A Brief History of The Age of the Earth

"The key difference between geologists and most other scientists is in their 
    attitude toward time."   (Earth, Press & Siever).
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Xenophanes of Colophon (570-470 B.C.):
recognized fossils as former life, inferred that sedimentary rocks originated 
as sea bottom sediments, thought that the rocks were "very old".


Herodotus (450 B.C.):
suggested that the Nile River Delta was formed was a series of floods, given the 
size of recent floods, it must have taken thousands of years to form the delta.


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.... some of these ideas were rediscovered by, for example,
     Leonardo da Vinci (-1519), Nicolaus Steno (1669)


... then, at the same time as Steno & Newton's contributions to Physics:


Archbisop Ussher (1664): used Bible to calculate that Earth was created at 
    9:00 am, Oct. 26, 4004 B.C.

            

...but the geologists wanted the Earth to be older


Comte de Buffon (1750's): from cooling rates of iron spheres, estimated 
        age of Earth to be 75,000 years.

            

...but the geologists wanted the Earth to be older, e.g.:
  
William Smith (1793) English engineer began to correlate fossils across England


von Helmholtz (mid 1800's): used Sun's energy output to estimate age of 
      Solar System as 20 to 40 million years.

            

...but the geologists wanted the Earth to be older, e.g.:
Re-emphasis of Uniformitarianism (Hutton, 1785, Lyell, 1830), rock sequences 
     must have taken much longer than 25 million years.


Lord Kelvin (late 1800's): used various arguments from Physics to estimate 
     age of Earth as 40,000 to 4 million years, later 20 million years old.


RADIOACTIVITY (discovered 1896 by Becqueral, also Rontgen, Marie Curie).
     
   1908: Rutherford suggests it can be used to date rocks
      
    by late 1950's: oldest Earth rocks are just less than 4 bilion years,
                       
                   age of Earth & Solar System is about 4.5 billion years.