ASTR 0088 – Stonehenge to Hubble: Key Flashcards (Aristarchus to Ptolemy)

A collection of flashcards summarizing key concepts and figures from the lecture on ancient astronomy, covering significant contributions from Aristarchus to Ptolemy.

Aristarchus of Samos

A Greek astronomer who proposed an early heliocentric model placing the Sun at the center.

Heliocentric model

The model suggesting Earth moves around the Sun instead of the Sun orbiting Earth.

Parallax

The apparent shift of a nearby object against a distant background when viewed from different angles.

Stellar parallax

Nearby stars would appear to shift relative to distant stars as Earth moves.

Detection of stellar parallax

Not seen in ancient times because stars are extremely far away, making the shift too small.

First and third quarter moon

The moon phases Aristarchus used to estimate distances.

Geometric relationship used by Aristarchus

A right triangle formed by the Earth, Moon, and Sun during the quarter moon.

Sun vs Moon size conclusion

Aristarchus concluded the Sun is larger than the Moon based on their apparent size and distance.

Earth's shadow on Moon

Aristarchus estimated the size of the Moon by observing Earth's shadow during a lunar eclipse.

Accuracy of Aristarchus’ measurements

The Earth–Moon size ratio was accurate within about 5%.

Eratosthenes

A Greek scholar who calculated the circumference of Earth using geometric methods.

Measurement of Earth’s circumference

Eratosthenes measured it by comparing the Sun’s angle at two cities at different latitudes.

Geography

The term coined by Eratosthenes.

Hipparchus

A Greek astronomer known for extremely precise observations of stars and the precession of Earth’s rotational axis.

Magnitude system

The system for classifying star brightness introduced by Hipparchus.

Retrograde motion

When a planet appears to move backward relative to the background stars.

Real cause of retrograde motion

It occurs when Earth passes another planet in its orbit.

Claudius Ptolemy

An astronomer who refined the geocentric model of the universe.

Ptolemy's explanation for retrograde motion

He used epicycles (small circles) moving along deferents (larger circles) to explain it.