Discovering the Universe - Chapter 2 Vocabulary (VOCABULARY)

A vocabulary set covering key terms from the chapter on gravitation and planetary motion, with concise definitions drawn from the lecture notes.

Copernicus

Polish astronomer who proposed the heliocentric model (Sun-centered universe); published De revolutionibus orbium coelestium and sparked the shift away from geocentrism.

Heliocentric cosmology

Model in which the Sun is at the center and planets orbit it; explains retrograde motion more simply than geocentric models.

Geocentric cosmology

Earth-centered model in which Earth is stationary and celestial bodies orbit it; basis of Ptolemy’s system.

Epicycle

A small circular motion of a planet around its deferent used in the Ptolemaic model to explain apparent retrograde motion.

Deferent

The large circular path around Earth along which an epicycle moves in the geocentric model.

Ptolemy

Ancient Greek astronomer who developed the geocentric model with epicycles and deferents to explain planetary motion.

Retrograde motion

Apparent backward motion of a planet against the background stars, caused by relative positions and motions of Earth and the planet.

Tycho Brahe

Danish noble and astronomer who made precise naked-eye measurements and proposed the Tychonic system; mentor to Kepler.

Tychonic system

Hybrid model: Earth remains stationary; the Sun and Moon orbit Earth, while other planets orbit the Sun.

Kepler’s First Law

Planets travel in ellipses with the Sun at one focus.

Kepler’s Second Law

A line connecting a planet and the Sun sweeps out equal areas in equal intervals of time.

Kepler’s Third Law

The square of a planet’s sidereal period is proportional to the cube of its orbit’s semimajor axis (P^2 ∝ a^3).

Ellipse

A closed curve; in astronomy the orbital path of a planet, with two foci defining its shape.

Focus

One of the two fixed points of an ellipse; the Sun is at a focus in planetary orbits.

Semimajor axis

Half of the major axis of an ellipse; a measures the size of the orbit and average distance from the Sun.

Perihelion

The point in a planet’s orbit nearest the Sun.

Aphelion

The point in a planet’s orbit farthest from the Sun.

Sidereal period

Orbital period measured relative to the distant stars (the true year for a planet).

Synodic period

Time between successive configurations as seen from Earth, measured with respect to the Sun.

Astronomical unit (au)

Average distance between the Earth and the Sun; about 1.5 × 10^8 km.

Light-year (ly)

Distance light travels in one year in vacuum; about 9.46 × 10^12 km (≈63,200 au).

Parallax

Apparent shift in position of a nearby object against distant background when viewed from different locations; used to measure distances.

Barycenter

Center of mass of a system of bodies; the motion of the barycenter is a straight line for an isolated system; closer to the more massive body.

Center of mass

The point where the mass of a system can be considered to be concentrated; dictates motion of the system’s barycenter.

Conic section

One of the curves obtained by intersecting a cone with a plane; includes ellipse, parabola, and hyperbola.

Parabola

A conic section representing a bound or unbound orbit in specific energy conditions; open curve.

Hyperbola

A conic section representing an unbound orbit where bodies escape each other.

Newton’s First Law

Law of Inertia: an object at rest stays at rest and an object in motion stays in motion unless acted on by a net external force.

Newton’s Second Law

Law of Motion: acceleration equals net force divided by mass (F = ma).

Newton’s Third Law

Action-reaction: forces come in equal and opposite pairs.

Newton’s Law of Universal Gravitation

Two masses attract with a force proportional to the product of their masses and inversely proportional to the square of the distance between them (F = G m1 m2 / r^2).

Mass

Quantity of matter in an object; a measure of inertia.

Weight

Gravitational force acting on a mass; weight depends on gravity.

Velocity

Speed with a specified direction; a vector quantity.

Acceleration

Rate of change of velocity with time.

Linear momentum

Momentum associated with straight-line motion: p = mv.

Angular momentum

Momentum of rotating/revolving motion; depends on rotation rate and distribution of mass.

Torque

Rotational effect of a force; tends to change an object's angular momentum.

Conservation of Linear Momentum

Total linear momentum of a closed system remains constant if no external forces act.

Conservation of Angular Momentum

Total angular momentum remains constant if no external torque acts.

Force

Interaction that can cause a change in motion; measured in newtons.

Gravity

Attractive force between masses; governs celestial motions.

Occam’s Razor

Philosophical principle that, among competing explanations, the simplest one is preferable.

Inferior conjunction

Planet lies between Earth and the Sun; typically not visible at night.

Superior conjunction

Planet lies beyond the Sun from Earth’s perspective; not visible at night.

Greatest eastern elongation

Planet is farthest east of the Sun as seen from Earth and is visible after sunset.

Greatest western elongation

Planet is farthest west of the Sun as seen from Earth and is visible before sunrise.

Opposition

Outer planet opposite the Sun in the sky; highest in the sky at midnight.