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1. History of Astronomy
Historic Periods in Astronomy
Prehistoric: before 500 BC, based on naked-eye observations of
motions of sun, moon, and stars; structures built that give evidence of astronomical knowledge.
Stonehenge, pyramids of Egypt
Prehistoric Astronomy
Before 500 B.C., what phenomena had been widely observed? Motions of:
Sun, Moon, and stars; constellations. Seasons—solstice, equinox, longer & shorter days. Tides, Eclipses,
Wandering Stars—planets, that sometimes go backwards (retrograde motion), Comets, meteors,
meteorites, supernovae, Pole star (North Star = Polaris),People devised calendar systems (year, month,
day),People built structures with astronomical alignments.
Stonehenge Built
The first stage of the construction of Stonehenge began in 2000 BC. Various
stones are aligned with Sun and Moon positions, but suggestions of eclipse prediction are overly
optimistic.
Historic Periods in Astronomy
Classical: 500 BC – 1400 AD; astronomers use geometry and
mathematics to try to explain what they see; religion strongly influences perceived knowledge of the
physical world. Ptolemy, Aristotle
Anaximander Draws Star Map
Around 550 BC in Greece, Anaximander drew a map of the sky and
proposed that planets move in separate orbits.
*Aristotle Proves Earth is Spherical
In Greece around 330 BC, Aristotle presented a formal ‘proof”
that Earth is spherical. During a lunar eclipse, the Earth’s shadow on the Moon always displays a round
edge—demonstrating Earth’s spherical shape. Vanishing sailing ships, and star positions also show this.
*Eratosthenese Measures Earth’s Diameter
In Egypt, around 200 BC, Eratosthenese accurately
measured the circumference of the Earth. On the summer solstice, the Sun is directly overhead Syrene,
but at an angle viewed from Alexander. Geometry gives the radius of the Earth.
Hipparchus Measures Distance to the Moon
In Greece, around 130 BC, Hipparchus determined the
distance to the Moon from measurements made during a solar eclipse. He also mapped the stars, and
devised the scale of stellar brightness that we still use today.
*Ptolemy Produces an Accurate Geocentric Model
Working in Greece, around 150 AD, Claudius
Ptolemy formalized an Earth-centered (geocentric) universe that explained retrograde motion of planets
with a system of epicycles. This was eventually accepted as church doctrine—stifling alternative
viewpoints.
Aryabhatta Suggests the Earth is Spinning
Many early observers probably suggested that the Earth
was spinning (as opposed to the entire universe orbiting around the Earth), but the earliest record of this
came from Aryabhatta in India, around 500 AD. The actual proof of a spinning Earth came much later.
Historic Periods in Astronomy
Renaissance: 1400AD – 1650 AD. The birth of modern science, in
which knowledge is based on observation and experimentation, instead of on pure thought sets the stage
for the renaissance. The Scientific Method sparks a boom of observational astronomy. Copernicus,
Galileo, Tycho Brahe, Kepler, Newton.
*Copernicus Proposes a Heliocentric Model
Polish Nicolas Copernicus wrote a detailed treatise
arguing in favor of a heliocentric (Sun-centered) solar system, titled, “On the Revolutions of the Celestial
Spheres.” Perhaps fearing reprisals, he didn’t have his book published until his death, in 1543, and he
dedicated it to Pope Paul III.
*Tycho Brahe Collects Longterm Planetary Data
In the late 1500s, Danish aristocrat Tycho Brahe
constructed large measurement devices that allowed him to record decades of precise astronomical data.
But his mathematical skills were lacking, and he was unable to interpret the motions of the planets.
Tycho lost his nose in a duel, and wore a metal replacement. He kept a human dwarf and a moose, as
“pets. “
Bruno is Executed for Heresy
Giordano Bruno vocally supported the heliocentric model of the solar
system, which was against church doctrine. He was imprisoned by the church for 7 years, and in 1600,
he was burned at the stake for heresy, earning him a reputation as a “martyr for science.”
*Galileo Revolutionizes Astronomy
Italian Galileo Galilei did not build the first telescope, but he
greatly improved the quality, and was first to observe celestial bodies with one. In 1609, he discovered
the moons of Jupiter (showing that the Earth was not the center of all things), and he observed the
phases of Venus (proving that planets orbit the Sun, not the Earth.)
*Kepler Developes Laws of Planetary Motion
German mathematician Johannes Kepler used
Tycho’s data to develop three laws of planetary motion, published in 1609 and 1619:
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1st –Planets orbit in ellipses (not circles) with the Sun at one focus.
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2nd –Planets move fastest when closest to the Sun.
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3rd –There’s a mathematical relationship between orbital distance and orbital period.
Historic Periods in Astronomy
Modern: 1650 AD to present. Development of telescopes, and other
technologies—spectroscope, film, CCD, radio astronomy, space probes, etc. Search for underlying
physical laws that describe the heavens. Astronomy becomes based on laws of physics. Newton,
Hubble, Sagan, Messier, Zwicky, Bell,
*Newton Separates Light
In 1672, British scientist and mathematician, Isaac Newton, separated white
light into colors—the start of spectroscopy. This began the scientific understanding of light.
Newton Quantifies Gravitation
Based on Kepler’s laws of Planetary Motion, Newton devised the law
of universal gravitation, published in 1687. This quantified the reasons for planetary movements.
(Along the way, Newton also invented calculus, formulated the basic laws of physics, invented a
practical reflecting telescope, and served as master of the Royal Mint.)
Halley Shows Comets are Periodic
In 1705, British mathematician, Edmund Halley, using historical
reports from 1456, 1531, 1607, and 1682, predicted the return of a large comet in 1758. This
established a new class of objects in the Solar System.
*Messier Catalogs Deep Sky Objects
In the 1700s, fame was established by comet discoveries.
Using improved telescopes, French astronomer Charles Messier made numerous discoveries of comets
and “non-comets.” In 1781, to assist others, he published a list of 110 astronomical objects, not to be
confused with comets. These eventually turned out to be galaxies, nebulae, and star clusters. His
designations, such as M31 (Andromeda Galaxy) have become standardized.
*Herschel Discovers Uranus
William Herschel, a British musician by day, and an amateur astronomer
by night, completed a survey of the entire night sky. In 1781, he observed a fuzzy blob, that he thought
was a comet. When the comet-expert, Messier, observed the object, he proclaimed it a planet.
Herschel originally named it “George,” but it was later named Uranus—the first planet that was ever
“discovered.”
Piazzi Discovers Ceres
Based on a mathematical pattern, known as the Titius-Bode Law,
astronomers began a search for a planet between Mars and Jupiter. In 1801, Italian Giuseppe Piazzi
discovered a new planet, called Ceres. After the discovery of Pallas (1802), Juno (1804), and Vesta
(1807), all these bodies were “demoted” from being called planets to being asteroids, or Minor Planets.
Good Fortune Leads to the Discovery of Neptune
Based on supposed gravitationally-induced
perturbations of the orbit of Uranus, mathematicians John Adams of Britain and Urbain La Verrier of
France both predicted positions for an eighth planet. In 1846, Johann Gottfried Galle, at the Berlin
Observatory, was the first to view this new planet, Neptune. Who deserves credit for the discovery is
still debated.
Struve Measures the Distance to the Stars by Parallax
Generally, determining distances in
astronomy is very difficult. The first successful measurement of the distance to a star (other than the
Sun) was announced by Wilhelm Struve of Germany in 1837.
The apparent motion (parallax) of the
star was measured as Earth moved in orbit around the Sun.
Foucault Creates a Pendulum that Demonstrates the Spinning Earth
In 1851, Leon Foucault created
a pendulum that proved to even non-scientists that the Earth spins. Science had already long accepted
the spinning Earth—based on geometry and the flattened poles—but the Foucault pendulum produced a
convincing argument to even the most skeptical.
Lockyer & Janssen Discover Helium in the Sun
During the solar eclipse of 1868, British scientist
Joseph Lockyer (and French scientist Pierre Janssen) observed an un-accounted for yellow spectral
line—which he attributed to a new element, which he called helium. Helium was first discovered on
Earth ten years later.
Einstein Revolutionizes Space, Time, and Gravity
In 1905, German theoretical physicist Albert
Einstein published his Theory of Special Relativity, predicting “strange” phenomenon at very high speeds.
Later work in General Relativity provided a warped-space explanation for gravitation.
*Swan Leavitt’s Study of Cepheid Variables Leads to Measurement of Galactic Distances
The
parallax method of distance measurement only works for “near” stars. A special type of pulsating stars,
Cepheid variables, has an absolute brightness correlated to its pulsation period, which can be used to
determine distance. Much of the analysis of Cepheid variable stars was performed by American
Henrietta Swan Leavitt in 1912.
Hertzsprung and Russell Identify Star Types
Working independently, Dane Ejnar Hertzsprung and
American Henry Russell created a chart of stellar classes (temperatures) versus luminosities (absolute
brightness) in 1913. This led to the identification of star types (red giants, white dwarfs, etc.) and an
understanding of stellar evolution.
Schwarzschild Analyzes Black Holes
Karl Schwarzschild of Germany did the first mathematical study
of black hole theory in 1916.
Jump Cannon Catalogs Thousands of Stars
Based at Harvard, Annie Jump Cannon, led a group of
women who compiled stellar classifications of 223,000 stars. Published between 1918 and 1924, this
became the basis of our understanding of stars.
*Payne-Gaposchkin Shows Stars Made of Hydrogen
Transplanted from Britain to the US, Cecilia
Payne-Gaposchkin published research in 1925 that demonstrated that stars are made primarily of
hydrogen.
*Hubble Proves Other Galaxies Exist
Using the 100-inch telescope at Mount Wilson, Edwin Hubble
was able to resolve individual stars of “nebulae,” and using Cepheid variable distance data, proved that
other galaxies exist in 1925.
*Eddington & Einstein’s Work Show how Stars Produce Energy
In 1927, British scientist Arthur
Eddington used Einstein’s energy-mass conversion, E=mc2 (1915), to explain how stars produce
energy.
*Hubble Detects the Expansion of the Universe In 1929, Edwin Hubble used redshift measurements to
show that other galaxies are moving away from one another. This first evidence of the expanding
universe (surprising Einstein) was the basis of the Big Bang model of the creation of the universe.
*Tombaugh Discovers Pluto
After years of work, comparing pairs of photographs at the Lowell
Observatory in Flagstaff, Arizona, Clyde Tombaugh discovered a ninth planet, Pluto, in 1930.
*Jansky Detects Cosmic Radio Energy
In 1932, while investigating sources of radio static for Bell
Labs in New Jersey, Karl Jansky discovered radio wavelength energy emitted from the center of the
galaxy. This began the study of radio astronomy—using non-visible “light” for studying the cosmos.
Oort Proposes a “Cloud” of Comets
In 1950, Jan Oort of the Netherlands postulated a distant group
of comets, now referred to as the Oort Cloud.
Kuiper Proposes a Belt of Minor Planets
In 1951, Dutchman Gerard Kuiper, working in the US,
proposed a large group of solar system objects located out past Neptune. This Kuiper Belt includes
Eris, Pluto, Haumea, Makemake, etc.
Sandage and Matthews Identify the First Quasar
In 1960, combining optical and radio astronomy,
Alan Sandage and Thomas Matthews identified the first quasar (quasi-stellar radio source)
*Penzias and Wilson Confirm the Big Bang
In 1963, while studying the usefulness of microwaves for
communication at Bell Labs in New Jersey, Arno Penzias and Robert Wilson detected the cosmic
microwave radiation left over from the Big Bang. This proved the validity of the Big Bang model of
creation of the universe.
*Bell Discovers the First Pulsar
pulsar in 1967.
Quaker British radio-astronomer Jocelyn Bell discovered the first
Startled by the oscillating intensity, she originally labeled the wave “little green men.”
Pulsars verified the existence of neutron stars. Two members of her team received the Nobel Prize for
this discovery.
Hawking Analyzes Black Holes
Despite having neuromuscular dystrophy, which limits his
communication to a touchpad, British cosmologist Stephen Hawking has been a world leader in his field.
In 1975, he proposed that black holes will eventually evaporate.
Voyager Flies Past the Giant Planets
Launched in 1977, the Voyager space probes completed the
famous Grand Tour of the outer planets (Jupiter, Saturn, Uranus, Neptune). This close-up data
dramatically increased our knowledge of the planets. Since then, probes have landed on Venus, Mars,
and Titan, and descended through the clouds of Jupiter.
Sagan Legitimizes the Search for Extraterrestrial Life
In the 1980s, American Carl Sagan popularized
and legitimized the search for extraterrestrial life. He helped devise the Viking life-searching tests
conducted on Mars, and he supported the project SETI (Search for Extraterrestrial Intelligence.)
Hubble Space Telescope Launched
Launched into Earth orbit in 1990, the Hubble Space Telescope
suffered a construction flaw that was repaired in 1993. It has collected data refining distances and the
age of the universe, and has shown that the expansion of the universe is accelerating. This leads to the
speculative existence of dark energy.
*Zwicky Detects Dark Matter
The “discovery” of dark matter has been a long process. Fritz Zwicky
(Swiss, MIT) conjectured its existence based on galactic rotation curves. Vera Rubin continued the work
in the 1970s. And in 2005 a galaxy made almost entirely of dark matter was unveiled.
Team Demonstrates Accelerating Expansion of the Universe In 1989, based on observation of distant
supernovae and the cosmic microwave background energy, the accelerating rate of the expansion of the
universe is observed. Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess won the In 2011 Nobel
Prize in Physics for this discovery, leading to the proposal of the existence of dark energy.
Pluto is Re-Classified
In 2006, after the discovery of Eris (a Kuiper Belt object larger than Pluto), the
International Astronomic Union, formalized the definition of “planet,” and created a new classification,
“dwarf planet,” which includes Pluto.
Deutschle Proposed Gravitational Fragmentation of Satellite Families
In a 2008 paper titled, “A
Gravity-Assisted Capture Mechanism of Planetary Irregular Satellites and A Tidal-Fragmentation Model of
Satellite Family Origin,” Salinas-based astronomy educator Philip Deutschle refutes the collisional
creation model of satellite family origin, proposing a tidal-fragmentation model.
Ongoing Detection of ExoPlanets
The Kepler Mission has detected over one thousand extrasolar
planets in orbit around other stars, using dips in stellar luminosity.
Questions to be Answered:
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What is astronomy? (What is the difference between astronomy and astrology?)
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What are our four major stages in the history of astronomy?
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What celestial (sky) phenomena have been witnessed since pre-history?
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What was our model of the universe during the period of classical astronomy?
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What development in thought triggered the renaissance of astronomy?
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How do we know: (a) the Earth is round? (b) the Earth spins? (c) planets orbit the Sun? (d)
distances to “near” celestial objects?
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What technological advancements fostered the period of modern astronomy?
What contributions to astronomy are credited to:
Ptolemy, Aristotle, Eratosthenes, Copernicus, Tycho Brahe, Johannes Kepler, Galileo Galilei, Isaac
Newton, Charles Messier, William Herschel, Henrietta Swan Leavitt, Cecilia Payne-Gaposchkin, Edwin
Hubble, Clyde Tombaugh, Karl Jansky, Arno Penzias and Robert Wilson, Jocelyn Bell, Fritz Zwicky