Ancient Astronomy: The Geocentric View - PowerPoint PPT Presentation

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.

• Sundials were used all over the world (Egyptian obelisk)
• Crescent moon angles predicted rainy seasons (Nigeria)

• 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

• 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

• 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

• 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.

• You see a 1st Quarter moon about 45above the Eastern horizon. The time is roughly: A. noon B. 3 PM C. 6 PM D. 9 PM E. 3 AM

• You see a 1st Quarter moon about 45above the Eastern horizon. The time is roughly: A. noon B. 3 PM C. 6 PMD. 9 PME. 3 AM

1st Quarter Moon rises at noon, highest at 6PM and sets at midnight (roughly)

• 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.

• 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.

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

• Earth at Center, then
• Moon
• Mercury
• Venus
• Sun
• Mars
• Jupiter
• Saturn
• Fixed stars on the celestial sphere

• 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 Western world.

• 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.

• 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?

• 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)

7.2O/360O = Alexandria--Syene distance/Earth circumference

• 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.

• 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.)

• 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.

• 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 andIndian knowledge (e.g., zero) were combined and preserved in centers like Baghdad.

Angles and relative positions of stars and planets were measured and recorded, as were “new stars”

• To determine the seasons
• To decide when to plant crops
• To navigate on long voyages
• All of the above
• 1 and 2 only

• To determine the seasons
• To decide when to plant crops
• To navigate on long voyages
• All of the above
• 1 and 2 only