astronomy

Pythagoras - was the first one to originate a spherical earth using observations such as: North and South Constellations; Sinking Ships; and Lunar Eclipse.

Plato - The earth is a form of a globe having its extremes in every direction equidistant from the center. The planet is:

• At the center of universe

• Perfect and eternal

• Has four elements

Aristotle- His work on 350 BC entitled, “On the Heavens” initiated the idea of a geocentric universe with fixed spherical earth at its center.

Three Arguments:

1. Rounded shadow which the earth projects on the moon

2. Change in the constellations as travelers voyage to southern and northern regions

3. Indirect idea on gravity

Eratosthenes - Ancient scholars tried to provide proof of a spherical Earth and its circumference through calculations. It was Eratosthenes who gave the most accurate size (through non-scientific calculations) during their time.

Geocentric (earth centered) Theorists: Plato, Aristotle, Ptolemy, Tycho Brahe

Heliocentric (sun centered) Theorists : Aristarchus of Samos, Nicolaus Copernicus, Galileo Galeli, Johannes Kepler

Ancient Astronomy

Plato (428-348 BCE)

He believed that the universe is perfect or ethereal and unchanging.

He further described the stars embedded in the outer space as eternal and divine.

“What uniform and ordered circular motion must be assumed for each of the planets to account for their apparently irregular annual paths?”

Eudoxus of Cnidus (390-337 BCE)

According to him, there are 27 interconnected geocentric spheres.

Aristotle (384-322 BCE)

In conjunction to Eudoxus’ model, he added 3 spheres to Jupiter and Mars; 4 spheres to Sun, Moon, Venus and Mercury.

Claudius Ptolemy (90-168 CE)

1. Earth is spherical object.

2. The stars are fixed bodies attached to a solid spherical exterior part of the universe.

3. Planets moved independently of the fixed stars and that sometimes, the planet seem to reverse their motion.

Aristarchus of Samos (310-230 BCE)

• He stated that sun and stars are fixed.

• Estimated the sizes of the Sun and Moon as compared to Earth's size (Moon's diameter was 0.32-0.40x the diameter of Earth and the Sun's diameter was 6.3-7.2x the diameter of Earth).

• Estimated the distances from the Earth to the Sun and Moon (sun is about 18-20x farther away from Earth than moon).

Modern Astronomy

2 major distinct characteristics that differentiate it from ancient are:

1. The observations became more quantitative in nature.

2. The theories formulated are based on logical and mathematical concepts

Nicolaus Copernicus (1473-1543)

He corrected the Geocentric theory and proposed the Heliocentric Theory with the following postulates:

1. Earth is considered as only one of the planets .

2. Earth rotates on its axis daily and revolves around the sun once a year.

3. Earth experiences an annual tilting of its axis.

4. The retrograde motion of the planets is explained by earth’s motion.

5. The distance from earth to sun is small compared to earth’s distance from the stars.

6. Tilting of the earth and four seasons

Tycho Brahe (1546-1601)

The following are his important contributions to modern astronomy:

1. He modified the geocentric model of the universe.

2. He designed and built huge instruments used to make precise measurements of the positions of the planets.

3. He discovered supernova in the constellation Cassiopeia in 1572.

4. He made observations of planetary motions that are important to the development of Kepler’s Laws and other models of solar system.

5. Tychonic System

Galileo Galilei (1564-1642)

Using his telescope, the following are what he observed:

1. He discovered sunspots and rough surface of the moon.

2. He discovered the 4 famous moons of Jupiter known as Galilean moons (Io, Europa, Ganymede, Callisto).

3. He discovered that Venus has phases like moon.

Johannes Kepler (1571-1630)

First Law

It states that the orbits of the planets are ellipses, with the sun at one focus.

Second Law

It states that an imaginary line drawn from the sun to a planet sweeps out equal areas in equal intervals

PERIHELION: Nearest to the sun

APHELION: farthest from the sun

Third Law

The cube of the radius of the orbit of a planet around the sun is proportional to the square of its period of revolution