Early (BC):


Little recorded information on early man's impression of the heavens, some drawings of eclipses, comets, supernovae such as the Pueblo Petrograph. Early man was frightened/overwhelmed by the sky ~30,000 B.C.

Early man also believed that the heavens held power over earthy existence (psychology of the unknown) => origins of astrology as an attempt to understand, predict and influence events.

The Carnac region, in France, contains many, many Menhir or upright stones (Carnac Stones). Some think these standing stones were signposts, others think they marked burial plots, still others think they were astronomical in nature. Research is going on now to try to find out if these stone markings served as sites for astronomy or whether they were aligned with the Sun or Moon in a special way ~4,500 B.C.

Stonehenge lies in Wiltshire, England. Stonehenge is so mysterious that it has alleged connection with the Druids, King Arthur's court, and even visitors from another world. These connections aren't quite true! The stones of Stonehenge date between 3,000 B.C and 2,000 B.C. So, Stonehenge is probably the marking of the Neolithic people. In southern England, the Neolithic period starts with early farming communities around 4,000 BC and ends with the start of the Bronze age around 2,000 BC. The original purpose of those is unclear. Some people think say it was bild as an astronomy observatory of sorts.

Babylonians kept records on clay tablets using a type of writing called cuneiform. At first this was generally for business purposes, in order to keep track of financial transactions and inventories. Several cuneiform tablets have been found, however, that focus on more scientific topics. One notable example is the Venus Tablet of King Ammizaduga, pictured below, which demonstrates the scientific methodology used by the Babylonian astronomers. The main topic of this tablet is the appearance and disappearance of the planet Venus as it goes from being an evening star to a morning star. Based on the detailed astronomical patterns mentioned in this tablet, modern scientists were able to use computers to determine that the Venus tablet was probably written in the year 1581 B.C. Another astronomical cuneiform tablet was found in the tomb of King Ashurbanipal of Ninevah, and also details the times of Venus' appearance and disappearance from the horizon.

Thus, Astronomy was the 1st science.


The first ancient culture that usually comes to mind as being more aware of the truth of their surroundings than other cultures of that time period are the Greeks witch inherited astronomical records from the Babylonians. In fact, our word astronomy comes from the Greek words meaning "law and order". The work of the Greek philosophers was widely distributed by the Romans and was the accepted authority on that subject for hundreds of years.

In the Ancient Greece (~500 B.C.) applied the data inherited from the Babylonians to construct a cosmological framework. Data was not just used for practical goals, such as navigation, but also to think of new experiments = natural philosopher.

Thales (~480 B.C.) used this data to predict eclipses.

Heraclides (330 B.C.) developed the first Solar System model, beginning of the geocentric versus heliocentric debate.

Note that orbits are perfect circles (for philosophical reasons = all things in the Heavens are "perfect").

Aristarchus (270 B.C.) developed the heliocentric theory.

Problems for heliocentric theory

Eratosthenes (220 B.C.) - The early Greeks knew the Earth was a sphere based on the shadow of Earth on the Moon during lunar eclipses. Eratosthenes proceeded to use this information to measure circumference of Earth in the following manner; he knew that on a certain date that a stick placed in the ground at Syene cast no shadow. Whereas, a stick at Alexandria has a small shadow. Using simple ratios he showed the following:

Hipparchus (135 B.C.) produced first star catalog and recorded the names of constellations.

Antique (AD):

Ptolemy (148 A.D.) was the Librarian of Alexandria who resurrected Heraclides geocentric theory and combined with centuries of data on planetary motions -> formulated complete description of the Solar System that explained/predicted the apparent motions. The Ptolemic system began the 1st paradigm or framework for our understanding of Nature.

Unfortunately, the Ptolemy framework was extremely complicated in order to explain retrograde motion.

The solution to retrograde motion was to use a system of circles on circles to explain the orbits of the planets called epicycles and deferents. The main orbit is the deferent, the smaller orbit is the epicycle. Although only one epicycle is shown in the figure below, over 28 were required to explain the actual orbits of the planets.

In the Ptolemaic system, deferents were large circles centered on the Earth, and epicycles were small circles whose centers moved around the circumferences of the deferents. The Sun, Moon, and planets moved around the circumference of their own epicycles. In the movable eccentric, there was one circle; this was centered on a point displaced from the Earth, with the planet moving around the circumference. These were mathematically equivalent schemes.

Although Ptolemy realized that the planets were much closer to the Earth than the "fixed" stars, he seems to have believed in the physical existence of crystalline spheres, to which the heavenly bodies were said to be attached. Outside the sphere of the fixed stars, Ptolemy proposed other spheres, ending with the primum mobile ("prime mover"), which provided the motive power for the remaining spheres that constituted his conception of the universe. His resulting solar system model looked like the following, although the planets had as many as 28 epicycles to account for all the details of their motion.

This model, while complicated, was a complete description of the Solar System that explained, and predicted, the apparent motions of all the planets. The Ptolemic system began the 1st mathematical paradigm or framework for our understanding of Nature.

Alexandria burns, Roman culture collapses, Dark Ages... but the Roman Catholic Church absorbs Aristotle's scientific methods and Ptolemy's model into its own doctrine. Thus, preserving the scientific method and Ptolemy's Solar System until the...


When new ideas became more important than dogma.

Copernicus (1500's) reinvented the heliocentric theory and challenged Church doctrine. The heliocentric model had greater depth than simply an improvement to solve retrograde motion, it also had social and political consequences. For example, the Bible states (Psalm 93, addressing God) "Thou hast fixed the earth immovable and firm", which implies a geocentric model. Also, Joshua commanded the Sun to stand still, not the Earth. Heliocentric theory questioned the authority of the most revered wise men of the ancient world and the outcry over his ideas had little to due with the impersonal concern for truth. By placing the Sun at the center of the Solar System, Copernicus forced a change in our worldview = paradigm shift or science revolution.

However, Copernicus also used circular orbits and had to resort to epicycles and deferents to explain retrograde motions. In fact, Copernicus was forced to use more epicycles than Ptolemy, i.e. a more complicated system of circles on circles. Thus, Copernicus' model would have failed our modern criteria that a scientific model be as simple as possible (Occam's Razor).

Tycho Brahe (1580's) was astronomy's 1st true observer. He built the Danish Observatory (using sextant's since telescopes had not been invented yet) from which he measured positions of planets and stars to the highest degree of accuracy for that time period (1st modern database). He showed that the Sun was much farther than the Moon from the Earth, using simple trigonometry of the angle between the Moon and the Sun at 1st Quarter.

Tycho's measurements were used to show that there was no detectable parallax with the naked eye, in support of the geocentric theory. So, even though his observations were the best for his time, his result was wrong, a lesson in how science is done.

Kepler (1600's) a student of Tycho who used Brahe's database to formulate the Laws of Planetary Motion which corrects the problems of epicycles in the heliocentric theory by using ellipses instead of circles for orbits of the planets.

The formulation of a highly accurate system of determining the motions of all the planets marks the beginning of the clockwork Universe concept, and another paradigm shift in our philosophy of science.

Galileo (1620's) developed laws of motion (natural versus forced motion, rest versus uniform motion). Then, with a small refracting telescope (3-inches), destroyed the the idea of a "perfect", geocentric Universe with the following 5 discoveries:

1 - spots on the Sun

2 - mountains and "seas" (maria) on the Moon

3 - Milky Way is made of lots of stars

4 - Venus has phases

5 - Jupiter has moons (Galilean moons: Io, Europa, Callisto, Ganymede)

Newton (1680's) developed the law of Universal Gravitation, laws of accelerated motion, invented calculus (math tool), the 1st reflecting telescope and theory of light.


In the 17th and 18th century, better instruments allowed the compilatin of more acurate and larger catalogs. A milestone was the Bonner Durchmusterung, created 1852-59 under Friedrich Wilhelm Argelander (1799-1875), which contains positions and magnitudes for 320,000 stars. This catalog was extended southward by the Cordoba Durchmusterung, compiled 1885-1892. Other important catalogs compiled visually include the Harvard Revised Photometry and the Potsdammer Durchmusterung, both published 1907.

The pioneering work of photographic photometry was Karl Schwarzschild's (1873-1916) Göttinger Aktinometrie, compiled 1904-1908. When spectroscopy came up, a first classification of 316 stars was published by the Italian Father Angelo Secchi (1818-1878) in 1867. A more comprehensive compilation of spectral classification was the Henry Draper Catalogue published 1918-24 at Harvard Observatory and containing data of 225,300 stars.

Catalogs of celestial objects:

Important early star catalogs:

For historic catalogs of deepsky objects, refer to the history of the discovery of the Deepsky objects. Notably, the first more comprehensive and reliable compilation was Charles Messier's (1730-1817) Catalogue of Nebulae and Star Clusters from 1781 containing 103 entries. William Herschel cataloged another 2500 objects, and his early 1782 catalog contained already 269 double stars, while John Herschel's General Catalog (GC) of nonstellar objects of 1864 contained over 5,000 objects, while J.L.E. Dreyer's New General Catalogue (NGC), together with its two supplements (Index Catalog I and II, IC) summarize over 13,000 deep sky objects.

When spectroscopy came up, a first classification of 316 stars was published by the Italian Father Angelo Secchi (1818-1878) in 1867. A more comprehensive compilation of spectral classification was the Henry Draper Catalogue published 1918-24 at Harvard Observatory and containing data of 225,300 stars.

New Astronomy:

In the second half of the 19th century, the following developments lead to significant changes in astronomy, and especially the upcoming of astrophysics (then often called "New Astronomy"):

Milky Way, Nebulae, and Stellar Systems:

Stellar Evolution:

Early models for stellar structure and evolution, though interesting in some cases, could not explain, e.g., why the Sun could emit an almost constant rate of energy for billions of years (although the Helmholtz-Kelvin model of a contracting star which radiates gravitational energy is interesting in the context of stellar formation and protostars). However, the true nature of stellar structure and evolution could be revealed only after the physical foundations had been established in the early 20th century.

An introduction to the current state of knowledge of stellar evolution is available.

Observations in the Invisible Light and Space Astronomy:

Space Exploration:

For a more accurate Astronomy timeline, refer to This One.

Solar System Live