Astronomers and How

they Shaped Astronomy

ERATOSTHENES OF CYRENE

 When most people believed the world was flat, the notable Greek
mathematician, astronomer and geographer Eratosthenes (276–195 B.C.)
used the sun to measure the size of the round Earth, according to NASA.
 His measurement of 24,660 miles (39,690 kilometers) was only 211 miles (340
km) off the true measurement.
 CLAUDIUS PTOLEMY
 In ancient Greece, astronomer and mathematician Claudius Ptolemy (A.D. 90–168) set
up a model of the solar system in which the sun, stars, and other planets revolved around
Earth. Known as the Ptolemaic system, it remained in place for hundreds of years, though
it turned out to be flat wrong.
 According to NASA, "Ptolemy represents the epitome of knowledge of Grecian
astronomy." As a mathematician, geographer and astronomer, he authored several
scientific texts which had considerable impact on Western intellectual thought.

In the 2nd century, Ptolemy published the Almagest, a comprehensive treatise on the
movements of the stars and planets. It expanded Hipparchus’ geometric model of
celestial motions, utilizing epicycles and eccentric circles in a geocentric theory which
placed the Earth at the center of the solar system. This Ptolemaic system presented
tables of information allowing convenient predictions of planetary locations. Ptolemy
also catalogued 48 constellations, the names of which are still in use at present.

Ptolemy's writings stood as authoritative for more than 1,200 years. However, his model,
which was incorrect, later fell out of use as the heliocentric view of the solar system
came into being.

Few details about Ptolemy's life survived to the present day.

CLAUDIUS PTOLEMY
ABD AL-RAHMAN AL-SUFI

Persian astronomer Abd al-Rahman al-

Sufi (903–986), known as Azophi to
Westerners, made the first known
observation of a group of stars outside of
the Milky Way, the Andromeda galaxy.
NICOLAUS COPERNICUS

 In 16th century Poland, astronomer Nicolaus Copernicus (1473–1543) proposed

a model of the solar system that involved the Earth revolving around the sun,
according to NASA. The model wasn't completely correct, as astronomers of
the time struggled with the backwards path Mars sometimes took, but it
eventually changed the way many scientists viewed the solar system.
 Copernicus, of Poland, felt the Ptolemaic view of the planets traveling in
circular orbits around the Earth was over-complicated with many smaller
circles, epicycles, needed to explain the intermittent retrograde motion of the
planets (in which they appear to move in the opposite direction of the the
stars). Copernicus published his book, De Revolutionibus Orbium Coelestium
("On the Revolutions of the Heavenly Spheres") when he was 70 and lay dying.

His ideas took almost a hundred years to gain credence, but Galileo's 1632
assertions that the Earth orbited the sun built upon the Polish astronomer's work,
cementing the Copernican revolution.
 JOHANNES KEPLER
 Using detailed measurements of the path of planets kept by Danish
astronomer Tycho Brahe, Johannes Kepler (1571–1630) determined that planets
traveled around the sun not in circles, as Copernicus had thought, but in ellipses. In
so doing, he calculated three laws involving the motions of planets that
astronomers still use in calculations today. However, closed minds put Kepler's work
at risk.
 Kepler defended and modified the Copernican view of the solar system with a
radical reformation that established him as one of the great lights of the Scientific
Revolution of the 16th-17th centuries.

Kepler's deduction that the planets travel elliptical orbits, with the sun at one of the
foci formed his first planetary law, which he published in 1609 with the second law
which stated that planets do not travel at the same rate throughout their orbits.

Kepler's third law, published a decade later, posited that the relationship between
the length of two planets' orbits is related to their distances from the sun. Though he
made other contributions to mathematics and optics, Kepler's three laws made him
a giant of astronomy.
 JOHANNES KEPLER
 "The era in which Kepler lived was one of tremendous upheaval and change,"
said Dan Lewis, curator of the history of science and technology at the Huntington
Library in San Marino, Calif. "Religious leaders were reluctant to relinquish their ideas
about the heavens.
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 Talk by astronomers of a sky filled with objects moving in non-circular orbits and
other phenomena that went against an Earth-centric model threatened their
beliefs. As a result, Kepler and his first wife, Barbara, created a code with which to
write letters to each other so that their correspondence would not put them at risk
of persecution."
 GALILEO GALILEI
 Born in Italy, Galileo Galilei (1564–1642) is often credited with the creation of the
optical telescope, though in truth he improved on existing models.
 According to the Rice University's Galileo Project, "Galileo made his first telescope in
1609, modeled after telescopes produced in other parts of Europe that could magnify
objects three times. He created a telescope later that same year that could magnify
objects twenty times.“
 The astronomer (also mathematician, physicist and philosopher) turned the new
observational tool toward the heavens, where he discovered the four primary moons
of Jupiter (now known as the Galilean moons), as well as the rings of Saturn.
 Though a model of the Earth circling the sun was first proposed by Copernicus, it took
some time before it became widely accepted. Galileo is most widely known for
defending the idea several years after Kepler had already calculated the path of
planets, and Galileo wound up under house arrest at the end of his lifetime because of
it.
 Galileo, born in Pisa, Italy, also made numerous scientific discoveries. He famously
proved that all falling bodies fall at the same rate, regardless of mass. Further he
developed the first pendulum clock.
GALILEO GALILEI
GIOVANNI CASSINI

Italian astronomer Giovanni Cassini (1625–

1712) measured how long it took the planets
Jupiter and Mars to rotate. He also
discovered four moons of Saturn and the
gap in the planet's rings. When NASA
launched a satellite to orbit Saturn and its
moons in 1997, it was fittingly dubbed Cassini.
 CHRISTIAAN HUYGENS
 Dutch scientist Christiaan Huygens (1629–1695) proposed the earliest theory about
the nature of light, a phenomenon that puzzled scientists for hundreds of years. His
improvements on the telescope allowed him to make the first observations of Saturn's
rings and to discover its largest moon, Titan.
 Developing improved telescopes, Huygens was able to make several important
astronomical discoveries. It was in 1655 that he proposed that a thin, flat ring circled
Saturn. His discovery of Titan marked the first moon spotted around the planet. He
made the first known drawing of the Orion Nebula.
 Elsewhere in his research, Huygens proposed a wave theory of light, which was
disputed by Newton, who preferred the particle theory. The modern theory of light
combines the two into a wave-particle duality model.

Recently, Huygens' legacy was commemorated in the probe named after him,
which parachuted on Titan in 2005.
CHRISTIAAN HUYGENS
 ISAAC NEWTON
 English astronomer Sir Isaac Newton (1643–1727) is most famous for his work on forces,
specifically gravity. Building on the work of those who had gone before him — he is quoted as saying,
"If I have seen further, it is by standing upon the shoulders of giants" — he calculated three laws
describing the motion of forces between objects, known today as Newton's laws.
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 The well-known Newtonian laws of motion are: 1) an object at rest tends to stay at rest and an object
in uniform motion tends to stay in uniform motion unless acted upon by a net external force. 2) The net
force on an object is equal to the rate of change of its linear momentum in an inertial reference
frame, or if a body is accelerating, there a force is acting on it. 3) For every action there is an equal
and opposite action.

In a story that has long since gone into the public consciousness, Newton supposedly found inspiration
for his theory of gravitation upon seeing an apple fall from a tree. From this he conjectured that
gravity's pull could extend outwards from the earth, even as far as the moon and further.
 Newton often commands respect as the most influential figure in all of science. He invented calculus,
as well as investigating optics, mechanics, experimental chemistry, alchemy, and theology. His
creation of the three universal laws of motion plus the invention of the theory of universal gravity
permanently altered the field of science.
 Newton's achievements have been celebrated in many ways, with statues and poems. Notably the
unit for force was named for him, the newton (N).
EDMOND HALLEY

Edmond Halley (1656–1742) was the British

scientist who reviewed historical comet
sightings and proposed that the comet that
had appeared in 1456, 1531, 1607, and 1682
were all the same, and would return in 1758.
Although he died before its return, he was
proven correct, and the comet was named
in his honor.
 CHARLES MESSIER
 French astronomer Charles Messier (1730–1817) composed a database of objects
known at the time as "nebulae," which included 103 objects at its final publication,
though additional objects were added based on his personal notes. Many of these
objects are often listed with their catalog name, such as the Andromeda Galaxy,
known as M31. Messier also discovered 13 comets over the course of his lifetime.
 Born in France, Messier developed an interest in astronomy at an early age, having
witnessed a 6-tailed comet at age 14 in 1744. Further he viewed an annular solar
eclipse in 1748.
 As a young comet hunter, he began to discover and note nebulas, as these
frequently were confused for comets. Thus began his famous catalog of deep-sky
objects, such as star clusters and galaxies. The first version in 1771 covered 45
objects, eventually expanded by Messier to 103 objects (though there is a debate
about M102). Later astronomers filled out the catalog to a total of 110 objects.
Today Messier's catalog is still used widely, though because of his location in France,
he only included Northern Hemisphere sky objects.
CHARLES MESSIER
WILLIAM AND CAROLINE HERSCHEL

 British astronomer William Herschel (1738–1822)

cataloged over 2,500 deep sky objects. He also
discovered Uranus and its two brightest moons, two of
Saturn's moons, and the Martian ice caps.
 William trained his sister, Caroline Herschel (1750–
1848), in astronomy, and she became the first woman
to discover a comet, identifying several over the
course of her lifetime.
WILLIAM AND CAROLINE HERSCHEL
HENRIETTA SWANN LEAVITT

 Henrietta Swann Leavitt (1868–1921) was one of

several women working as a human "computer" at
Harvard College, identifying images of variable stars
on photographic plates.
 She discovered that the brightness of a special
flashing star known as a Cepheid variable was related
to how often it pulsed. This relationship allowed
astronomers to calculate the distances of stars and
galaxies, the size of the Milky Way, and the expansion
of the universe.
HENRIETTA SWANN LEAVITT
ANNIE JUMP CANNON

 Annie Jump Cannon (1863–1941), who became known as the "census taker
of the sky", was an American astronomer who classified around 350,000
stars manually. She developed the Harvard spectral system, which is used
to classify stars today.
 Before Cannon, stars were classified alphabetically, from A to Q, based on
their temperatures. Cannon realized that a star's elements create different
wavelength and determine its color. In 1901, she had improved the
classification system with ten categories that also reflected a star's color
and brightness.
 Cannon inspired many women to work in astronomy, at a time when
gender biases greatly favored men in the space industry.
ANNIE JUMP CANNON
 ALBERT EINSTEIN

 In the early 20th century, German physicist Albert Einstein (1879–

1955) became one of the most famous scientists ever after
proposing a new way of looking at the universe that went
beyond current understanding. Einstein suggested that the laws
of physics are the same throughout the universe, that the speed
of light in a vacuum is constant, and that space and time are
linked in an entity known as space-time, which is distorted by
gravity.
 In a lecture given in 1966, fellow scientist Robert Oppenheimer
said, "Einstein was a physicist, a natural philosopher, the greatest
of our time."
ALBERT EINSTEIN
 EDWIN HUBBLE
 At the same time Einstein was expanding man's view of the
universe, American astronomer Edwin Hubble (1899–1953)
calculated that a small blob in the sky existed outside of the
Milky Way.
 Prior to his observations, the discussion over the size of the
universe was divided as to whether or not only a single galaxy
existed. Hubble went on to determine that the universe itself
was expanding, a calculation which later came to be known
as Hubble's law. Hubble's observations of the various galaxies
allowed him to create a standard system of classification still
used today.
EDWIN HUBBLE
HARLOW SHAPLEY

 American astronomer Harlow Shapley (1885–1972)

calculated the size of the Milky Way galaxy and
general location of its center. He argued that the
objects known as "nebulae" lay within the galaxy,
rather than outside of it, and incorrectly disagreed
with Hubble's observations that the universe boasted
galaxies other than the Milky Way.
FRANK DRAKE

 Frank Drake (born 1930) is one of the pioneers in the

search for extraterrestrial intelligence. He was one of
the founders of the Search for Extraterrestrial
Intelligence (SETI) and devised the Drake equation, a
mathematical equation used to estimate the number
of extraterrestrial civilizations in the Milky Way galaxy
able to be detected.
FRANK DRAKE
 CARL SAGAN
 American astronomer Carl Sagan (1934–1996) may not have been a great scientist in
comparison to some on this list, but he is one of the most famous astronomers. Sagan not only
made important scientific studies in the fields of planetary science, he also managed to
popularize astronomy, according to NASA Science.
 His charismatic teaching and boundless energy influenced people around the world as he
broke down complicated subjects in a way that interested television viewers even as he
educated them. Sagan founded the Planetary Society, a nonprofit organization devoted to
advancing space science and exploration.
 Sagan was born in Brooklyn, New York. He served as professor of astronomy and space
sciences, and director of the Laboratory for Planetary Studies at Cornell University. He made
many scientific discoveries, including explaining the high temperatures of Venus and the
seasonal changes on Mars.
 However, Sagan's most known contribution to astronomy was as a educator and popularizer
of science. He published numerous articles and books, including "Cosmos," which became a
television show viewed by a billion people in sixty countries. As host of the show, he even
spawned his own catchphrase — the oft-parodied “billions and billions” — based on his
distinctive inflection, though he never uttered that phrase during the show. Sagan also
penned the science-fiction novel “Contact, later adapted into a motion picture starring
Jodie Foster. Many tributes and memorials were dedicated to Sagan following his death,
illustrating how deeply his persona pervaded the cultural landscape.
CARL SAGAN
WILLIAM K. HARTMANN

 American astronomer William K. Hartmann (born 1939)

put forth the most widely accepted theory on the
formation of the moon in 1975.
 He proposed that, after a collision with a large body
scooped, debris from the Earth coalesced into the
moon.
WILLIAM K. HARTMANN
 STEPHEN HAWKING
 Stephen Hawking (1942– 2018) made many significant insights into the field of cosmology. He
proposed that, as the universe has a beginning, it will likely also end. He also suggested that it
has no boundary or border.
 Despite being viewed as one of the most brilliant minds since Einstein, many of
Hawking's books and lectures are steered toward the general public as he seeks to educate
people about the universe they live in.
 A theoretical physicist and cosmologist, Hawking was considered one of the greatest scientific
minds since Einstein. Although motor neurone disease related to amyotrophic lateral sclerosis
(Lou Gehrig’s disease) hobbled him since the age of 20, Hawking completed his doctorate in
cosmology at Cambridge.
 Hawking's primary discovery stated that since the universe began (at the Big Bang), it must
come to an end. Hawking demonstrated (with Roger Penrose) that since Einstein’s general
theory of relativity suggested that space and time began at the birth of the universe, and ends
within black holes. This results unites general relativity and quantum theory. Further, Hawking
predicted that black holes do emit radiation, called Hawking radiation.

Hawking wrote about these and other discoveries in several books, including the best-seller A
Brief History of Time. His wheelchair-bound appearance and his speech-synthesized voice (he
is now completely paralyzed) are familiar to the public from appearances on Star Trek: The
Next Generation, The Simpsons, Futurama, and The Big Bang Theory.
STEPHEN HAWKING