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Not to be confused with Horoscopes and Zodiac signs.
ASTRONOMY
The subject of Astronomy
 Astro- means “pertaining to stars or celestial bodies”.
 -nomy means “distribution, arrangement, or management”
 The field of astronomy began when the earliest humans
gazed to the heavens and pondered just what it was that
they were seeing.
 The earliest astronomers concocted stories that attempted
to explain how terrestrial events were tied to celestial
events.
 In this way, both astronomy and astrology were born.
 Astronomy differs from Astrology in that one is based on
observable and quantifiable data. The body of knowledge
in this field has grown through the centuries due to
measurable and reproducible investigations. The other is
more subjective.
Early Astronomers
 The earliest astronomers were the Greeks and their
study of the heavens occurred between 600 BCE–
150 CE.
 The early Greeks astronomers held a geocentric
view, believing the Earth was stationary and all
known bodies (the moon, sun, Mercury, Venus,
Mars, Jupiter, and Saturn) were in orbit around the
Earth.
 They believed there existed beyond the planets a
transparent, hollow sphere (celestial sphere) on
which the stars traveled daily around the Earth.
Geocentric view
Early Astronomers (continued)
 The Greeks believed the Earth was too large to
move, and exhibited no evidence of motion, so
this was the basis of their view.
 The planets observed were known to wander
(planetai is Greek for “wanderer”) across the sky,
with the rest of the universe seeming frozen in
place.
 Aristotle (384-322 BCE) deduced the Earth must
be spherical, because it always cast a curved
shadow on the Moon during an eclipse.
Breaking with tradition
 Aristarchus (312-230 BCE) proposed a
heliocentric model for what we know as our
solar system.
 He also used basic geometric principles to
measure the size of the Earth and Moon, as
well as their relative distance to the Sun.
 Unfortunately, because his calculations were
in error, all proposals made by Aristarchus
were roundly dismissed as incorrect and the
geocentric model persisted.
More about the early Astronomers
 Eratosthenes (276-194 BCE) was the first to accurately
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measure the size of the Earth.
He measured the angle of the Sun in two Egyptian
cities that lie roughly North and South of one another
(Aswan and Alexandria).
The difference in angles was 7 degrees, or 1/50 of a
circle, so he deduced that the circumference of the
Earth was 50 times the distance between the 2 cities.
His estimate was the Earth’s circumference is 24,428
miles.
The actual circumference is 24,902 miles.
This percent error is 1.9%.
More about the early Astronomers
 The greatest and most productive Greek
astronomer was Hippocarpus.
 He charted more than 850 stars, developed a
method of measuring brightness, and
grouped them according to that brightness.
 He also determined, to within minutes, the
actual length of a year.
 All of these discoveries were done prior to the
development of the telescope, the calculator,
a computer, and an accurate time-piece.
The Ptolemaic System
 Ptolemy published Almagest (“the great
work”) in 141 CE, which was able to
accurately predict planetary motion.
 In this model, the planets were believed to
orbit the Earth in perfect circles (which the
Greeks considered the pure and perfect
shape).
 It was though this tome that retrograde was
identified and explained.
Planetary retrograde
Modern Astronomy
 Nicholas Copernicus (1473-1543) first proposed a
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model that made the Earth one of the planets
orbiting the Sun.
Further, after observing planetary motion that
defied a circular orbit, Copernicus predicted elliptical
orbits for some of the planets.
Copernicus published his predictions and findings in
a manifesto that was published as he lay dying.
These predictions were determined by the Church to
be heretical.
Giordano Bruno attempted to defend the findings of
Copernicus, but was tried during the Inquisition,
found guilty of heresy, and burned at the stake.
Modern Astronomy (continued)
 Tycho Brahe (1546-1601) established an
observatory in Copenhagen, Denmark. This
observatory served as a model for accurate and
precise measurements of celestial bodies.
 Brahe also noted that a phenomenon of a
parallax, which made stars appear to move
slightly over time.
 When the Ruler of Denmark died, Brahe lost his
patron and moved to the Czech Republic to
continue his work.
Parallax
Modern Astronomy (continued)
 Johannes Kepler worked under Brahe and
continued his work after his death.
 Kepler developed three laws of planetary
motion –
 The path of each planet is an ellipse with the Sun
at one of the foci.
 Each planetary orbit covers an equal area in an
equal amount of time.
 The orbits of planets and their distance to the Sun
are proportional. (p2 = d3)
Kepler’s 1st and 2nd Laws
Modern Astronomy (continued)
 Galileo Galilei (1564-1642) heard about a Dutch
invention that used lenses to magnify distant
objects.
 Without ever seeing an example, Galileo
constructed his own telescope (30x) and began
making observations of the night sky.
 With this telescope, Galileo discovered:
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Four moons orbiting Jupiter,
That planets possess a circular shape (Earth-like),
Venus has phases just like the Moon,
The pock-marked surface of the Moon,
Sunspots.
Galileo (continued)
 Galileo proposed, but never demonstrated from the
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Leaning Tower of Pisa, that objects fall at the same rate.
(Gravity is a constant)
In 1630, Galileo published his opus (Dialogue of the Great
World Systems) and traveled to Rome to ask for permission
to publish.
After publication, it was realized just what he was
proposing and Galileo was ordered to refute his work.
He refused, was arrested, tried, and convicted of
proclaiming doctrines contrary to religious doctrine. He
was sentenced to house arrest and died 10 years later.
In 1992, the Church formally overturned Galileo’s
conviction.
Sir Isaac Newton
 Born the year of Galileo’s death (1643),
Newton was the first to accurately identify,
describe, and quantify the force of gravity.
 He also defined inertia and described its
effect on the movement of objects.
 Additionally, he explained variances in
planetary periods (perturbation) by
accounting for the gravitational forces by
nearby planets.
Other Astronomers
 The Mayan people combined astronomical
observations with religious beliefs.
 Along with the Aztecs, Mayan astronomers
refined other Mesoamerican calendars.
 Many of their observations dealt with periods.
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Modern (days)
Lunar month 29.53059
Period of Venus 583.93
Period of Mars 779.94
Solar year 365.24198
Maya in days
29.53086
583.92027
780
365.242
Ptolemy
29.53337
583.94267
779.94
365.24667
Other Astronomers
 Aztecs refined a calendar, which allowed for
ceremonial observations on specific days.
 Druids appeared more than 300 years BCE.
 They left many structures with which
astronomical measurements could be made.
 Much speculation surrounds their culture and
practices, but their observatories leave little
doubt that they were keen astronomers.
Druid structures
Egyptian Astronomers
 Egyptian Astronomers are recorded as early as 5000 BCE.
 Early observations were concerned with plotting movement of
celestial objects during the pre-Dynastic period using stone
circles much like Stonehenge.
 During the Dynastic period, a 365 day calendar was developed
and used to chart, record, and predict annual flooding of the
Nile.
 The pyramids at Giza are aligned with (what was then) the
pole star. Because of variations in the tilt of the Earth, our
current pole star (Polaris) is different from what was the pole
star during this period.
 The Temple of Amun-Re was aligned with the midwinter Sun.
 These structural observatories allowed early Egyptians to
accurately predict seasons and perform ceremonial rites
during specific periods.
Egyptian Astronomers (cont.)
 During the last century BCE, Alexandria,
Egypt was a primary site for scientific
investigation.
 The brightest supernova (exploding star) was
noted and described in 1006 from a location
in Egypt.
 Many astronomical instruments used in Egypt
and elsewhere were developed by Egyptian
mathematicians.