Chapter 3 The Science of Astronomy

Chapter 3
The Science of Astronomy
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3.1 The Ancient Roots of Science
Our goals for learning:
• In what ways do all humans employ
scientific thinking?
• How did astronomical observations
benefit ancient societies?
• What did ancient civilizations achieve in
astronomy?
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In what ways do all humans
employ scientific thinking?
• Scientific thinking is based on everyday
ideas of observation and trial-and-error
experiments.
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How did astronomical observations
benefit ancient societies?
• In keeping track of time and seasons
— for practical purposes, including agriculture
— for religious and ceremonial purposes
• In aiding navigation
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What did ancient civilizations
achieve in astronomy?
• Daily timekeeping
• Tracking the seasons and calendar
• Monitoring lunar cycles
• Monitoring planets and stars
• Predicting eclipses
• And more…
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Modern measures of time come
directly from ancient observations of
motion in the sky
• Length of day = time it takes Sun to
make one full circle in sky
• Length of month = Moon’s cylce of
phases
• Length of year = Cycle of seasons
• 7 days in a week = named after
7objects that could be seen with the
naked eye = Sun, Moon and 5 planets
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Ancient people of central Africa (6500 B.C.) could
predict seasons from the orientation of the crescent
moon.
They noticed that the orientation of the ‘horns’ of the
crescent relative to the horizon is closely related to
the local rainfall patterns.
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Days of the week were named for Sun, Moon,
and 5 visible planets.
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Time of Day
• Use of sticks to cast
shadows and tell
time:
– Egyptian obelisk
• Can trace origins of
modern clock to
ancient Egypt about
4000 yrs ago.
– Divided daylight into
12 equal parts
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Marking Seasons
• Many ancient cultures built structures to help them
mark the seasons
• England: Stonehenge (2750 BC, completed
around 1550 B.C.)
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• Mexico: model of the Templo Mayor
- Sun rose directly through the notches between
temples on the equinoxes (March and September)
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New Mexico: Anasazi kiva aligned north–south
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SW United States: Amazi people carved a 19-turn spriral
“Sun Dagger” on a vertical cliff in Chaco Canyon:
Sun’s rays form a dagger of sunlight that pierces the
center of the carved spiral only once a year
 mark summer solstice (around June 21 when receive
most direct sunlight)
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Macchu Pichu, Peru: structures aligned with solstices
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The Temple at Abu Simbel,
Aswan, Egypt (13th century BC)
The massive facade of the main temple is dominated by
the four seated colossal statues of Ramesses. These
familiar representations are of Ramesses II himself.
Each statue, 67 feet high, is seated on a throne and
wears the double crown of Upper and Lower Egypt.
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The temple at Abu Simbel,
Aswan, Egypt
The axis of the temple is arranged so that on two days
of the year, in February and October (around
equinoxes), the rising sun shoots its rays through the
entrance and through the halls until it finally
illuminates the sanctuary statues.
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Peru: lines and patterns, some aligned with stars
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South Pacific: Polynesians were very skilled in the art of
celestial navigation.
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"On the Jisi
day, the 7th day
of the month, a
big new star
appeared in the
company of the
Ho star."
"On the Xinwei day the new star dwindled."
Bone or tortoiseshell inscription from the 14th century B.C.
China: earliest known records of supernova explosions (1400 B.C.)
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Lunar calendar
Some ancient civilizations used lunar phases for basis
as a calendar (29 or 30 days) <29.5 days>
A 12 month lunar calendar has 354 – 355 days (11
days < solar calendar),
e.g. Muslim calendar shifts 11 days every year
Lunar calendars can be synchronize with solar
calendars through interesting coincidence:
19 yrs solar = 235 months lunar
 Lunar phases repeat on same dates about every 19
years (Metonic cycle)
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Scotland: 4,000-year-old stone circle
 Moon rises (as shown here) every 18.6 years.
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France: Cave paintings from 18,000 B.C. may
suggest knowledge of lunar phases (29 dots).
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What have we learned?
• In what ways do all humans employ
scientific thinking?
— Scientific thinking involves the same type
of trial-and-error thinking that we use in
our everyday lives, but in a carefully
organized way.
• How did astronomical observations benefit
ancient societies?
— Keeping track of time and seasons;
navigation
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What have we learned?
• What did ancient civilizations achieve
in astronomy?
— To tell the time of day and year, to track
cycles of the Moon, to observe planets
and stars. (Many ancient structures
aided in astronomical observations.)
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Astronomy of Babylonians
• What were the purposes of Astronomy
in ancient Babylonia?
• What are the scientific breakthroughs of
the Babylonians?
• How did the western astronomy benefit
from these breakthroughs?
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Our mathematical and scientific heritage originated
with the civilizations of the Middle East.
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Babylonian context
Mesopotamia is the large area associated with the Tigris and
Euphrates rivers, in what is now Iraq. Civilization had developed in
this fertile valley by 4000 BCE, with irrigation for growing crops,
domesticated animals, houses, temples, an army, and all the other
features of civilization.
•Mesopotamia is generally considered by historians the cradle of
western civilization, and it is from here we begin to trace the
development of western astronomy.
•From ca. 2000 to 500 BCE the city of Babylon was the capital of
the Babylonian empire.
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The Ancient Near East
Tigris R.
Euphrates R.
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Babylonian context
•There was continual fighting for supremacy among various tribes in
the region over the centuries (Sumerians, Akkadians, Amorites,
Hittites, Assyrians, Chaldeans, Persians).
•In ca. 1890 BCE the Amorites, a tribe from the Syrian desert to the
West reached ascendancy. The sixth Amorite king was the famous
Hammurabi (ca. 1792-1750 BCE). Hammurabi introduced the
famous code of laws: “an eye for an eye, a tooth for a tooth.” He
also introduced a common calendar for the empire, and this was
based on astronomical measurements.
•His capital city was Babylon, on the Euphrates River about 55 miles
south of present-day Baghdad. The large empire was administered
from this central capital.
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Cuneiform writing
•The Babylonians also invented a system of
writing called “cuneiform:” wedge-shaped marks
made on clay tablets, which were then baked in
the Sun.
•Our knowledge of Babylonian astronomy is
derived from such cuneiform tablets, many of
which give positions of celestial objects much like
a modern ephemeris.
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Cuneiform tablets
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Hammurabi
This column shows Hammurabi
receiving his code of laws from
the god Shamash (the god of
justice, seated).
Below the two figures is the
cuneiform text of Hammurabi’s
code.
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Ishtar Gate
The city of Babylon was
famous throughout the
ancient world. This is one of
8 gates leading into the city
of Babylon, and the most
magnificent. The walls of the
city were 85 ft thick. This
gate stood next to the
palace of Nebuchadnezzar.
Just south of the palace was a
ziggurat (pyramidal temple)
300 ft high, probably the
Tower of Babel mentioned in
the Bible.
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Babylonian astronomy
The Babylonians had two main interests in observing the heavens:
• to establish and maintain a reliable calendar.
This was essential to administer their large, land-based empire.
• to make astrological predictions.
Their astronomical observing program:
•They acquired long and continuous records of observations of the stars
and planets over many 100s of years
•They looked for cycles or patterns in these.
•They also looked for patterns in deviations from cycles or patterns.
•They used these patterns to extrapolate to the future.
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Purposes of Astronomy in ancient
Babylonia
• As in most ancient cultures, astronomy was actually
practiced as astrology.
• Astronomical events, whether they were every-day
occurrences or rare incidents, had a deep religious meaning
for the people.
• It was believed that all things happened for a reason!
Lives were lived according to the advice of these
astronomers/astrologists!
• Orientation of the constellations was used to mark seasons
for harvesting or sowing crops.
• Certain constellations were noted for their yearly rising or setting
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times, and provided an accurate clock by which time could be
measured.
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What are the scientific breakthroughs of
the Babylonians?
• Babylonians not only recognized Venus as the same
object whether it appeared in the morning or evening,
but they actually developed a method for calculating
the length of the Venus cycle!
• Babylonians were able to predict solar and lunar
eclipses. They applied a simple method, which made
future predictions based on past observations!
• They almost certainly knew about the saros cycle; the
general pattern of eclipses repeats every 18 years (and 11.3
days).
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• Babylonians created the zodiac–”the circle of little
animals”–which marked the twelve constellations that
the Sun, Moon, and planets travel between during
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What are the scientific breakthroughs of
the Babylonians? (Cont.)
• Constellations that we still use today, such as Leo,
Gemini, Capricorn, etc. were invented by the Sumerians
between 2,000-3,000 BC. A great deal of astronomical
mythology was handed down from the Sumerians.
• Babylonian system of mathematics was sexagesimal, or
a base 60 numeral system. From this we derive the
modern day usage of 60 seconds in a minute, and 60
minutes in an hour.
• Babylonians invented the degree system to distinguish
positions in the sky (360 degrees, 60’ in 1 degree, etc.).
• Greeks adopted the degree system and also many of the
Babylonian constellations, which they renamed in Greek!
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How did western astronomy benefit from
these breakthroughs?
• The Babylonian underpinnings of western astronomy
are extensive!
• It was from the Babylonians that the Greeks gained
their knowledge of the five visible planets and the
constellations of the zodiac, and centuries of recorded
astronomical observations.
• Greeks also adopted the idea that the motions of the
planets could be predicted with accuracy!
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Artist’s reconstruction of the Library of Alexandria
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3.2 Ancient Greek Science
Our goals for learning:
• Why does modern science trace its
roots to the Greeks?
• How did the Greeks explain planetary
motion?
• How did Islamic scientists preserve and
extend Greek science?
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Why does modern science trace its roots to
the Greeks?
• Greeks were the first
people known to make
models of nature.
• They tried to explain
patterns in nature
without resorting to
myth or the
supernatural.
Greek geocentric model (c. 400 B.C.)
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Special Topic: Eratosthenes measures the Earth
(c. 240 B.C.)
Measurements:
Syene to Alexandria
• distance ≈ 5,000 stadia
• angle = 7°
Calculate circumference of Earth:
7/360 × (circum. Earth) = 5,000 stadia
⇒ circum. Earth = 5,000 × 360/7 stadia ≈ 250,000 stadia
Compare to modern value (≈ 40,100 km):
Greek stadium ≈ 1/6 km ⇒ 250,000 stadia ≈ 42,000 km
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How did the Greeks explain planetary
motion?
Underpinnings of the Greek geocentric
model:
• Earth at the center of the universe
• Heavens must be “perfect”—objects
move on perfect spheres or in
perfect circles.
Plato
Aristotle
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But this made it difficult to explain the apparent
retrograde motion of planets…
Review: Over a period of 10 weeks, Mars appears to stop, back
up, then go forward again.
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Mars Retrograde Motion
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The most sophisticated
geocentric model was that of
Ptolemy (A.D. 100–170) —
the Ptolemaic model:
• Sufficiently accurate to
remain in use for 1,500
years
• Arabic translation of
Ptolemy’s work named
Almagest (“the greatest
compilation”)
Ptolemy
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So how does the Ptolemaic model explain retrograde motion?
Planets really do go backward in this model.
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Ptolemaic Model
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