Ancient to Modern Astronomy

The Greeks and the Spherical Earth

Ancient Greeks were among the first to suggest that Earth is a sphere based on careful observation and reasoning.

Ship’s Hull Disappearing First

As ships sailed away, the hull vanished before the sails, showing that Earth’s curved surface explains the gradual disappearance.

Phases of the Moon and Lunar Eclipses

Earth’s shadow on the Moon during a lunar eclipse is always round, proving that only a spherical object can consistently cast a round shadow.

Eratosthenes’ Measurement of Earth’s Size (c. 240 BCE)

Eratosthenes, a Greek scholar and librarian in Alexandria, made the first known accurate calculation of Earth’s circumference using the Sun’s angles in Syene and Alexandria and basic geometry.

Eratosthenes

A Greek scholar who calculated Earth’s circumference as approximately 40,000 km, close to the modern value.

Ancient Astronomical Phenomena

Ancient astronomers observed the sky and recorded patterns in stars and planets, helping shape understanding of the cosmos.

Planetary Movement

The word “planet” comes from the Greek word planētēs, meaning “wanderer.” Planets move across the night sky unlike stars and sometimes appear to move backward, called retrograde motion.

Retrograde Motion

The apparent backward movement of planets in the sky.

Ptolemy (c. 150 AD)

An astronomer who proposed the geocentric model, where Earth is at the center of the universe.

Geocentric Model

A model where Earth is considered the center of the universe.

Epicycles

Small circular orbits placed on larger orbits in Ptolemy’s model to explain Mars’ retrograde motion.

Nicolaus Copernicus (1543)

Proposed the heliocentric model, where the Sun is at the center and Earth and other planets orbit it.

Heliocentric Theory

A model stating that the Sun is at the center and planets orbit around it.

Copernican Model

A heliocentric model that naturally explained retrograde motion as Earth overtaking other planets in orbit.

Solar Eclipse

Occurs when the Moon blocks the Sun’s light and casts a shadow on Earth.

Lunar Eclipse

Happens when Earth casts a shadow on the Moon during a full moon.

Polaris (North Star)

A star located nearly above Earth’s North Pole that remains almost fixed in the sky and helps navigators determine true north.

Tycho Brahe (1546–1601)

A Danish astronomer known for accurate naked-eye observations of stars and planets. His data, especially on Mars, helped Kepler develop the laws of planetary motion.

Johannes Kepler (1571–1630)

A German mathematician who used Tycho Brahe’s data to develop three laws of planetary motion supporting the heliocentric model.

Kepler’s Three Laws of Planetary Motion

Laws that explain how planets move around the Sun.

Law of Ellipses

Planets orbit the Sun in an ellipse, with the Sun at one focus, correcting the belief that planetary orbits are perfect circles.

Law of Equal Areas

A line from a planet to the Sun sweeps out equal areas in equal times; planets move faster when closer to the Sun and slower when farther away.

Law of Harmonies

The square of a planet’s orbital period is proportional to the cube of its average distance from the Sun, explaining why farther planets take longer to orbit.