Ancient Astronomy and Planetary Motion

Vocabulary flashcards covering the historical development of the spherical Earth concept, ancient astronomical observations, and Kepler's laws of planetary motion.

Pythagoras

The first philosopher to propose the Earth's spherical shape with his pupils; he believed the Universe is mathematical and that the Sun, Moon, and Earth are spherical.

Anaxagoras

Supported the spherical Earth proposal by observing that the circular shadow the Earth cast on the Moon during a lunar eclipse was reflected on the Moon's surface.

Plato

Used mathematical symmetries to demonstrate perfect shapes like the sphere and circle, and believed celestial spheres were crystalline.

Eudoxus

Modified previous models of the universe by adding auxiliary spheres to provide an appropriate tilt to the paths of the planets.

Aristotle

Proved the Earth is a sphere by noting its shadow on the moon during lunar eclipses is always an arc of a round circle rather than an ellipse.

Eratosthenes

An ancient scholar who calculated the circumference of the Earth to be approximately $250,000$ stadia ($40,000$ kilometers) by comparing the Sun's rays in Alexandria and Syene.

Stadia

A unit of measurement used in ancient times to describe the size of a typical stadium, used by Eratosthenes to calculate the Earth's size.

North Star (Polaris)

A star believed to be at a fixed position; the Greeks noticed it moved closer to the horizon as they traveled nearer the equator, providing proof of a curved Earth.

Gnomon

A primitive version of a sundial used by Babylonian and Egyptian civilizations to systematically observe the motion of the sun.

Diurnal Motion

The daily motion of the sun, moon, stars, and planets as they appear to move across the sky.

Annual Motion

The apparent yearly movement of stars as observed from Earth, which is a direct effect of the Earth's revolution around the Sun.

Precession of the Equinoxes

The observable phenomena of the rotation of the heavens which spans a period of $25,920$ years.

Equinox

Occurs when the Sun is directly above the equator, resulting in an equal length of time for day and night.

Solstice

Refers to the longer length of time of day and night; includes the Summer solstice (longest day) and Winter solstice (shortest day).

Lunar Phase Cycle

The period of $29.5$ days during which the moon changes its path and appearance from a thin semi-circular disk to a full circular disk.

Solar Eclipse

Occurs when the moon is positioned between the Sun and the Earth, partially or completely blocking the Sun's light.

Lunar Eclipse

Occurs when the Earth is between the Sun and the Moon, and the Moon moves into the Earth's shadow.

Umbra

The inner, darkest part of the shadow formed during an eclipse where the light source is completely blocked.

Penumbra

The outer portion of the shadow where the light source is only partially blocked.

Planetes

The Greek word meaning "wanderer," which is the origin of the word "planet."

Hans Lippershey

A Dutch eyeglass maker widely credited as the first person to patent a telescope in $1608$.

Kijker

The name of the device invented by Hans Lippershey, meaning "looker," which could magnify images up to $3$ times.

Galileo Galilei

In $1609$, he built his own telescope and made significant improvements, reaching magnifications of around $20$ times.

Tycho Brahe

A Danish astronomer and the last major naked-eye astronomer; his accurate observations of Mars provided the data needed for Kepler’s laws.

Johannes Kepler

The assistant of Tycho Brahe who analyzed planetary positions to explain the backward movement of Mars and formulated the three laws of planetary motion.

Law of Ellipses

Kepler's First Law, stating that the orbit of each planet around the Sun is an ellipse with the Sun at one of the foci.

Law of Equal Areas

Kepler's Second Law, stating that an imaginary line from the Sun to a planet sweeps out equal areas in equal intervals of time.

Law of Harmonies

Kepler's Third Law, stating the ratio of the squares of the periods of any two planets is equal to the ratio of the cubes of their average distances from the sun.