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
- Earth in motion??? can't feel it
- no parallax seen in stars
- geocentric = ego-centric = more "natural"
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.
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:
- 1603 - Johann Bayer (1572-1625), Uranometria; note this appeared still before the introduction of the telescope in astronomy in 1609! In this work, Bayer introduced the naming of stars for their constellation and by Greek letters in the approximate order of (apparent) brightness, e.g. Alpha Centauri.
- 1661 - Johann Hevelius (1611-1687), Sternverzeichnis
- 1679 - Edmond Halley (1656-1742) compiled the first southern star catalog
- 1712 - John Flamsteed (1646-1719), Historia Coelestis Britannica, edited by Edmond Halley. In this catalog, the labelling of stars by the so-called Flamsteed numbers (e.g., 21 Tauri) was introduced.
- 1725 - John Flamsteed, Stellarum Inerrantium Catalogus Britannicus. Flamsteed's own corrected and official version, without the numbers.
- 1762 - James Bradley (1693-1762), Star Catalog.
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.
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"):
- Stellar Photometry
- Until early 19th century, stellar magnitudes were estimated approximately and inacurately by visual observers, to at most 0.2 magnitudes acuracy. In 1861, Karl Friedrich Zöllner (1834-82) in Berlin introduced the visual photometer. Further improvement was achieved by introducing photoelectric cells for photometry, and by measuring photographic plates.
- Already in 1666, Isaac Newton had shown that sunlight can be decomposed into a spectrum when passing a prism. In 1802, British chemist and physicist William Hyde Wollaston (1766-1828) found dark lines in the Solar spectrum.
- In 1818, Joseph Fraunhofer (1787-1826) was the first to take a good spectrum of the Sun and discovered 576 dark lines in it; he labelled the more prominent lines with letters A to K. He later discovered that the light from Moon and planets show the same spectral features as the solar spectrum, that the spectra of star differ from this spectrum, and developed the diffraction grating, one of his had 3,625 lines per centimeter.
- In 1832, David Brewster showed that cold gasses produce dark absorption lines in continuous spectra. In 1847, John W. Draper found that hot solids emit light in continuous spectra while hot gasses produce line spectra. In 1859, Gustav Robert Kirchhoff (1824-87) and Robert Bunsen (1811-99) discovered that each chemical element (and compound) shows a characteristic spectrum of lines, which are at the same wavelengths in emission and absorption spectra. Thus, the chemical composition of a light source (including celestial bodies) can be determined from spectral analysis; Kirchoff published a study of the chemical constitution of the Sun im 1859.
- Anders Jonas Angstrom (1818-74) published his map of the Solar spectrum with identifiication of lines corresponding chemical elements in 1863.
- In 1864, British amateur William Huggins (1824-1910) published his investigations of spectra of stars and nebulae (thereby finding the gaseous nature of diffuse and planetary nebulae). The same year, Giovanni Batista Donati showed that comet spectra contain emission lines. The first spectrogram (photo of a spectrum) of a star, Vega (Alpha Lyrae), was obtained in 1872 by American amateur Henry Draper (1837-82).
- Christian Doppler (1803-53) had discovered that moving bodies show shifted spectral lines, so that radial velocities can be determined spectroscopically with high acuracy. William Huggins stated in 1868 that because of this effect, spectral lines of moving celestial objects should appear shifted. The first measurements of this effect were obtained in 1888 by Hermann Carl Vogel (1841-1907).
- Of the early spectral classifications schemes, that of Edward Charles Pickering (1846-1919) and Annie Cannon (1863-1941), used in their Henry Draper Catalogue, was finally adopted by the IAU.
- Astronomical Photography
- The first photo of the Moon was obtained in 1841 by J.W. Draper on Daguerre plates. The first solar eclipse photos were obtained on July 18, 1851. W.D. Bond obtained the first photos of stars in 1857. The major breakthrough was finally the invention of dry photographic plates by R.L. Maddox in 1871 which made durable photos possible.
- The power of photography for every branch of astronomy was quickly demonstrated; early pioneering work was done by Isaac Roberts, Edward Emerson Barnard (1857-1923) and Max Wolf (1863-1932) especially for the Milky Way, star clusters, and nebulae.
- Better large telescopes
- During the 18th and early 19th century, small refractors and larger metal mirror reflectors (up to Lord Rosse's 72-inch Leviathan of 1845) were the telescopes available for observers.
- Telescope optics was notably improved by Fraunhofer when he developed the achromatic objective in 1824, which led to the construction of larger refractors up to the Yerkes 102 cm.
- The reflector techniques was significantly improved by the invention of glass mirrors by Steinheil in 1857 who built a 10 cm reflector, succeeded by Faucault's 33-cm and Lassell's 60-cm glass mirrors. Almost all big telescopes of the 20th century are reflectors with glass mirrors. The first telescope exceeding Lord Rosse's Leviathan of 1845 in aperture was the 100-inch Mount Wilson telescope constructed 1917, followed by the Palomar 200-inch in 1948, and the limitedly successful 6.1-meter Selenchukskaya telescope in 1976.
Milky Way, Nebulae, and Stellar Systems:
- After Galileo's discovery that Milky Way and some nebulous patches were composed of stars, it was generally assumed that all "nebulae" were actually distant star clusters and should be resolved in sufficiently powerful telescopes; a notable and exotic exception was Edmond Halley's view of "nebulae" as lucid "holes in space". William Herschel first found this view confirmed by his observations, as with his large telescopes, he could resolve also dense globular clusters, but later realized that some "nebulae", notably the Orion Nebula M42 and planetary nebulae, were probably made up of a "shining fluid of a nature totally unknown to us". William's son John Herschel, on the other hand, seriously doubted the existence of nebulae which were not made up of stars in the late 1840s.
- In 1845, William Parsons, third Earl of Rosse (1800-67) discovered the spiral pattern of M51, and later of M99 and 13 other "nebulae" which were since known as "spiral nebulae".
- The essential event marking the discovery of gaseous nebula came when William Huggins observed their spectra in 1864 and found them to be emission line spectra. Now there was a simple and unique criterion distinguishing them from star clusters, which like the stars composing them, show a continuous spectrum (with overlaid absorption and sometimes emission lines). Spiral "nebulae", however, show continuous spectra like stars.
- It was known since Herschel that the Milky Way forms a system of stars one of which is our Sun. Since Kant and Herschel, it was speculated that there might be other similar stellar systems; some believed Rosse's spiral nebulae could be candidates. By 1900, Easton proposede a model of the Milky Way as a spiral nebula. Another fraction of astronomers, including astrophotographer Isaac Roberts who interpreted his photo of the Andromeda "nebula" M31, thought these nebulae were solar systems in formation (with the companions M32 and NGC 205 [M110] supposed as forming Jovian planets).
- Stellar statistical methods, invented by Herschel and improved by H. von Seliger and J. Kapteyn, indicated that the Solar System was, presumably by chance, situated close to the center of the Milky Way Galaxy. In 1904, interstellar reddening and absorption were found; nevertheless, it was longly believed to be a minor effect only.
- In 1912, Vesto M. Slipher of Lowell Observatory discovered the nature of the nebulae in the Pleiades star cluster M45 as reflection nebulae. In 1914, he found that the spiral and some elliptical "nebulae" are moving at very high radial velocities so that their membership in the Milky Way got questionable, and in 1915 he determined the rotational velocity of the edge-on "nebula" M104 to be about 300 km/s. The view that spiral "nebulae" might be galaxies like our Milky Way was stressed by Heber D. Curtis of Lick Observatory, also on the basis of nova observations and as absorption could explain why spirals "avoid" to be seen near the galactic plane, but opposed in particular by Adriaan van Maanen (1884-1946) who erroneously believed to have found internal proper motions in spirals which would have indicated observable rotation.
- In 1912, Henrietta Leavitt found the period-luminosity relation of Cepheid variables in the Magellanic clouds. Using this relation, Harlow Shapley, in 1918, determined distances in the Milky Way, and in particular of globular clusters, which he found centered aroung a location in Sagittarius: He concluded that the center of the Galaxy should be located there, with the solar system lying in an outer region of Milky Way; however, as he significantly underestimated the influence of interstellar absorption, he overestimated the size of the Milky Way by a factor of about 3.
- In 1924, Edwin Hubble resolved the outer part of the Andromeda "Nebula" M31 into stars and found novae and Cepheid variables, thus establishing its nature as an external star system or galaxy.
- In 1926, Bertil Lindblad and Jan Oort developed the theory of kinematics and dynamics of the Milky Way Galaxy.
- In 1929, Hubble derived his distance - redshift relation for galaxies, indicating the expansion of the universe.
- In 1930, Robert Julius Trumpler (1886-1956) of Lick observatory found from investigations of open clusters that the interstellar absorption had been signficantly underestimated, and the Milky Way Galaxy was correspondingly smaller. In 1937, interstellar molecules (CO_2) were found as absorption lines.
- In 1943, Carl Seyfert discovered that certain galaxies (now called Seyfert Galaxies) have "active" nuclei with peculiar nonthermal spectra. In 1944, Walter Baade discovered that the stellar population in different regions of galaxies varies and there are two different stellar populations: Young Population I in spiral arms and irregular galaxies, and old Population II stars in elliptical (and lenticular) galaxies, globular clusters, and the bulges and nuclei of spiral galaxies.
- In 1951, the 21-cm radio radiation of neutral hydrogen was discovered. Observations of the Milky Way in this wavelength provided first direct evidence of the spiral structure of our Galaxy.
- In 1952, Baade found that Cepheids of two classes exist: Type I Cepheids ("classical" Delta Cephei stars) which are members of population I and Type II Cepheids (W Virginis stars) which are 4 to 5 times fainter. This discovery implied that the intergalactic distance scale had to be revised, moving the galaxies to more than double distance away, and thus removing discrepancies of Milky Way size compared to external galaxies. Since, the distance scale had been subject to minor modifications on various occasions, last due to revision of the Cepheid distances found by the astrometric satellite Hipparcos in early 1997.
- In 1963, the first quasar was discovered by Maarten Schmidt.
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.
- 1906 - Karl Schwarzschild modeled the solar atmosphere from theory of thermodynamical equilibrium; R. Emden published his theory of gaseous spheres; 1906-12 temperature-luminosity relation of stars discovered (Hertzsprung-Russell diagram)
- 1923 - Enjar Hertzsprung discovered the mass-luminosity relation
- 1925-30 - Theory of stellar atmospheres (C.H. Payne), stellar structure (A.S. Eddington) and convective transport (A. Unsöld) developed
- 1934 - Hypothesis of neutron stars proposed by W. Baade and F. Zwicky
- 1938 - Nuclear reactions proposed as stellar energy sources by H. Bethe and C.F. v. Weitzsäcker
- 1951 - Carbon formation reaction ("Triple-Alpha process") investigated by E.J. Öpik and E.E. Salpeter
- 1955 and later - Computer modelling of stellar structure and evolution, introduced by M. Schwarzschild and F. Hoyle.
An introduction to the current state of knowledge of stellar evolution is available.
Observations in the Invisible Light and Space Astronomy:
- As early as 1800, William Herschel discovered the infrared radiation beyond the red end of the visible spectrum with a thermometer. One year later in 1801, J.W. Ritter discovered UV radiation by demonstrating that they produce chemical effects. However, observation of this radiation was restricted to the Sun until the 20th century.
- In 1931, K.G. Jansky discovered radio radiation from the Milky Way. In 1939, G. Reber found this radiation concentrated within the galactic plane and toward the galactic center. In 1942, J.S. Hey and J. Southward found the first extragalactic radio radiation.
- Individual radio sources were identified in the early 1950s, and the first radio galaxies in 1954.
- With upcoming space missions, astronomy became possible in those parts of the electromagnetic spectrum for which Earth's atmosphere is not transparent. In 1960, cosmic X-rays (fronm the Solar corona) were observed for the first time by an aerobee rocket. In 1965, the first cosmic X-rays were discovered (E.T. Byram, H. Friedman, T.A. Chubb); U.S. satellite Uhuru discovered 160 X-ray sources in 1970.
- In 1963, radio astronomers discovered the first quasar (M. Schmidt), and in 1967, the first pulsar (J. Bell and A. Hewish).
- Since, astronomical satellites have become a powerful tool to investigate astronomical objects in every spectral range; for more detail, look at the list of orbiting astronomical observatories (astronomy satellites).
- 1957 - Sputnik 1 (USSR) first artificial satellite
- 1961 - Venera 1 (USSR) first mission to another planet (Venus)
- 1969 - Apollo 11 (USA) first men land on the Moon
- 1976 - Viking 1 and 2 (USA) first successful unmanned landing on Mars
- 1979 - Voyager 1 and 2 (USA) fly by Jupiter
- 1980/81 - Voyager 1 and 2 fly by Saturn
- 1986 - Voyager 2 flyby of Uranus
- 1989 - Voyager 2 flyby of Neptune
- 1990 - Hubble Space Telescope (USA/ESA) launched
For a more accurate Astronomy timeline, refer to This One.
Solar System Live