Geodesia

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EARLY IDEAS OF EARTH
Earth stood still

HISTORY OF GEODESY
Robert Burtch Surveying Engineering Ferris State University

Otherwise, we would feel ourselves moving

World center of universe
Looking up one saw the moon and stars revolving about earth

Must be flat
Otherwise we would fall off

Immense - stretching into unknown oceans and lands

Each society thought itself as thecenter of universe
called ompalos syndrome - Greek for navel

FLAT DISK EARTH
Homer (9th Century BC) - flat earth supporting the sky over hemisphere Thales of Miletus - earth disk resting on water Anaximenes - earth held fixed over air of infinite thickness which could not escape upward due to the large extent of the disk

CYLINDRICAL EARTH
Anaximander
first to hold that the earth was held atthe center of a celestial sphere
• Being in the center, earth needed no support because it was equidistant to the sphere at all points • No preference between directions to fall toward one end and no up and down

Earth cylinder - 3:1 ratio of diameter to height
• Top location of inhabited world • Axis oblique to path of sun

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SPHERICAL EARTH
Pythagoras (6th Century BC)
Fromaesthetically speculative considerations, suggested earth as sphere Sphere and number 10 considered perfect therefore, a perfect world would consist of 10 planetary bodies of spherical form First to suggest the earth rotated in a circular motion around the sun

ARISTOTLE (4th CENT. BC)
Spherical earth based on gravity
All heavy bodies tend to fall towards the center and seek the lowest place in thisprocess thus they are eventually pressed into spherical ball

Observational experience
Ships at sea, superstructure is last seen going away from land Changes in observable stars going north/south Earth cast round shadows during lunar eclipses

ERATOSTHENES (276-195 BC)
“Father of Geodesy” First to make measurement of size of earth Measured length of shadow from a gnomon at noon in Alexandria atthe summer solstice In Syene, which was assumed to be on same meridian and known to lie under the Tropic of Cancer, sun’s rays reached the bottom of a well

ERATOSTHENES

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ERATOSTHENES
How was distance measured?
Camel caravan traveled 100 stadia/day and took about 50 days to travel distance Probably determined by Egyptian cadastral maps made by ‘bematists’ (clerks who measured distancesby walking evenly and counting their steps)

ERATOSTHENES
Difficulty in converting stadia into meters therefore one estimate is the circumference is too large by 16% and another 0.6% too small Errors
Sun could not be directly overhead at time of measurement -- off by about 22’ Alexandria and Syene not on same meridian Unit conversion problem

Calculated distance 5000 stadia thereforecircumference of earth 250,000 stadia

POSIDONIUS & AL-MAMUM
Posidonius (135-50 BC)
Used star Canopus to determine size of earth Found star grazed horizon at Rhodes but came up 1/4 of zodiacal sign over Alexandria (1/48 of circle) Reduced size to 180,000 stadia which was used by Columbus in voyage

WILLEBRORD SNELLIUS
1580 - 1626 One of first to use triangulation Measured 33 triangles alongnearly meridional arc between Alkmaar and Bergen op Zoom in Netherlands with 1’ angular precision and astronomic latitudes at ends about 0.1% different from current average

Caliph al-Mamum (9th century)
Measured size of earth but measured length using wooden rods over a degree of latitude (56 2/3 Arabic miles) near Bagdad

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CASSINIS
Jean Picard, 1669, measured meridional arc from Malvoisinenorth to Amiens using triangulation and astronomic latitude at ends Continued north to Dunkerque & south to Collioure by Philippe de Lahire & Dominique and Jaques Cassini between 1683 and 1716

CASSINIS
Found 1o meridional arc decreased northward Earth was pointed at poles, or egg-shaped Called prolate spheroid - s2 > s1 First evidence earth not sphere

NEWTON
Suggested earth should be...
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