The History of Astronomy

Vocabulary flashcards covering the major figures, models, observations, and laws in the history of astronomy from ancient Greece to Isaac Newton.

Pythagoras (~500 BC)

Greek philosopher who taught that "numbers govern nature," asserted that the Earth and Moon are spherical (deduced from lunar phases), and believed the planets were spheres because the gods liked spheres.

Aristotle's Proofs of a Spherical Earth

Evidence proposed around ~350 BC including the observed altitudes of celestial objects, the behavior of falling bodies, and the circular shadow cast by the Earth on the Moon during lunar eclipses.

Eratosthenes (~230 BC)

Scholar who determined the tilt of the Earth and calculated its circumference to be $252,000\text{ stadia}$ by assuming a distant Sun with parallel rays striking a spherical Earth.

Prograde Motion

The eastward, forward, or direct apparent motion of a planet across the background sky.

Retrograde Motion

The westward, backward, or reverse apparent motion of a planet, during which planets always appear at their brightest.

Geocentric Model

Early Greek planetary model supported by Eudoxes of Koidus and Aristotle that placed Earth at the center with circular orbits and celestial spheres.

Primum Mobile

The grand moving sphere in the ancient Greek geocentric celestial system.

Almagest

The major astronomical work written by Claudius Ptolemy (~140 AD) outlining the geocentric model with circular orbits.

Deferent

The primary, larger circular orbit in the Ptolemaic planetary system along which an epicycle travels.

Epicycle

A secondary smaller circular orbit whose center moves along the deferent, used to account for retrograde planetary motion.

Eccentric

An off-center circular planetary orbit in ancient geocentric systems.

Epicyclet

A combination of three distinct motions or orbits used to explain planetary movements.

Alphonsine Tables

An astronomical reference book sponsored by King Alphonso X of Castille in 1252 AD.

Ockham's Razor

Methodological principle introduced by William of Ockham (~1340 AD) stating that the best model is the simplest one based on the fewest assumptions.

Heliocentric Model

Model developed by Nicholas Copernicus stating that the Earth is a planet, all planets orbit the Sun in circular paths, and correctly establishing the planets' order, relative distances, and periods of revolution.

Copernican Explanation of Retrograde Motion

An optical illusion and relative motion resulting from planets orbiting at different speeds and different distances from the Sun.

De Revolutionibus Orbium Coelestium

Seminal scientific text published by Nicholas Copernicus presenting the heliocentric theory of the solar system.

Uraniborg

Tycho Brahe's astronomical observatory, meaning "castle of the stars."

Kepler's First Law of Planetary Motion (1609)

Law stating that all planets orbit the Sun in elliptical orbits, with the Sun situated at one focal point and nothing at the other.

Perihelion

The point in an elliptical planetary orbit where the planet is closest to the Sun.

Aphelion

The point in an elliptical planetary orbit where the planet is furthest from the Sun.

Orbital Eccentricity ($\epsilon$)

The ratio calculated as the distance between focal points divided by the length of the major axis; for Earth, $\epsilon = 0.0167$.

Kepler's Second Law (Law of Equal Areas, 1610)

Law stating that a line connecting a planet to the Sun sweeps out equal areas of its orbit in equal intervals of time.

Kepler's Third Law (1618)

Law establishing that the square of a planet's period of revolution is proportional to the cube of its average distance from the Sun: $(\text{period of revolution})^2 \propto (\text{average distance from sun})^3$.

Galileo's Telescopic Observations

Empirical discoveries showing that the Milky Way consists of stars, the Sun has blemishes, the Moon has a rough surface, Jupiter has orbiting bodies, Saturn has an odd shape, and Venus changes size and exhibits phases.

Newton's First Law of Motion

Physical law stating that an object in motion stays in motion unless an external force is applied to it.

Newton's Second Law of Motion

Physical law stating that the sum of all forces equals mass multiplied by acceleration ($\sum F = ma$).

Newton's Third Law of Motion

Physical law stating that for every action, there is an equal and opposite reaction.

Newton's Law of Universal Gravitation

The gravitational force equation between two bodies: $F = G \frac{M_1 M_2}{d^2}$.