Abscissa

The horizontal axis of a graph.

Acceleration

The time rate of change of velocity. A vector quantity.

Antiperistasis

Aristotle's explanation of projectile (violent) motion. When a body is struck or thrown into motion a void is created behind it due to the motion. Air from the front of the motion is pushed aside and rushes in to fill the void, thus providing the push to maintain the motion. Aristotle was not sure of this and suggested an alternative solution.

Areal Law

The second of Johannes Kepler's laws of planetary motion. It states that the area swept out by the line joining sun and a planet as the planet moves along an elliptical orbit is the same for equal time intervals no matter where the planet is along its path. Often stated as "The radius vector sweeps out equal areas in equal time intervals." First stated in 1609

Aristarchus of Samos

(310 - 230 BC) Determined the relative earth-moon and earth-sun distances. Suggested a heliocentric planetary model that was resoundingly rejected by his contemporaries.

Aristotle

(384 - 322 BC) Developed a theory of mechanical motion and planetary motion. Used 56 homocentric spheres. Divided universe into the celestial (perfect) and the sub lunar (imperfect). All sub lunar matter is fire, air, earth, water. Celestial matter was called aether. Introduced four types of motion: local, increase and decrease, creation and destruction, and alteration. Law of motion: v µ (F/R)

Astronomical Units

The units of measure which have the earth-sun distance equal to one distance unit and the period of revolution of earth (the year) as the time unit.

Atom

From the Greek meaning not to cut and thus means indivisible

Atomist Theory

Originated by Leucippus and popularized by Democratus

Celestial Sphere

The sphere which contains the "fixed stars."

Celestial Motion

The perfect motion in the celestial realm of Aristotle.

Celestial Planets

Mars, Jupiter, Saturn, Uranus, Neptune, Pluto

Claudius Ptolemy

lived about AD 150. Wrote the Almagest (majestic) and developed a cosmology which lasted nearly 1500 years. Used the eccentric, equant, deferent circle, and the epicycle.

Conservation of Momentum

Momentum remains fixed. One of two conservation laws of Newton

Conservation Law

The total amount of a physical quantity remains fixed.

Conservation of Mass

Mass remains fixed. One of two conservation laws of Newton. Also assumed by Aristotle.

Deductive Logic

A conclusion which proceeds from a few rules. Due to René Descartes (AD 1596 - 1650)

Deferent Circle

A large circle whose circumference carries the center of an epicycle.

Displacement

A vector quantity that gives both the distance and direction an object moves from a starting point.

Dynamics

The combination of kinematics and kinetics.

Eccentric

The position of the Earth in Ptolemy's model. Also used in other astronomical models.

Eclipse

The shadow of the moon on the earth (Solar Eclipse) or the shadow of the earth through which the moon passes (Lunar Eclipse).

Ecliptic

The path of the sun across the celestial sphere. The ecliptic makes an angle of approximately 23½ degrees with the earth’s equatorial plane.

Ellipse

The geometric figure that is rather egg shaped and has two foci. According to Kepler’s theory of planetary motion, the planets move on the circumference of ellipses.

Empedocles

(c. 500 - 430 BC) An Ionian scientist who said all matter is composed of fire, air, earth, and water. Introduced two forces, love the combiner and strife the destroyer.

Epicycle

A circle whose center moves along the circumference of deferent circle.

Equant

A point somewhat displaced from the center of the deferent circle so that the radius vector (a line from the equant to the center of the epicycle) rotates at a constant rate as epicycle moves along the circumference of the deferent circle. The length of this line changes as the planet moves. Despised by Copernicus.

Equinox

The two times of the year when the sun crosses the celestial equator. There is equal daylight and night time. Vernal—about March 21, Autumnal—about September 21

Eratothanes

(276 - 195 BC) Determined the earth's diameter.

Eudoxus of Cnidus

(409 - 356 BC) Introduced a 27 sphere theory to explain planetary motion.

Fixed Stars

The name given by ancient astronomers to stars thought to be attached to the celestial sphere. They have the same position in the sky at the same time each year.

Four Prime Substances

Fire, Air, Earth, Water. Fire and air naturally rise, earth and water naturally fall. According to the Greeks all matter comes from these four. Due to Empedocles (c. 500 - 430 BC)

Galileo Galilei

(1564 - 1642) The first modern scientist. The first to use a telescope to study the heavens. Studied mechanics, free fall, temperature.

Geocentric

The earth is at the center of the planetary system.

Gravity

The force between two material bodies; it is directly proportional to the product of the masses of the two bodies and inversely proportional to the square of their separation. This force permeates all space.

Greek Science

The study of the natural world by Greek philosophers. First determine the reality (truth) behind the way things appear and then reason a solution from axioms. Restricted to academic problems.

Hans Lippershey

The man who built the first telescope. A Dutch lens maker.

Harmonic Law

The third of Johannes Kepler's laws. This law states that the square of a planets period (the time to complete one orbit is equal to the cube of the planet's radius when Astronomical units are used. First stated in 1619

Heliocentric

The sun is at the center of the planetary system.

Hipparchus

Introduced six star magnitudes (brightnesses) and made a useful star catalogue. Introduced the epicycle and eccentric into planetary models.

Homocentric

A geocentric model of the universe which has all the circles (or spheres) concentric (having the same center).

Inductive Logic

A general rule which is reasoned from the results of experiments. Championed by Francis Bacon (AD 1561 - 1626)

Inertia

The resistance that a body has to a change in its motion.

Inferior Planets

Planets that have orbits smaller than earth’s orbit. Mercury and Venus

Inverse Square Law

Any physical law that has the square of a separation distance in the denominator of its defining equation. Inversely proportional to the square of some physical quantity, often a distance.

Ionian School

Considered water to be the "prime substance" from which all else emerged.

Isaac Newton

(1642 - 1727) Developed a mechanical universe, three laws of motion, and the Law of Universal Gravitation. One of the greatest physicists of all time.

Jean Burdin

(AD 1295 - 1358) A scholar of motion who provided counter examples to Aristotle's Antiperistasis.

Johannes Kepler

(1571 - 1630) Developed a planetary model based on Brahe's observation stated in three laws, the elliptical law, the areal law, and the harmonic law.

John Philoponus (John the Grammarian)

An early (circa AD 500) critic of Aristotle's theory of projectile motion.

Kinematics

The branch of mechanics which deals with the description of motion without regard to the causes of motion.

Kinetics

The branch of mechanics which deals with the causes of motion.

Law of Ellipses

The first of Johannes Kepler's laws. This law states planets move in elliptical paths with the sun at one focus of the ellipse. First stated in 1609.

Local Motion

Up and down motion in the sub lunar realm of Aristotle.

Mass

A measure of the inertia of a body, i.e. its resistance to change in motion.

Mechanics

The combination of dynamics and statics.

Mertonians

Scholars belonging to Merton College of Oxford University in England. They studied and developed kinematics in the 13th century.

Momentum

A property of motion given as the product of mass and velocity. Originally called Quantity of Motion.

Natural Motion

The motion of a body from less perfect toward more perfect, according to Aristotle.

Newton's First Law

A body in a uniform state of motion will remain in a uniform state of motion unless acted upon by an impressed force

Newton's Second Law

An applied force will cause an acceleration of a body proportional to the force and inversely proportional to the mass of the body

Newton's Third Law

For every action (Force) there is an equal and opposite reaction (Reaction Force)

Nicholas Copernicus

(1473 - 1543) A Catholic cleric and astronomer. The first modern man to develop a heliocentric planetary model. Used the Ptolemaic eccentric, deferent circle, and epicycle. Hated and eliminated the equant. All works banned by the Church until 1835.

Ockham's Razor

A rule whereby simple explanations are preferred to complicated ones.

Ordinant

The vertical axis of a graph.

Perfect Vacuum

The absence of all matter. Thought to be impossible by Aristotle because motion of objects would be very large and that was not observed.

Philolaus

Introduced 10 heavenly objects and a "central fire" to explain planetary motion. This is not a heliocentric theory.

Planet

The name from Greek for the "wandering" stars.

Plato

(429 - 348 BC) Introduced the concept that the ideal is the ultimate reality. Decreed that planetary motion must be circular.

Prograde Motion

Motion or rotation from west to east. The normal motion of the planets.

Projectile Motion

According to Aristotle this is motion with no apparent force to actively propel an object. In modern physics projectiles have both a horizontal and a vertical component of motion. These two independent motions can be combined by superposition as first described by Galileo.

Pythagoreans

A school of philosophers who used concentric spheres to explain the planetary system. They used friction to cause planetary motion. Planetary motion gave rise to celestial music which could be heard only by the most gifted ears.

Radius Vector

In Ptolemaic theory it is the line between the equant and the center of the epicycle which rotates at constant speed. In Keplerian theory it distance from the sun to a planet.

Retrograde Motion

The apparent reversal of motion of a planet from its normal direction. This motion is east to west.

Scalar

A quantity which has size only. It may have units.

Scientific Method

A cyclic combination of observation, hypothesis, and test.

Solstice

One of the two times of the year when the sun is either at its highest or lowest position in the sky. At these two times the sun does not have northward or southward motion. Summer—about June 21, Winter—about December 21

Speed

The time rate of change of distance along some path.

Statics

The branch of mechanics which deals with the forces required to keep a body motionless.

Superior Planets

Planets that have orbits greater than earth’s orbit.

Telescope

A device which optically enlarges objects so that they appear closer or bigger. The amount of enlargement is called the power of the telescope and is designated by a number followed by X.

Terrestrial Planets

Mercury, Venus, Earth. Earth-like planets, mostly rocky

Thales

An early Ionian philosopher-scientist. May have founded the Ionian School. Introduced rational thought and made science possible.

Tycho Brahe

(1546 - 1601) A late 16th century astronomer who collected a large amount of very accurate data (for that time) of planetary motions. Developed a compromise cosmology with the earth at the center. The sun rotated about earth and the planets about the sun. Hired Johannes Kepler as an assistant.

Uniform Circular Motion

Motion along the circumference of a circle at a constant speed. The angle between a radius and the positive x axis changes at a constant rate.

Uniform Motion

Motion along a straight line and at a constant speed.

Uniform Velocity

A constant speed along a straight line.

Uniformly Accelerated Motion

Any motion where the time rate of change of velocity is a constant. The acceleration is a constant value (has both a constant magnitude and constant direction).

Vector

A quantity which has both magnitude and direction.

Velocity

The time rate of change of displacement. A vector (magnitude and direction) which specifies the speed and direction of an object at some point in space or time.

Violent Motion

According to Aristotle it is unnatural motion. That is, motion of a body in the unnatural direction