Astronomy: From Geocentric to Newtonian Mechanics – Vocabulary Flashcards

Vocabulary flashcards covering key terms from the lecture notes.

Geocentric model

Earth-centered model of the solar system in which Earth is at the center with other bodies orbiting it.

Heliocentric model

Sun-centered solar system; planets orbit the Sun, while the Moon orbits Earth.

Copernican Revolution

Shift from a geocentric to a heliocentric view of the solar system; Earth is not the center.

Epicycle

A small circle whose center moves along a larger circle (deferent) to explain planetary motion in Ptolemaic theory.

Deferent

The large circle in the Ptolemaic model on which the center of an epicycle moves.

Ptolemy

Ancient Greek astronomer who proposed the geocentric model with epicycles and deferents.

Parallax

Apparent shift of a nearby object against distant background when viewed from different positions.

Phases of Venus

Different observed shapes of Venus; evidence supporting a heliocentric system.

Inferior planet

A planet whose orbit lies closer to the Sun than Earth’s (Mercury, Venus).

Superior planet

A planet whose orbit lies farther from the Sun than Earth’s (Mars, Jupiter, Saturn).

Inferior conjunction

When an inferior planet lies between Earth and the Sun.

Superior conjunction

When an inferior planet is on the far side of the Sun from Earth.

Opposition

Planet is opposite the Sun in the sky; Earth lies between the Sun and the planet.

Conjunction

Two bodies line up with the Sun; for planets, the Sun-Earth-planet line is aligned.

Retrograde motion

Apparent backward motion of a planet due to the relative motion of Earth and the planet.

Ellipse

An oval-shaped orbit; a conic section with two focal points.

Focus

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

Eccentricity

Measure of how elongated an ellipse is (0 = circle; closer to 1 = more elongated).

Perihelion

Point in a planet’s orbit closest to the Sun.

Aphelion

Point in a planet’s orbit farthest from the Sun.

Semimajor axis

Half of the ellipse’s major axis; a measure of the orbit’s size.

Major axis

The longest diameter of an ellipse.

Kepler’s First Law

Planets move in ellipses with the Sun at one focus.

Kepler’s Second Law

A line from the Sun to a planet sweeps out equal areas in equal times.

Kepler’s Third Law

P^2 ∝ a^3; the orbital period squared equals the semimajor axis cubed (P in years, a in AU).

Astronomical Unit (AU)

Mean distance from the Earth to the Sun; a unit for measuring solar-system distances.

Mass of the Sun

Mass inferred from planetary motions; approximately 2.0 × 10^30 kg.

Gravitational constant (G)

Universal constant in Newton’s law of gravitation: G ≈ 6.67×10^-11 N m^2/kg^2.

F = G m1 m2 / r^2

Newton’s law of gravitation: gravitational force between two masses decreases with the square of distance.

Newton’s Laws

Three laws of motion: inertia, F = ma, and action–reaction (every force has an equal and opposite reaction).

Escape speed

Minimum speed needed to break free from a planet’s gravity without further propulsion.

Center of mass

Balance point of a system; the barycenter around which bodies orbit (Earth–Moon, Pluto–Charon examples).

Pluto’s reclassification

Pluto reclassified as a dwarf planet; not considered a major planet due to its small size and orbit.

Galileo Galilei

Early telescope astronomer who observed Moon topography, Sunspots, Saturn’s rings, Jupiter’s moons, and Venus’s phases.

Moon’s mountains and valleys

Rugged lunar surface features observed on the Moon’s surface.

Sunspots

Dark, cooler areas on the Sun’s surface indicating magnetic activity and rotation.

Saturn’s rings

Rings around Saturn observed by Galileo and others.

Jupiter’s moons

Moons observed orbiting Jupiter, supporting heliocentric ideas.

Tycho Brahe

Astronomer whose precise planetary observations aided Kepler’s laws.

Stellar parallax

Apparent shift of stars due to Earth's orbit; not observed by Galileo with his instruments.

Phase of Venus evidence

Venus phases observed in telescope support heliocentric model over geocentric.