Early Astronomy & Retrograde motion

Egyptian Calendar

A calendar based on a $365\text{-day}$ year, adopted around $3000$ years ago.

Predawn Rising of Sirius

Tracked by Egyptians to predict the annual flooding of the Nile River.

Chinese Astronomy Contributions

Recorded comets, meteors, sunspots, and 'guest stars' during the same era as Egyptians.

Mayan Calendar System

A complex calendar developed based on the orbital motion of Venus.

Celestial Navigation

Technique used by Polynesians to travel hundreds of kilometers across open ocean.

Stonehenge

Built in Prehistoric Britain around $2800\text{ BCE}$ to align with solar and lunar motions.

Cosmology

The study of the basic structure, origin, and evolution of the universe.

Spherical Earth Argument by Pythagoras

Suggested Earth was spherical based on the belief that spheres represent perfect geometric forms.

Aristotle's Evidence for Spherical Earth

Observed that Earth's shadow on the Moon during lunar eclipses is round.

Stellar Visibility Argument

Noted that moving south reveals stars not visible further north, indicating a curved surface.

Ships Disappearing Hull-First

Observation that ships sailing into the distance disappear hull-first over the horizon.

Stellar Parallax

The apparent shift in direction of an object caused by the change in position of the observer.

Heliocentric Model

Proposed by Aristarchus of Samos, suggesting that Earth rotates and orbits the Sun.

Eratosthenes

Successfully measured Earth's circumference around $200\text{ BCE}$ using geometry.

Calculation of Earth's Circumference by Eratosthenes

Calculated to be $250{,}000$ stadia, close to the actual value of $40{,}000$ km.

Hipparchus's Star Catalog

Created an accurate star catalog containing approximately $850$ stars.

Apparent Magnitude System

Developed by Hipparchus to classify stars by visual brightness.

Precession

The gradual shift in the orientation of Earth's rotational axis, completing a cycle every $26{,}000$ years.

Ptolemy's Almagest

Compiled Greek astronomical knowledge into a comprehensive work.

Retrograde Motion

The apparent westward motion of a planet caused by faster-moving Earth overtaking an outer planet.

Geocentric Model

Earth-centered view of the universe preserved by Ptolemy, requiring circular motions.

Epicycle and Deferent

Epicycle is a small circle for planet orbiting, and deferent is the larger circle that the epicycle travels around.

Equant Point

An offset center point used in Ptolemy's model to explain non-uniform motion while maintaining circularity.