Chapter 2: Kepler's Laws and Newton's Laws

Sun, Moon, Stars, and Five Planets (Mercury, Venus, Mars, Jupiter, and Saturn)

What celestial objects did ancient astronomers observe?

Sun, Moon, and Stars

What celestial objects are observed to have simple motions in the night sky?

Planets

What celestial objects have complex motions?

Apparent backward motion of planets along the celestial sphere.

What is retrograde motion?

Earth-centered model and everything revolves around it.

What is the geocentric model of the solar system?

A basic version of a geocentric model used to explain planetary motions.

What is an epicycle?

The path of the center of the epicycle.

What is a deferent?

Ptolemy

Who used epicycles to explain the retrograde motion of planets while maintaining a geocentric model of the solar system?

The simplest model is often the correct model. If the model is too complex and/or there’s too many assumptions made to establish a model, it’s probably not the best model.

What is Occam’s razor?

A model where Earth spins on its axis and revolves around the Sun.

What is the heliocentric model of the solar system?

Nicholas Copernicus

Who first proposed the heliocentric model of the solar system in the 1500’s?

The presentation of this new Sun-centered model and its foundations to society.

What was the Copernican Revolution?

Galileo

Who first used telescopic observations to support the idea of a heliocentric model of the solar system?

Moon has mountains and valleys, Sun gas sunspots and rotates, Jupiter has moons, and Venus has phases.

What are the four observations Galileo made with a telescope?

It cannot be explained by the geocentric model.

Why were the observations Galileo made with the telescope shocking to those of the time?

Tycho Brahe

Whose observations did Kepler use to derive his laws of planetary motion?

Planetary orbits are ellipses, with the sun at one focus.

What does Kepler’s first law state?

An imaginary line connecting the sun and the planet sweeps out equal areas in equal times.

What does Kepler’s second law state?

When the orbiting object is furthest away from the planet.

What is aphelion?

It’s moving the slowest because it’s less influenced by the parents’ gravity.

How does the orbiting object’s speed at the aphelion compare to its speed at other points in its orbit?

When the orbiting object is closest to the parent.

What is perihelion?

It’s moving the fastest because it’s more influenced by the parents’ gravity.

How does the orbiting object’s speed at the perihelion compare to its speed at other points in its orbit?

The square of the period of a planet’s orbital motion is proportional to the cube of it’s semi-major axis.

What does Kepler’s third law state?

The time it takes to complete one orbit.

What is the period of a planet’s orbital motion?

The average distance between the orbiting object and its parent.

What is the semi-major axis of a planet’s orbit?

The average distance between the Earth and the Sun.

What is an astronomical unit (AU)?

1 AU

What is the distance between the Earth and the Sun?

A measure of how much an orbit deviates from a perfect circle and how elongated it is, expressed as a number between 0 and 1.

What is the eccentricity of a planet’s orbit?

Planets going around other stars, a moon/satellite going around a planet, and comet/debris going around the Sun.

What celestial objects does Kepler’s third law pertain to?

Remain in the same orbit.

What would happen to the Earth if the Sun were suddenly replaced by a solar-mass black hole?

Newton

Who discovered WHY the planets orbit the Sun the way they do?

Kepler

Who discovered HOW the planets orbit the Sun the way they do?

An object in motion stays in motion, and an object at rest stays at rest, unless acted upon by an external force.

What is Newton’s first law of motion?

The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass, expressed as F=ma.

What is Newton’s second law of motion?

For every action, there is an equal and opposite reaction.

What is Newton’s third law of motion?

Sir Isaac Newton

Who discovered gravity?

The masses of the two objects and the distance between their centers.

What physical variables does the gravitational force between two objects depend on?

Gravitational pull of the Sun and Inertia

What two phenomena keep planets in their orbits?

The center of mass is closer to the more massive object.

Massive objects orbit around their common center of mass, which is closer to or farther from the more massive object?

Kepler's Third Law.

Which of Kepler’s laws did Newton modify?

The orbit of a planet around the sun is an ellipse, with the center of mass of the planet-Sun system at one focus.

What is Newton’s modified version of Kepler’s Third Law?

A perfect circle

If a planet’s orbit is 0, what is its shape?

A straight line

If a planet’s orbit is 1, what is its shape?