Editing Pendulum (section)

=== 1656: The pendulum clock ===
{{multiple image
| align    = left
| direction = horizontal
| footer   = The first pendulum clock
| image1   = Huygens first pendulum clock - front view.png
| width1   = 120
| image2   = Huygens first pendulum clock.png
| width2   = 112
}}{{Main|Pendulum clock}}
In 1656 the Dutch scientist [[Christiaan Huygens]] built the first [[pendulum clock]].<ref>although there are unsubstantiated references to prior pendulum clocks made by others: {{cite book
 | last=Usher | first=Abbott Payson
 | title=A History of Mechanical Inventions
 | year=1988
 | publisher=Courier Dover
 | pages= 310–311
 | isbn=978-0-486-25593-4
 | url=https://books.google.com/books?id=xuDDqqa8FlwC&pg=PA312
}}</ref>  This was a great improvement over existing mechanical clocks; their best accuracy was improved from around 15 minutes deviation a day to around 15 seconds a day.<ref>{{cite book
 | last = Eidson | first = John C.
 | title = Measurement, Control, and Communication using IEEE 1588
 | publisher = Burkhausen
 | year = 2006
 | page = 11
 | url = https://books.google.com/books?id=jmfkJYdEANEC&q=%22accuracy+of+clocks%22&pg=PA11
 | isbn = 978-1-84628-250-8
}}</ref>  Pendulums spread over Europe as existing clocks were [[retrofitted]] with them.<ref>Milham 1945, p.145</ref>

The English scientist [[Robert Hooke]] studied the [[conical pendulum]] around 1666, consisting of a pendulum that is free to swing in two dimensions, with the bob rotating in a circle or ellipse.<ref name="OConnor2002">{{cite web
  | last = O'Connor
  | first = J.J.
  | author2 = E.F. Robertson
  | title = Robert Hooke
  | website = Biographies, MacTutor History of Mathematics Archive
  | publisher = School of Mathematics and Statistics, Univ. of St. Andrews, Scotland
  | date = August 2002
  | url = http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Hooke.html
  | access-date = 2009-02-21
  | archive-date = 2009-03-03
  | archive-url = https://web.archive.org/web/20090303081753/http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Hooke.html
  | url-status = dead
}}</ref>  He used the motions of this device as a model to analyze the [[orbital motion]]s of the [[planet]]s.<ref>{{cite conference
  | first = Michael
  | last = Nauenberg
  | title = Robert Hooke's seminal contribution to orbital dynamics
  | book-title = Robert Hooke: Tercentennial Studies
  | pages = 17–19
  | publisher = Ashgate Publishing
  | year = 2006
  | isbn = 0-7546-5365-X
  }}</ref> Hooke suggested to [[Isaac Newton]] in 1679 that the components of orbital motion consisted of inertial motion along a tangent direction plus an attractive motion in the radial direction. This played a part in Newton's formulation of the [[law of universal gravitation]].<ref>{{cite journal
 | last=Nauenberg
 | first=Michael
 | title=Hooke and Newton: Divining Planetary Motions
 | journal=Physics Today
 | year=2004 | volume=57 | issue=2 | page=13
 | url=http://scitation.aip.org/journals/doc/PHTOAD-ft/vol_57/iss_2/13_1.shtml
 | access-date=2007-05-30
 | doi=10.1063/1.1688052 |bibcode = 2004PhT....57b..13N | doi-access=free
 | url-access=subscription
 }}</ref><ref>{{cite web
 |author=The KGM Group, Inc. 
 |year=2004 
 |url=http://www.sciencemaster.com/space/item/helio_4.php 
 |title=Heliocentric Models 
 |publisher=Science Master 
 |access-date=2007-05-30 
 |url-status=dead 
 |archive-url=https://web.archive.org/web/20070713175810/http://www.sciencemaster.com/space/item/helio_4.php 
 |archive-date=2007-07-13 
}}</ref> Robert Hooke was also responsible for suggesting as early as 1666 that the pendulum could be used to measure the force of gravity.<ref name="OConnor2002" />

During his expedition to [[Cayenne]], [[French Guiana]] in 1671, [[Jean Richer]] found that a [[pendulum clock]] was {{frac|2|1|2}} minutes per day slower at Cayenne than at Paris. From this he deduced that the force of gravity was lower at Cayenne.<ref>{{cite conference
  | first = Victor F.
  | last = Lenzen
  |author2=Robert P. Multauf
  | title = Paper 44: Development of gravity pendulums in the 19th century
  | book-title = United States National Museum Bulletin 240: Contributions from the Museum of History and Technology reprinted in Bulletin of the Smithsonian Institution
  | pages = 307
  | publisher = Smithsonian Institution Press
  | year = 1964
  | location = Washington
  | url = https://books.google.com/books?id=A1IqAAAAMAAJ&pg=RA2-PA307
  | access-date = 2009-01-28}}</ref><ref>{{cite book
 | last=Richer
 | first=Jean
 | year=1679
 | title=Observations astronomiques et physiques faites en l'isle de Caïenne
 | publisher=Mémoires de l'Académie Royale des Sciences | bibcode=1679oaep.book.....R
 }} cited in [https://books.google.com/books?id=A1IqAAAAMAAJ&pg=RA2-PA307 Lenzen & Multauf, 1964], p.307</ref>  In 1687, [[Isaac Newton]] in ''[[Philosophiæ Naturalis Principia Mathematica|Principia Mathematica]]'' showed that this was because the Earth was not a true sphere but slightly [[Oblate spheroid|oblate]] (flattened at the poles) from the effect of [[centrifugal force]] due to its rotation, causing gravity to increase with [[latitude]].<ref>[https://books.google.com/books?id=A1IqAAAAMAAJ&pg=RA2-PA307 Lenzen & Multauf, 1964], p.307</ref>  Portable pendulums began to be taken on voyages to distant lands, as precision [[gravimeter]]s to measure the [[Gravity of Earth|acceleration of gravity]] at different points on Earth, eventually resulting in accurate models of the [[Figure of the Earth|shape of the Earth]].<ref>{{cite book
  | last = Poynting
  | first = John Henry
  |author2=Joseph John Thompson
  | title = A Textbook of Physics, 4th Ed
  | publisher = Charles Griffin & Co.
  | year = 1907
  | location = London
  | pages = [https://archive.org/details/bub_gb_TL4KAAAAIAAJ/page/n30 20]–22
  | url = https://archive.org/details/bub_gb_TL4KAAAAIAAJ
  }}</ref>