Editing Milutin Milanković (section)

====Polar wandering====
{{Main|Polar wander}}

Conversations with [[Alfred Wegener|Wegener]], the father of [[continental drift]] theory, got Milanković interested in the [[Internal structure of Earth|interior of the Earth]] and the movement of the poles, so he told his friend that he would investigate polar wandering. In November 1929, Milanković received an invitation from Professor [[Beno Gutenberg]] of [[Darmstadt]] to collaborate on a ten volume handbook on geophysics and to publish his views on the problem of the secular variations of the Earth's rotational poles. In the meantime, Wegener died in November 1930 during his fourth expedition to [[Greenland]].<ref>{{cite journal |last1=Hoffman|first1=P. F.|title=The Tooth of Time: Alfred Wegener|journal=Geoscience Canada|url=https://journals.lib.unb.ca/index.php/gc/article/view/19504/23194|date=2012 |volume=39|issue=3|pages=102–111|doi=}}</ref> Milanković became convinced that the continents 'float' on a somewhat fluid subsurface and that the positions of the continents with respect to the axis of rotation affect the [[centrifugal force]] of the rotation and can throw the axis off balance and force it to move.<ref>{{cite encyclopedia|url=http://www.encyclopedia.com/doc/1G2-2830905919.html |title=Milanković (Milankovitch), Milutin – Dictionary definition of Milanković (Milankovitch), Milutin &#124; Encyclopedia.com: FREE online dictionary |encyclopedia=Encyclopedia.com |date=12 December 1958 |access-date=15 August 2012}}</ref> Wegener's tragedy additionally motivated Milankovich to persevere in solving the problem of polar wandering.

Milanković began working on the problem of the shape of the Earth and the position of the Earth's poles in 1932 and 1933 at the suggestion of Alfred Wegener. The Earth as a whole he considered as a [[Fluid dynamics|fluid body]], which in the case of short-duration forces behaves as a [[Rigid body|solid body]], but under an influence behaves as an [[elastic body]].<ref name="elib.mi.sanu.ac.rs"/> Using vector analysis he made a mathematical model of the Earth to create a theory of secular motion of the terrestrial poles. He derived the equation of secular trajectory of a terrestrial pole and also the equation of pole motion along this trajectory. His equation, also known as Milankovitch's theorem, is ''v = c grad Ω''.<ref>{{cite book|author=Adrian E. Scheidegger|title=Principles of Geodynamics|url=https://books.google.com/books?id=5PnuCAAAQBAJ&dq=Milankovitch+theorem&pg=PA129|date=2013|publisher=Springer Berlin Heidelberg|isbn=978-3-66-201532-2|page=176}}</ref>

He drew a map of the path of the poles over the past 300&nbsp;million years and stated that changes happen in the interval of 5&nbsp;million years (minimum) to 30&nbsp;million years (maximum).<ref>{{Cite web|url=http://servo.aob.rs/eeditions/CDS/Srpsko%20bugarska%20konferencija/6/pdfs/07.pdf|title=Milankovic's "End of the World" by Vlado Milicevic p. 7/85}}</ref> He found that the secular pole trajectory depends only on the configuration of the [[Crust (geology)|terrestrial outer shell]] and the instantaneous pole position on it, more precisely on geometry of the Earth mass.<ref name="elib.mi.sanu.ac.rs"/> On this basis he could calculate the secular pole trajectory. Also, based on Milanković's model, the continental blocks sink into their underlying "fluidal" base, and slide around, 'aiming to achieve' [[Isostasy|isostatic equilibrium]].<ref name="Grubić">{{cite journal |last1=Grubić|first1=A.|title=The astronomic theory of climatic changes of Milutin Milankovich|journal=Episodes Journal of International Geoscience|url=https://www.episodes.org/journal/view.html%3Fuid%3D1529|date=2006 |volume=29|issue=3|pages=197–203|doi=}}</ref> In his conclusion about this problem, he wrote: For an extraterrestrial observer, the displacement of the pole takes place in such a way that the ... Earth's axis maintains its orientation in space, but the Earth's crust is displaced on its substratum.<ref name="Grubić" /> Milankovitch published his paper on the subject entitled "Numerical trajectory of secular changes of pole’s rotation" in Belgrade in 1932.

Milanković wrote four sections of Gutenberg's "Handbook of Geophysics" (Handbuch der Geophysik):

* ''Stellung und Bewegung der Erde im Weltall'', No I,2 - 1931, (The Earth's Position and Movement in Space)
* ''Drehbewegungen der Erde'', No. I,6 - 1933, (Rotational Movement of the Earth)
* ''Säkulare Polverlagerungen'', No. I,7 - 1933, (Secular shift of the Poles)
* ''Astronomiche Mittel zur Erforschung der erdgeschichtlichen Klimate'', No. IX, 7 - 1938, (Astronomic Means for Climate Study during the Earth's history)

The lecture on the apparent shift of poles was held at a congress of Balkan mathematicians in [[Athens]] in 1934. That same year, 
held a lecture dedicated to the work of Alfred Wegener under the title ''Moving of the Earth's Poles – A Memory to Alfred Wegener'' in Belgrade, which was also published under the same name. Wegener's untimely death ended the collaboration between them on this subject.

Milankovitch's work on this topic was criticized from the beginning.<ref>{{cite journal |last1=Umbgrove |first1=Johannes Herman Frederik|title=Recent Theories on Polar Displacement|url=https://ajsonline.org/article/61351-recent-theories-on-polar-displacement/attachment/139702.pdf |journal=American Journal of Science |volume=244|date=1946 |issue=2 |pages=105–113|doi=10.2475/ajs.244.2.105 }}</ref> Milankovitch's trajectory of polar wandering was a topic of discussion after World War II.<ref>{{cite journal |first1=E. R.|last1=Deutcsch|title=Polar Wandering a Phantom Event?|url=https://ajsonline.org/api/v1/articles/58976-polar-wandering-a-phantom-event.pdf |journal=American Journal of Science |volume=261|date=1963 |issue=2 |pages=194–199|doi=10.2475/ajs.261.2.194 }}</ref> In the 1950s, paleomagnetic data showed different results than Milankovitch's theoretical numerical values for polar wandering trajectory.<ref>{{cite journal |first1=J|last1=Hospers|title=Rock Magnetism and Polar Wandering|url=https://www.jstor.org/stable/pdf/30066137.pdf |journal=The Journal of Geology |volume=63|date=1955 |issue=1 |pages=59–74|doi=10.1086/626226 |jstor=30066137 }}</ref>