What are some of the things Hipparchus and Aristarchus were known for?
Aristarchus of Samos: 3rd Century BC
- The first Heliocentric Astronomer
- proposed that all of the planets revolve around the Sun
Hipparchus: 2nd Century BC
- One of the founder of astronomy as a mathematical science
- trigonometry, discovered precession of equinoxes
What is retrograde motion?
Planets track back moving east to west/right relative to the background stars. They stop, then back up for several weeks or months.
What is the Ptolemaic model?
How did it explain retrograde motion?
An explanation for everything on the sphere, it involved an intricate mechanism of epicycles.
Ptolemy (Alexandria, Egypt 1900 years ago) had the concept that each planet is assumed to move in a small circle called an epicycle, the center of which moves in a larger circle called a deferent, whose center is offset from Earth.
It explained retrograde motion because the planet is seen in direct motion (left or eastward) against the background stars throughout most of the year, but when the planet is on the part of its epicycle nearest Earth, its motion along the epicycle subtracts from the motion of the epicycle along the deferent. The planet appears to be slow and then halt its usual movement to the left among the constellations, and then moves to the right.
What is the Copernican model?
How did Copernicus explain retrograde motion?
Hypothesis:
Sun is at the Center!
Observations could now determine the order and relative positions of the planets around the Sun. What we observe are configurations.
Copernicus concluded that Mercuries and Venus’ orbits must lie inside Earth’s.
Elongation: Inferior planets: Inferior Conjunction vs. Superior Conjunction
Superior planets: Conjunction vs. Opposition
Observing configurations, Copernicus was able to calculate the planet’s periods and distances from the Sun.
What are (very basically) planetary configurations? What are the Superior vs. Inferior Planets?
The geometric relationships between Earth, any other planet, and the Sun.
Inferior Planets - planets between Earth and the Sun
Superior Planets - planets on the opposite side of the Sun from Earth
What did Copernicus use the planetary configurations for?
To distinguish between the periods of each planet
Who was Tycho Brahe?
- 16th century ad
- the last and the most accurate of the old observational astronomers
- De Stella Nova - book
- made meticulous observations of the the motion of the planets on the celestial sphere
- reasoned that if a new star is nearby, its position should shift against the background stars over the course of a night
What did Kepler do with what Brahe left him (leading into his laws)?
- 17th Century AD
- Founder of Modern Astronomy as a Mathematical Science
- Kepler corrected Copernicus’ model by turning the planetary orbits from circles into Ellipses with the Sun at one of the foci.
- elliptical orbits
What are Kepler’s laws (can you state them without memorization) and what do they mean?
- The orbit of a planet around the Sun is an ellipse with the Sun at one focus
- A line joining a planet and the Sun sweeps out equal areas in equal intervals of time
- The square of a planets sidereal period around the Sun is directly proportionals to the cube of the length of its orbits semi major axis
What did Galileo do and what did it mean for the Copernican model?
17th century AD
- Founder of modern observational astronomy
- supported conclusion that Venus orbits the Sun, not Earth
- made dramatic new observations using new telescope
- discovered 4 moons orbiting Jupiter
- provided further proof that Earth is not the center of the Universe`
What are Newton’s laws (can you state them without memorization) and what do they mean?
- Inertia is the property of matter that keeps an object at rest or moving in a straight line at a constant speed unless acted upon by a net external force
- The acceleration of an object is directly proportional to the net force acting on it and is inversely proportional to its mass.
F=ma - Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object
How do Newton’s laws relate to Kepler’s laws?
Newton found that he could mathematically explain Keplers laws instead of by trial and error
What is Newton’s gravitational law, and how are the planets orbiting the Sun instead of falling in?
2 objects attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them
What is linear momentum and angular momentum?
What does the amount of each depend on?
Can you give an example of two things/processes with roughly the same momenta?
Linear - moving in a straight/non-closed line
Angular - moving in a circle or closed path, a measure of how much energy is stored in an object due to its rotation and revolution
What does it mean that either kind of momentum is conserved?
Can you give a real life example?
It means planets neither spiral into the Sun nor fly away from it. As the spread of mass decreases, the rotation rate must increase.
Real life example would be a twirling ice skater, brings in arms and legs in order to spin faster.
