The History and Practice of Ancient Astronomy

By James Evans

The heritage and perform of old Astronomy combines new scholarship with hands-on technological know-how to deliver readers into direct touch with the paintings of old astronomers. whereas tracing principles from historic Babylon to sixteenth-century Europe, the ebook locations its maximum emphasis at the Greek interval, while astronomers constructed the geometric and philosophical rules that experience decided the next personality of Western astronomy. the writer ways this background throughout the concrete information of old astronomical perform. rigorously equipped and generously illustrated, the booklet can educate readers easy methods to do genuine astronomy utilizing the tools of historical astronomers. for instance, readers will learn how to expect the following retrograde movement of Jupiter utilizing both the arithmetical tools of the Babylonians or the geometric equipment of Ptolemy. they're going to the best way to use an astrolabe and the way to layout sundials utilizing Greek and Roman suggestions. The e-book additionally includes supplementary workouts and styles for making a few operating astronomical tools, together with an astrolabe and an equatorium. greater than a presentation of astronomical equipment, the booklet presents a severe examine the facts used to reconstruct old astronomy. It contains large excerpts from old texts, meticulous documentation, and energetic discussions of the function of astronomy within the numerous cultures. obtainable to a large viewers, this e-book will attract a person drawn to how our realizing of our position within the universe has replaced and constructed, from precedent days throughout the Renaissance.

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Measuring the Obliquity of the Ecliptic The Su n strikes approximately 2 three half° north or south o f the equator. Th e ar c betwee n the tw o tropic s i s therefor e abou t forty seven° . How ca n thi s angl e b e ensure d from commentary s o f the solar ? This parameter, on e o f the main fundamenta l for th e developmen t o f astronomy , i s additionally one o f th e easies t to degree . easily degree the noo n altitud e of the Su n a t summe r solstice , maybe with th e ai d of a gnomon (sec . 1. four and fig. 1. 9). Wait six months an d measur e the noo n altitud e o f th e Su n a t winte r solstice .

Although, th e shado w is actuall y a bi t wider . I n Almagest V, 14 , Ptolem y say s tha t th e shado w i s 2 3/5 time s th e widt h o f th e Moon . Th e reade r ca n mak e a n independen t estimate b y usin g figur e 1. 29 . permit u s se e what end result s from thes e tw o enhance d values . W e leav e al l o f Aristarchus's different hypothese s unchanged. O (half the angula r diameter of the solar) i s then 1/4° . The whol e diameter o f th e shado w i s 2 3/5 X half° = 13/10° . hence, T (hal f th e shadow ) i s 13/20°.

1—determinin g the latitud e fo r whic h a n ancien t horizonta l plan e dia l wa s designed . Th e challenge i s advanced b y th e absenc e o f th e origina l gnomons. therefore , tw o similar query s mus t b e answere d concurrently : what wa s the heigh t o f the missin g gnomon , an d fo r what latitud e wa s the dia l designed ? We shal l remedy thi s proble m fo r th e tw o dial s foun d a t Rom e an d Delos , illustrate d i n figures three. four an d three. five . The Rome Dial determine three. 15 . We begi n wit h th e dia l fro m Rome , becaus e th e proble m i s less complicated fo r thi s dial.

Then, sinc e z = L - eight , we receive size o f shado w = lengt h o f gnomo n X tan (L — 5). This rul e makes i t eas y to fin d th e lengt h o f the shado w fo r a given position o n Earth a t equino x o r a t summe r o r winte r solstice . for instance, take the cas e o f Athens, range 38° N. A t summer season solstice , the sunlight' s declinatio n i s nearly +2 three 1/2°, an d w e have shadow = gnomon = X tanl zero. 2 6 X gnomon . hence, at Athens at the summer season solstice, th e midday shadow is set one-fourt h the lengt h o f the gnomon .

Th e angl e L tha t thi s lin e make s wit h the equator i s our range. At neighborhood midday, the Sun's zenith distance i s z, that is th e angl e betwee n a ra y o f ligh t arrivin g a t A an d th e zenit h course . Draw a moment ray of sunshine CB , parallel t o th e first one, bu t passin g during the cente r o f the Eart h C . The angl e tha t thi s ra y makes wit h th e equato r is the Sun's declination eight . Now, angl e ACB is the same as z, so we've the subsequent uncomplicated end result : The relation received above for choosing the range at the day of equinox (i.

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