the Sun

10. The Universal Law of Gravitation
Every particle in the Universe attracts every other particle.
The force is proportional to the product of their masses and inversely proportional to the square of the distance between them

11. Newton’s Laws 1st Law (Law of Inertia)
A body at rest remains at rest unless made to change by a force
A body in uniform motion (in a straight line) remains that way unless made to change by forces acting on it.
Uniform motion means both speed and direction are unchanged
A body not moving in a straight line or not moving at a constant speed must be experiencing a force.

12. Acceleration
Acceleration can be a change in speed (faster or slower) or a change in direction
You can experience an acceleration due to a force.

13. Mass and Weight are not the same
Mass - the amount of matter a body contains.
Weight - a measure of the gravitational force on an object
On the Moon we would weigh less but have the same amount of mass

14. Newton’s Laws 2nd Law a=F/m
The amount of acceleration (a) that a force (F) can produce depends on the mass (m) of the object being accelerated
a=F/m

15. Newton’s Laws
3rd law
When two bodies interact, they create equal and opposite forces on each other
The force of gravity the Earth experiences due to the Sun is the same as the force the Sun experiences due to the Earth.

16. Newton’s Version of Kepler’s 3rd Law
Newton generalized Kepler’s 3rd Law so that it can be applied anywhere in the Universe not just to planets going around the Sun. (Remember P2(years) = a3(AU) ?)
Newton’s version includes the mass of the two objects. So if you know the period of the orbit and distance you can determine the mass.
By knowing the period of the orbit of one of Jupiter’s moons and its distance from Jupiter you can “weigh” Jupiter.
We can use this formula to weigh anything (stars, galaxies, black holes, etc.)