Chapter 1 Terms Flashcards
Retrograde motion
Backwards, westward loop traced out by a planet with respect to the fixed stars.
Geocentric model
A model of the solar system that holds that Earth is at the center of the universe and all other bodies are in orbit around it. The earliest theories of the solar system were geocentric.
Epicycle
A construct of the geocentric model of the solar system that was necessary to explain observed planetary motions. Each planet rides on a small epicycle whose center in turn rides on a small epicycle whose center in turn rides on a larger circle (the deferent).
Deferent
A construct of the geocentric model of the solar system that was needed to explain observed planetary motions. A deferent is a large circle encircling Earth, on which an epicycle moves.
Ptolemaic model
Geocentric solar system model, developed by the second century astronomer Claudius Ptolemy. It predicted with great accuracy the positions of the then known planets.
Heliocentric model
A model of the solar system that is centered on the Sun, with Earth in motion about the Sun.
Retrograde motion
Backward, westward loop traced out by a planet with respect to the fixed stars.
Copernican revolution
The realization, towards the end of the 16th century, that Earth is not at the center of the universe.
Ellipse
Geometric figure resembling an elongated circle. An ellipse is characterized by its degree of flatness, or eccentricity, and the length of its long axis. In general, bound orbits of objects moving under gravity are elliptical.
Focus
One of two special points within an ellipse, whose separation from each other indicates the eccentricity
In a bound orbit, planets orbit in ellipses with the Sun at one focus.
Semimajor axis
One-half of the major axis of an ellipse. The semimajor axis is the way in which the size of an ellipse is usually quantified.
Eccentricity
A measure of the flatness of an ellipse, equal to the distance between the two foci divided by the length of the major axis
Perihelion
The closest approach to the Sun of any object in orbit about it.
Aphelion
The point on the elliptical path of an object in orbit about the Sun that is most distant from the Sun.
Orbital period
time taken for a body to complete one full orbit around another.
Astronomical unit (AU)
The average distance of Earth from the Sun. Precise radar measurements yield a value for the AU of 149,603,500 km.
Radar
Acronym for RAdio Detection And Ranging. Radio waves are bounced off an object, and the time taken for the echo to return indicates its distance.
Newtonian mechanics
The basic laws of motion postulated by Newton, which are sufficient to explain and quantify virtually all of the complex dynamical behavior found on Earth and elsewhere in the universe.
Inertia
The tendency of an object to continue moving at the same speed and in the same direction, unless acted upon by a force
Mass
A measure of the total amount of matter contained within an object
Acceleration
The rate of change of velocity of a moving object
Gravity
The attractive effect that any massive object has on all other massive objects. The greater the mass of the object, the stronger its gravitational pull.
Gravitational force
Force exerted on one body by another due to the effect of gravity. The force is directly proportional to the masses of both bodies involved and inversely proportional to the square of the distance between them.
Inverse-square law
The law that a field follows of its strength decreases with the square of the distance. Fields that follow the inverse-square law decrease rapidly in strength as the distance increases, but never quite reach zero.
Center of mass
The “average” position in space of a collection of massive bodies, weighted by their masses. For an isolated system this point moves with constant velocity, according to Newtonian mechanics.
