QUARKS 2 COSMOS CH 2 TERMS

Retrograde motion

Backward, westward loop traced out by a planet with respect to the fixed stars

Geocentric

The earth lay at the center of the universe and all other bodies moved around it

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 larger circle

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

The sun is the center of the universe and all other bodies move around it

Aberration of starlight

Small shift in the observed direction to a star, caused by the Earth’s motion perpendicular to the line of sight

Laws of planetary motion; what are they?

Three laws derived by Kepler describing the motion of the planets around the sun

1. The orbital paths of the planets are elliptical (not circular), 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

   • A planet moves fastest in its orbit when it is closest to the Sun

3. The square of a planet’s orbital period is proportional to the cube of its semimajor axis — P² (in Earth’s years) = a³ (in astronomical units)

   • P is the planet’s sidereal orbital period, a is the length of its semimajor axis

   • Planets on larger orbits take longer to complete one trip around the Sun; the orbital period increases faster than the size of the orbit

Ellipse

Geometric figure resembling an elongated circle, 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

Cus

Foci of the ellipse; two fixed points located on the major axis of the ellipse, where the sum of the distances from any point on the ellipse to these two points is always constant

Major axis

The long axis of the ellipse, containing the two foci

Semimajor axis

One-half of the major axis of an ellipse; 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

Sidereal orbital period

The time needed for the planet to complete one circuit around the sun

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; what are they?

The basic laws of motion which are sufficient to explain and quantify virtually all of the complex dynamic behavior found on Earth and elsewhere in the universe

1. Every body continues in a state of rest or in a state of uniform motion in a straight line, unless it is compelled to change that state by a force acting on it

   • Simply put, a moving object will move forever in a straight line, unless some external force — a push or a pull — changes its speed or direction of motion

   • It requires no force to maintain motion in a straight line with constant speed (velocity), contrasting Aristotle who maintained that the natural state of an object was to be at rest

2. When a force F acts on a body of mass m, it produces in it an acceleration a equal to the force divided by the mass. Thus, a = F/m, or F = m*a

   • The greater the force acting on the object or the smaller the mass of the object, the greater is the acceleration of the object

     • If two objects are pulled with the same force, the more massive on will accelerate less; if two identical objects are pulled with different forces, the one acted on by the greater force will accelerate more

3. To every action, there is an equal and opposite reaction

   • In other words, if body A exerts a force on body B, then body B necessarily exerts a force on body A that is equal in magnitude, but oppositely directed

Force

Action on an object that causes its momentum to change. The rate at which the momentum changes is numerically equal to the force

Weight

The gravitational force exerted on you by Earth (or the planet on which you happen to be standing)

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

• The greater an object’s mass, the more inertia it has, and the greater is the force needed to change its state of motion

Velocity

Displacement (distance plus direction) per unit time, written in m/s

• Object’s velocity includes both its speed (in mph or meters per sec) and its direction in space (up, down, etc.)

  • Different from speed; consider a rock tied to a strong moving at a constant rate in a circle — a rock’s speed is constant, but its direction of motion, and hence its velocity, is continually changing

Acceleration

The rate of change of velocity of a moving object (speeding up, slowing down, or simply changing direction); written in m/s²

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 if 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

• Example: tripling the distance makes the force 3² = 9 times weaker, whereas multiplying the distance by five results in a force 5² = 25 times weaker

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

What does Kepler’s first law become when considering the planet and the Sun orbit their common center of mass?

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

Escape speed

The speed necessary for one object to escape the gravitational pull of another. Anything that moves away from a gravitating body with more than the escape speed will never return

Unbound

An orbit that does not stay in a specific region of space, but where an object escapes the gravitational field of another. Typical unbound orbits are hyperbolic in shape