Chapter 1, The Copernican Revolution

Aphelion

The point on the elliptical path of an object in orbit about the Sun that is most distant from the Sun.

Deferent

A construct of the geocentric model, a large circle encircling Earth, on which an epicycle moves.

Ptolemaic Model

A geocentric solar system model developed by Ptolemy. It predicted with great accuracy that positions of the known planets.

Eccentricity

A measure of the flatness of an eclipse, equal to the distance between the two foci divided by the length of the major axis, the flatter the eclipse the closer to one it is, the rounder the eclipse the closer to zero. Ranges from 0-1

Geocentric Model

A model of the solar system that holds the Earth at the center of the universe with all other bodies orbiting around it.

Describe the astronomical contributions of Copernicus.

Copernicus believed in the heliocentric model of the universe. He believed that the Earth spun on an axis, and like all other planets, orbited the sun.

Astronomical Unit

The average distance of Earth from the Sun. (AU)

Retrograde Motion

The backward westward loop traced out by planets in respect to the fixed stars.

Perihelion

The closest approach to the Sun of any object in orbit about it.

How did the geocentric model explain retrograde motion?

The planets moved along small circles that moved on larger circles around the Earth.

How did the heliocentric model explain retrograde motion?

The planets orbit the Sun at different speeds, retrograde motion is natural and it occurs when the Earth appears to overtake or pass another planet in its orbit. More distant planets move more slowly across the sky than planets that are closer.

Copernican Model

The realization toward the end of the 16th century that Earth is not at the center of the universe.

Period

The time needed for an orbiting body to complete one revolution about another body.

Describe the astronomical contributions of Tycho Brahe.

Tycho observed the motion of the stars and planets without a telescope. Tycho created instruments for measuring celestial objects and built his own observatory. All of his data was given to Kepler after his death which enabled Kepler to discover the laws of planetary motion.

Be able to state and explain each of Kepler's three laws of planetary motion.

1. The orbital paths of the planets are elliptical with the Sun at one focus. 2. An imaginary line connecting the Sun to any planet sweeps out equal areas of the ellipse in equal intervals of time. 3. The square of a planet's orbital period is proportional to the cube of it's semimajor axis. p^2(in earth years)= a^3(in Astronomical Units)

Epicycle

A construct of the geocentric model that was necessary to explain observed planetary motions. Each planet rides on a small epicycle whose center, in turn, rides on a larger circle (the deferent).

Heliocentric Model

A model of the solar system that is centered on the sun with Earth orbiting around the Sun.

List Galileo's observations and explain why they did not support the geocentric model.

Galileo's observations: -The sun has sunspots -Venus has phases. -The moon has mountains, valley's, and craters -Jupiter has moons In Ptolemy's model, everything has to rotate around the Earth, so it would be impossible to have moons rotating around Jupiter. This proved that the heliocentric model had to be right Galileo's observations about the different phases of Venus did not support the geocentric model.

Describe the astronomical contributions of Kepler.

He developed 3 laws of planetary motion. He also found that elliptical oval orbits describe motion of planets.

Explain why the geocentric model predicted only crescent phases of Venus while the heliocentric model predicted that Venus would exhibit a complete cycle of phases, including gibbous.

In the geocentric model, Venus orbits Earth, moving around a smaller circle on its larger orbital circle. The center of the smaller circle lies on the Earth-Sun line. The geocentric model believed that Venus only experienced quarter to crescent phases. In the heliocentric model, Venus orbits the Sun, allowing Earth to see many different phases of Venus.

Semi-Major Axis

One-half of the major axis of an ellipse. The semi-major axis is the way in which the site of an ellipse is usually quantified.

Describe the astronomical contributions of Ptolemy.

Ptolemy created the Ptolemaic model of the universe. The Ptolemaic model was geocentric and the most accurate model of the time. His model of the solar system made sufficiently accurate predictions of planetary positions and remained in use for many centuries