1. Ancient Astronomy:
The Geocentric View
People have been fascinated by the
night skies since the beginning of
civilization.
Astronomy must be counted among
the first of the sciences.
It was intimately related with the
development of basic mathematics.
2. Telling Time and Seasons
• Sundials were used all over the world (Egyptian obelisk)
• Crescent moon angles predicted rainy seasons (Nigeria)
3. Special Seasonal Alignments
• Stonehenge (England -- and others) had many
alignments for solstices, equinoxes and bright stars
• Sun Dagger (New Mexico) shows noon on summer
solstice; other effects in winter & equinoxes
4. Mesopotamian Astronomy
• MESOPOTAMIANS built observatories starting
~6000 years ago:
• the ziggurats had seven levels, one for each
wandering object in the sky:
• Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn
• Thus 7 days to the week
• They tracked stars --- groups rising before sun at
different times of year implied seasonal
beginnings for planting and harvesting (zodiac).
• Divided circles in 360 degrees,
each degree into 60 minutes and
each minute into 60 seconds -- we still use!
• Left written records in cuneform so we understand
them better
5. Mesopotamian Astronomy and Influence
• By 2000 BC Ur and other Sumerian and Babylonia
cities had large temples, or ziggurats, usually aligned
N-S, like most Egyptian pyramids
• Egyptian and Mesopotamian knowledge spread to
Europe, Asia and Africa
6. Other Ancient Accomplishments
• Mesopotamians could predict planetary positions --
synodic periods, e.g., Mars returns to same location
roughly every 780 days:
22 synodic periods = 47 years, so
• records of old planetary positions could give good
locations.
• Knew about the SAROS cycle 2700 years ago: lunar
eclipses definitely occurred every 18.6 years.
• Chinese, Indians and Mayans also knew these
patterns
• Egyptians used astronomical events to forecast Nile
floods and harvest times.
7. PEER INSTRUCTION QUESTION
1. You see a 1st Quarter moon about
45above the Eastern horizon. The
time is roughly:
A. noon
B. 3 PM
C. 6 PM
D. 9 PM
E. 3 AM
8. PEER INSTRUCTION QUESTION
1. You see a 1st Quarter moon about
45above the Eastern horizon. The
time is roughly:
A. noon
B. 3 PM
C. 6 PM
D. 9 PM
E. 3 AM
1st Quarter Moon rises at noon, highest at
6PM and sets at midnight (roughly)
9. Greek Astronomy: The Earth at the
Center
• While they may have built upon Egyptian &
Mesopotamian results (not much preserved beyond
calendars and orientation of temples),
• Greeks tried to EXPLAIN and UNDERSTAND,
not just
• PREDICT based upon repetitive cycles of motions.
• Thales (624--547 BCE) was claimed to have
predicted a solar eclipse.
• Anaxamander (611--547 BCE) of Miletus (Asia
Minor) produced a model: Earth as a cylinder,
Sun, Moon and stars are fire filled wheels --
precursor of non-mythical explanations.
10. Early Greek Ideas of the Cosmos
• Anaximenes of Miletus (585--526 BCE) believed stars
were fixed to a solid, crystalline vault surrounding the
Earth -- the concept of the Celestial Sphere.
• Pythagoras (582--500 BCE) and his students in Croton
(S. Italy) argued that:
• Earth and all heavenly bodies are perfect SPHERES.
• All celestial motions were perfect CIRCLES.
• By then, it was understood that moonshine was
reflected sunlight.
• Eudoxus (408--355 BCE) had planets moving on
multiple spheres, all surrounding the Earth. These
could explain RETROGRADE LOOPS in the orbits of
MARS, JUPITER and SATURN -- but didn't account for
diversity thereof or for variations in brightness of
planets, since their distance from Earth was fixed.
11. Retrograde Loops Planets usually
go in same
direction as
stars but at
different speeds.
BUT sometimes
go backwards.
Mars in main
figure and time
lapse of several
planets (in
planetarium)
above.
Greek Model
12. Heavenly Spheres: Geocentrism
• Earth at Center, then
• Moon
• Mercury
• Venus
• Sun
• Mars
• Jupiter
• Saturn
• Fixed stars on the
celestial sphere
13. Aristotle: “The Authority”
• Aristotle (384--322 BCE) gave PROOFS that the
Earth was SPHERICAL:
objects all fell towards its center yet perpendicular
to ground sphere (but it could still be a cylinder).
• Noted shadows cast on moon during eclipse were
always round -- they sometimes wouldn't be if the
earth were disk-like (or cylindrical).
• But he also argued that since everything fell toward
the earth, it was the heaviest thing around, therefore it
shouldn't move -- EARTH at CENTER of the
UNIVERSE -- the GEOCENTRIC COSMOS
This was certainly LOGICAL, but by no means a
PROOF (as he thought it was).
His opinions on this and many other subjects which he
studied and wrote about were considered
authoritative between 12th & 17th centuries in the
14. Aristarchus: Real Distance Measurements
• Aristarchus (310--230 BCE) of Samos applied Euclid's geometry
to get the distance to the Moon.
• The angular diameter is measured directly; the linear diameter
comes from seeing how much of the Earth's shadow the moon
occupies during a lunar eclipse (about 3/8).
• If the Earth's diameter is known, this allows the Moon's to be
found at about 3/8ths of Earth's.
• Used geometry to estimate that the Sun was 19 times further than
the Moon (19 times larger, since angular sizes are the same).
• Aristarchus then could estimate that Sun was about 7 times the
diameter of the Earth (19 x 3/8)
• THIS LED HIM TO PROPOSE A HELIOCENTRIC COSMOLOGY
--- with the BIG SUN at REST, SMALL EARTH MOVING AROUND
IT.
• His lunar size was a little too big and his distance to the sun much
too small, because of inaccurate measurements, but the
techniques were clever and were major advances.
16. Ancient Objections to a Heliocentric
Picture
• A moving earth should yield a powerful wind
that would blow us off.
• Stars didn't show measurable parallax
(Greeks couldn't think of them being SO
much further away than planets).
• It sure seems like we're standing still and
everything in the sky is moving, doesn't it?
17. The Size of the Earth
• Eratosthenes (276--195 BCE) used geometry and
simple astronomy to make an accurate
measurement of the Earth's radius.
• He realized the difference in the altitude of the
noonday Sun in Syene and Alexandria equaled
the latitude difference between the cities.
• That gave the ratio:
• circumference of the Earth / 360 = distance / 7.2
• Accuracy determined by distance in stadia ---
measured by foot and uncertain, but around
40,000 km, and probably good to 10%
• (Correct value: 40,074 km or 24,890 miles)
19. Stellar Catalogs and Precession
• Hipparchus (190--125 BCE) utilized
• Aristarchus' method to get the Moon to be 59 Earth
radii away (60 is correct!)
• He made a better measurement of the length of the
year.
• Hipparchus also saw a NOVA and this caused him to
make a CATALOG of bright stars.
• Comparing his locations to those recorded about 170
years earlier he found a difference of about 2 degrees,
• and concluded that there was PRECESSION -- his
estimate of a 28,000 year period was very good.
20. Ptolemy: The Peak of Greek Astronomy
• Ptolemy worked in Alexandria from 127--151 CE.
• As a geographer, he is the first one known to have
used latitude and longitude on earth.
• His astronomy book, "”-- megiste -- or
``The Greatest'' -- Arabic name: Almagest.
• Catalog of over 1000 stars w/ brightnesses, using the
MAGNITUDE SYSTEM.
• Ptolemy's influence on astronomy was immense for
he published a detailed GEOCENTRIC MODEL.
• (It’s not clear how much was original to him.)
22. Detailed Geocentric Model
• Ptolemy showed the simple system of Hipparchus,
with just a DEFERENT and EPICYCLE was
inadequate.
• His model added an EQUANT -- the motion of the
center of the epicycle is uniform only if viewed
from the equant.
• However, Ptolemy's greatest contribution was the
publication of his text, a summary of all earlier
Greek astronomical knowledge.
• While complex, IT WORKED (to the accuracy the
Greeks could measure) and was USED for 1500
years! Certainly OPERATIONALLY successful.
24. Arabic Astronomy & Knowledge Transmission
• Hypatia (370--415 CE) of Alexandria, built better
instruments and made more accurate positional
measurements. She was murdered by monks who
objected to her paganism and her astrology.
• After the burning of the Alexandria library and the fall
of Rome, Astronomy in Europe withered, with only
parts of Greek and Roman knowledge retained.
• The rise of Islam (esp. the need to know direction to
Mecca) led to large observatories in Samarkand,
Persia, Turkey and eventually Spain with more
careful observations and improved instruments.
• Big catalogs were produced: more stars, more
accurate locations
• Greek and Indian knowledge (e.g., zero) were
combined and preserved in centers like Baghdad.
25. Turkish and Chinese Instruments
Angles and relative positions of stars and planets were
measured and recorded, as were “new stars”
26. How was careful observation of the
sky used in early cultures?
• To determine the seasons
• To decide when to plant crops
• To navigate on long voyages
• All of the above
• 1 and 2 only
27. How was careful observation of the
sky used in early cultures?
• To determine the seasons
• To decide when to plant crops
• To navigate on long voyages
• All of the above
• 1 and 2 only
