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History of Astronomy
How our understanding of the
Universe evolved…
History of Astronomy
Big Questions:
• Why is astronomy often considered the
“oldest science”
• What are some of the major discoveries that
have shaped our understanding of the
• What is the value of learning about early
astronomers like Ptolemy and Aristotle, since
their models to describe the universe were
Astronomy: The oldest science?
• As far back as the stone and bronze
ages, human cultures realized the
cyclical nature of the motions in the sky.
• Monuments dating back to ~ 3000 BCE
show alignments with astronomical
• Those monuments were probably used
as calendars or even as ancient
Stonehenge, England (3000-2500 BCE)
•Alignments with locations of
sunset, sunrise, moonset and
moonrise at summer and
winter solstices
• Possibly used as a calendar.
Pyramid of Khufu, Giza
(Egyptian Culture, 2600-2400 BCE)
• Shafts from the King's chamber indicate location of Polaris 5000
years ago
• Former position of Orion's belt
• The pyramid is also aligned perfectly N-S and E-W
Caracol Temple, Mexico
(Mayan culture ~ 1000 CE)
• Solstice and equinox alignments
• Star alignments
• Alignments with Venus
Big Horn Medicine Wheel, Wyoming
(Plains Indians, approx. 1200 CE)
• alignments to the rising and setting of the Sun at summer solstice
• alignments to rising places of Aldebaran, Rigel, Orion, and Sirius
(all bright stars associated with the Solstice)
Mesopotamian Astronomy ~ 1200 BCE
• Earliest Babylonian star
catalogues recorded
observations on clay tablets
• Babylonian astronomy was
the basis for much of what
was done in Greek astronomy
Greco-Roman Astronomy
Astronomy in ancient Greece
Greek models were based on wrong “first
principles”, believed to be obvious and not
1. Geocentric Universe: Earth at the center
of the Universe.
2. “Perfect Heavens”: Motions of all celestial
bodies described by motions involving
objects of “perfect” shape, i.e., spheres or
3. “Unchanging Heavens”: always was and
always will be the same
Major Greek Astronomers
• Aristotle (384-322 B.C.E.) – major authority in
ancient Greece, promoted the idea of a
geocentric universe and first principles
• Eratosthenes (276 - 194, B.C.E.) – calculated
the circumference of Earth using shadows and
• Ptolemy (85--165 C.E.) -- Wanted to create a
mathematical model of the universe and
explain retrograde (backward) motion of the
Motions of the “Wanderers”
• The ancient Greeks tracked seven objects that
seemed to move against the background of fixed
stars in the sky: the Sun, the Moon, Venus,
Jupiter, Saturn, Mercury, and Mars
• They named them “planets” (the Greek word for
wanderers), and much of ancient astronomy was
centered around explaining the motion of these
seven bodies
Ptolemy’s Epicycles
The Ptolemaic system (still geocentric) was
considered the “standard model” of the
Universe for over 1000 years, through the
middle ages
Astronomy in the Renaissance
Major Renaissance Astronomers
• Nicolaus Copernicus (1473 – 1543) – caused
controversy by “changing our place in the universe”, leading
to what is known as the Copernican Revolution
• Tycho Brahe (1546 –1601) – Invented instruments for
measuring angles in the sky, made very precise
• Johannes Kepler (1571 – 1630) – used Brahe’s observations
to study planetary motion mathematically, discovered that
planets’ orbits are elliptical, not perfect circles
• Galileo Galilei (1594 – 1642) – greatly improved the
telescope and made many observations in support of the
heliocentric model of the solar system
The Copernican Revolution
• Heliocentric Universe
– The Sun at the center
– Earth was a planet, and the moon revolved around
Earth while the earth revolved around the sun
– His model still accepted Aristotle’s idea of uniform
circular orbits, so it did not perfectly predict the
motion of the planets
Copernican explanation of retrograde
motion in a heliocentric universe
• Retrograde (backward) motion
of a planet occurs when the
Earth “passes” the planet.
Arrangement of the Planets
• Copernicus made observations that allowed
him to determine the order of the 5 planets
known at that time
– Mercury and Venus can only be observed fairly near to the
sun at dawn or dusk (their orbits must lie inside of Earth’s
orbit, since they are always in the same direction as the
– Mars, Jupiter and Saturn can be observed at night when
the sun is far below the horizon (only possible if Earth is
between the sun and these planets)
– By measuring the time it took planets to return to the
same position in the sky, he determined their distances
from the sun
Kepler and Galileo
Refining Heliocentric Cosmology
Johannes Kepler (1571-1630)
• Kepler used Tycho Brahe’s meticulous
observations to deduce a mathematical
explanation for the motion of the planets
• Determined that Copernicus made a mistake-planets do not orbit the sun in perfect circles!
Kepler’s Laws of Planetary Motion
1. The orbits of the planets are ellipses with the
sun at one focus
2. A line from a planet to the sun sweeps over
equal areas in equal intervals of time.
3. A planet’s orbital period (P) squared is
proportional to its average distance from the
sun (a) cubed:
Kepler’s First Law (orbital shapes)
• The orbits of the planets are ellipses with the
sun at one focus.
Eccentricities of Ellipses
• Eccentricity = how far from circular an ellipse is
Eccentricity e = c/a
Eccentricity of Ellipses
• Planet’s orbits are very close to circular with
small eccentricities
e = 0.4
e = 0.6
Kepler’s Second Law (changing speeds)
• A line from a planet to the sun sweeps over
equal areas in equal intervals of time.
• Planet’s move faster when closer to the sun
Eccentricity of ellipse
is exaggerated for
Kepler’s 2nd law
Kepler’s Third Law (length of years)
• There is a mathematical relationship
between the distance from the sun and the
time it takes the planet to complete its orbit.
• A planet’s orbital period (P) squared is
proportional to its average distance from the
sun (a) cubed:
P2 ∝ a3
P = period in years
a = distance in AU
∝ = is proportional to…
Kepler’s 3rd Law
Galileo Galilei (1564- 1642)
• Greatly improved upon the newly invented
telescope and made discoveries in support of
the Heliocentric model of the solar system
Major Discoveries of Galileo
• Moons of Jupiter (4 Galilean moons)
Major Discoveries of Galileo
• Surface structures on the moon;
• first estimates of the height of mountains on the
• moon isn’t a perfect
Major Discoveries of Galileo
• Sun spots proving that
the sun is
not perfect
Major Discoveries of Galileo
• Phases of Venus- proving that Venus orbits
the sun and not the Earth!
Conflict of Religion and Science
• Galileo’s observations and evidence in support of
the Copernican Theory created a conflict
between a faith-based system and the scientific
ways of thinking based on observation and
• His methods and conclusions are considered the
birth of the modern view of science
A New Era of Science
A New Era of Science
• Astronomy continued to evolve after the
• Mathematics as a tool for understanding the
physics of the universe became the central
• Evolving telescopes made new observations
Isaac Newton (1643 - 1727)
• Added physics interpretations to the
mathematical descriptions of astronomy by
Copernicus, Galileo and Kepler
• Major Achievements:
• Invented Calculus as a necessary tool to
solve mathematical problems related to
• Discovered the three laws of motion
• Discovered the universal law of mutual
Newton’s Laws of Motion
Newton’s 1st Law of Motion
A body at rest
continues to rest, and
a body in motion will
continue in uniform
motion in a straight
line unless it is acted
upon by some net
(AKA- Inertia)
Newton’s 2nd Law of Motion
Acceleration (a) of a
body is inversely
proportional to its
mass (m), directly
proportional to the net
force (F), and in the
same direction as the
net force.
a = F/m  F = m a
Newton’s 3rd Law of Motion
• To every action, there is an equal and
opposite reaction.
Newton’s Universal Law of Gravity
• Any two bodies are attracting each other
through gravitation, with a force proportional
to the product of their masses and inversely
proportional to the square of their distance:
m1 * m2
F = force
∝ = is proportional to
m1 = mass of first body
m2 = mass of second body
d = distance
• More Mass = more gravitational force
• More Distance = less gravitational force
Mass vs. Weight
• Because of Newton’s Law of Gravity, we know
– Your weight is a measure of the force of gravity
acting on your body
• Your weight will be different on the moon because it
has a different mass, and therefore a different
gravitational “pull”
– Your mass is a measure of how much matter you
• Your mass will be the same or the moon
Charles Messier
• Published a catalog in 1774 that contains over
one hundred deep sky objects, including
nebulae and galaxies.
• These are now known as Messier Objects
• Also discovered 13 comets (his true interest)
Cosmic Times???????
• Replace modern astronomy section with
Cosmic Times lessons in jigsaw groups?
Modern Astronomy
in the 20th and 21st centuries
Albert Einstein
• Einstein observed that Newton’s laws of
motion are only correct in the limits of low
velocities and masses
• Space and time are not absolute and are
affected by large masses and an observer’s
relative motion
– Theory of Special Relativity (E = mc2) published
in 1905
– Theory of General Relativity (modification of
Newtonian Gravity) published in 1916
Henrietta Leavitt
• In 1912, Leavitt worked at Harvard College
Observatory as a “computer,” one of several
women paid 25 to 30 cents per hour to extract
data from thousands of photographic plates
• She identified a pattern in the brightness and
periods of Cepheid variable stars
• Her calculations formed the basis of how
astronomers now calculate distances in the
Edwin Hubble
• In 1925 he calculated that M31 was much
further away previously though, and therefore
not part of the Milky Way galaxy
– We now know that M31 is actually the
Andromeda Galaxy and that there are many other
galaxies beyond the Milky Way
• In 1929 published he findings that the
universe is expanding
– The farther away a galaxy is, the faster it is moving
away-- a relationship now known as Hubble's Law.
Arno Penzias and Robert Wilson
• Discovered cosmic microwave background
radiation (CMB) in 1965
• This “leftover” energy is evidence of the Big
Bang and the origin of the universe
Stephen Hawking
• In 1970 the English physicist provided theories
indicating that black holes are actually a
common feature of general relativity.
• He also predicts that black holes should in
theory emit radiation (known today as
Hawking radiation)
Michel Mayor and Didier Queloz
• Swiss astronomers detected the first
exoplanet orbiting a sun-like star in 1995
• The planet is officially called 51 Pegasi b, but
also known as Bellerophon
• Since that discovery, nearly 2000 extrasolar
planets have been identified
Astronomy in Action
to study
What will the NEXT great
discovery be?