8/12/2019 D Ancient Astronomy

http://slidepdf.com/reader/full/d-ancient-astronomy 1/2

Unit D – Ancient Astronomy

This handout merely consists of the information inthe PowerPoint presentation for this unit. You mayneed to take additional notes.

You should be reading the appropriate parts of thetext (Chapter(s) 2.3-2.4, 3.2).

The origins of astronomy are probably practical:timekeeping, navigation, surveying, etc.Most people throughout history have had amagical/supernatural view of the universe. It’s notsurprising that astrology – the belief that the

positions of celestial objects influence humanevents – is widespread.Astronomy and astrology slowly separated duringthe rise of modern scientific astronomy. Eratosthenes (3 rd century BCE) and the size of theEarth

Most ancient literate cultures developed reasonablygood methods of calculating future planetary

positions from past observations – these were justcalculational recipes, with no underlying physicalmodel of the universe.The Greek approach probably also started as acalculational recipe, but they did the calculationsusing geometry – which will naturally lead to

thinking about the actual geometry of the universe. Ancient Greek models of planetary motion typicallyhad the following constraints:1. The Universe is geocentric – the Earth isimmobile at the center of the universe.2. All celestial motion is represented by acombination of uniform circular motions. The most successful ancient Greek model was byClaudius Ptolemy (2 nd century CE). Shown here isan illustration of Ptolemy’s arrangement of the

planets from Cosmographia by Peter Apian (1542).

Ptolemy’s explanation of planetary motion

www.astronomynotes.com

http://www.nordita.dk/~steen/fysik51/ast/astt16_files/AT40202_files/AACHCJU0.JPG

Ptolemy’s book on astronomy is known by a hybridGreek-Arabic title, the Almagest .His basic model of planetary motion, with somerefinements, was accepted for ~ 1400 years.

8/12/2019 D Ancient Astronomy

http://slidepdf.com/reader/full/d-ancient-astronomy 2/2

The Ptolemaic model can predict planetary positions to about 1º, which was consideredsufficient precision at the time.One ancient astronomer did consider the possibilityof a heliocentric (Sun-centered) universe – Aristarchus (3 rd century BCE).

Other ancient astronomers rejected this idea for both observational and philosophical reasons.They looked for annual parallax of stars but didn’tsee it. Why not? Medieval Europeans saw the universe as aChristianized fusion of Ptolemy and Aristotle: thedeferents and epicycles are interlocking crystallinespheres in which the planets are embedded. Here is

an illustration of the cosmos as described in Dante’s Divine Comedy (c. 1310-1314).

http://ircamera.as.arizona.edu/NatSci102/images/dante1.gif

Most important advances between the fall of theWestern Roman Empire and the Renaissanceoccurred in the Middle East, particularly in the 9thand 10th centuries. This is reflected in the Arabicnames of many stars and terms such as azimuth,zenith, nadir, etc.

Astronomers writing in Arabic preserved andrefined Ptolemy during the “Dark Ages” in Europe. The most important advances were mathematical:- Development of algebra (from al-jebr , “thereunion of broken parts”) - Discovery of five of the six basic trigonometricfunctions- Arabic (decimal) numerals (originally from India,

but refined and transmitted by Islamicmathematicians) Arabic (decimal) numerals:

26

12

52

260

312

Roman numerals:

XXVI times XII

Make two columns

XXVI XII

Successively divide left column by 2 whilemultiplying right column by 2.

XXVI XII

XIII XXIVVI LXVIIIIII XCVII CXCII

Cross out lines where left column is even.

XXVI XIIXIII XXIVVI LXVIIIIII XCVI

I CXCII

Add remaining right column entries to get theanswer.

XXIV + XCVI + CXCII = CCCXII (312)