A Brief History of Astronomy

Asian and Greek Astronomy

(4000 BC to 100 AD)


Flat Earth
Ancient views of a flat Earth

4000 BC

Early peoples think that the world is flat with a crystalline sky overhead. The Sun is thought to be a god that rode across the sky in a chariot, travelling beneath the Earth at night.

The world is thought to have been created in a small amount of time by a deity or deities.

Astronomical Records
Astronomical records
from Mesopotamia

2000 BC

Mesopotamian priests begin keeping systematic astronomical records. Observations of the stars and planets made in India and China.

Chinese Star Map
Chinese star map

1500 BC

The Sumarians, Babylonians, Indians, Chinese and Egyptians develop astronomy. The stars appear to form patterns in the sky that are visible every year. These patterns are considered fixed and are called constellations. The Chinese divide the sky into 28 constellations; the Indians into 27.

The length of the day, the month and the year is known. The five naked eye planets are known.

600 BC

In Greece, Anaximander notices that the stars appear to rotate around a pole. He suggests that the sky is a complete sphere around the Earth. He thinks that the Earth's surface must be curved after hearing that travellers saw new stars appearing when moving north or south. He pictures the Earth as a cylinder.

The planets are known to move against the background of the stars which appear fixed to a crystal sphere. The word planet means "wanderer".

Pole Star
Stars rotating around the pole

Pythagoras
Pythagoras

500 BC

Pythagoras and his followers teach that the Earth is a sphere. The idea came about from observations of Lunar Eclipses - the Earth's shadow on the Moon is always circular.

The Pythagoreans think that the motions of the planets are mathematically related to musical sounds and number. These ideas are called "The Music of the Spheres".

Rotating Globe
Rotating Earth

350 BC

Heracleides suggests that the daily motion of the Sun, Moon, planets and stars around the Earth could be explained if the Earth rotated on its axis once every day.

Aristotle
Aristotle

330 BC

Aristotle writes a series of books which contain ideas that will influence humanity for 1800 years.

He talks about the four elements (earth, fire air and water) which he says are only found on Earth. These elements each have their own tendencies: earth is heavy and falls, fire is light and rises. Motion is in straight lines. The heavier the object, the faster it falls.

A fifth element, the Aether, is only present in the objects of the sky. Its natural motion is circular so celestial objects travel around the Earth in perfect circles. Aristotle assumes that light travels infinitely fast.

The Earth and the heavens are, therefore, subject to different natural laws. Things on Earth are corrupted and subject to change while the heavens are incorruptible and unchanging.

Eratosthenes
Eratosthenes

250 BC

Eratosthenes measures of the size of the Earth from observations of the Sun in different parts of the Earth. On the longest day of the year, the Sun is overhead in southern Egypt but 7° from the vertical in northern Egypt. Eratosthenes takes the distance between these two points and multiplies it by the ratio between a full circle (360°) and the 7°.

His measurement is within 1% of the correct value.

Eratosthenes' measurement of the size of the Earth
Measuring the size of the Earth from the altitude of the Sun at two locations

Aristarchus
Aristarchus

Aristarchus accurately measures of the distance to the Moon using trigonometry applied to Lunar eclipses. He correctly shows that the moon is 25% as large as the Earth.

Distance to the Moon
Distance to the Moon

He makes the first attempt to find the distance to the Sun. His theory is good but the measurements are difficult and his figure (19 times further than the Moon - 5% of the correct value) is too low. Even so, the Sun is shown to be larger than the Earth.

Distance to the Sun
Distance to the Sun

Aristarchus even suggests that the Earth goes around the larger Sun. This idea does not take root because of lack of evidence and will not become accepted for 1800 years.

Hipparchus
Hipparchus

140 BC

Hipparchus refines the distance between the Earth and Moon using Trigonometric Functions which he had invented.

He thinks that he had observed positional changes amongst the so called "fixed stars" but he is unsure. He creates a very accurate map of the 1000 or so brightest stars. This map will play an important role in astronomical history 1800 years later.

During his research he discovers that there are two types of year. The Tropical Year and the Sidereal Year differ by 20 minutes. This causes the position of the Celestial Pole to move in a circle taking 26,000 years to complete one cycle. This phenomenon is called the Precession of the Equinoxes.

130 BC

Seleucus thinks that the Moon is somehow responsible for the tides. This idea would not be proved for nearly 1800 years until the time of Newton.

The astrological symbol for Scorpio
The astrological
symbol for
Scorpio

115 BC

Poseidonius recalculates the Earth's circumference as 70% of the correct value. This figure would become accepted until modern times. 1500 years later, Christopher Columbus would use this figure when searching for finance for his expedition across the Atlantic.

Poseidonius also measures the distance between the Earth and the Sun to an accuracy of 43%.

He also popularises astrology.


Ptolemy

100 AD

Ptolemy writes a book (known by its Arabic name, The Almagest - The Greatest) which summarises the astronomical knowledge of the ancients, especially that of Aristotle.

The cosmology is based on Earth being the centre of the Universe with the Sun, Moon, planets, and stars (all set on crystal spheres) revolving around the Earth in a series of circles called Epicycles. The planets, Mercury and Venus always lie close to the line joining the Earth and the Sun.

Epicycle
An Epicycle
The system is cumbersome but could be used to predict the motions of the planets to naked eye accuracy. Tables are created that predict the positions of the planets in the future. He republishes the star map of Hipparchus and names the (48) classical constellations with the names they are still known by in the West.

Ptolemy writes that the sphere of the stars is 200 times further away than the Moon.

The book also contains a summary of geographical knowledge with estimates of latitudes and longitudes for places in Europe. These would not be improved for 800 years.

The book is one of the few to survive the chaos of the European Dark Ages. After the fall of the Roman Empire, the book would be translated into Arabic in the Islamic world, and, later, into Latin and will play a part in Europe's Renaissance

Constellations
The (Western) Constellations


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KryssTal Related Pages

An easy-to-understand scaling of the Universe in space. Distances in space are represented by the time light takes to travel there.

An easy-to-understand scaling of the Universe in time. The chronology of the Universe is compared to a real year.

A listing of the 20 brightest stars as well as explanations of the terms used.

Information about the planets and satellites of the Solar System with explanations of the terms used.

A historical account of the discovery of the electromagnetic spectrum and its uses in Astronomy. Radio waves, infra-red, visible light, ultra violet, X-rays and gamma rays are explained.

An account of how various properties of stars can be measured by studying starlight. Includes brightness, distance, luminosity, temperature, mass, radius, density and an introduction to the H-R Diagram.

An account of how stars evolve and change the chemistry of the Universe.

The force that moves apples and planets. A short introduction to the ideas of Kepler and Newton that culminated with the theory of Universal Gravitation.

This looks at the history of inventions and the various civilisations of the world.

Selected biographies of people from around the world including scientists and astronomers.


External Links

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History of Science A large collection of resources looking at the history of astronomy, physics, chemistry and mathematics.