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The Eulerian nutation of the Éarth's axis; 
by G. H. Darwin, Cambridge. 
The latitude of any place is the mean of the altitudes 
of a star at its two transits above and below the pole, 
and the meridian is the great circle through the zenith 
bisecting the small circle described by a circumpolar 
star. Thus both the latitude and the meridian depend 
on the instantaneous axis of the Earth’s rotation, the 
observed latitude being the altitude of the instantaneous 
axis, and the meridian a great circle passing though that 
axis and the zenith of the place of observation. 
But the meridian may also be taken to mean a great 
circle fixed in the Earth passing through the geogra- 
phical pole and the place. We may call these two cireles 
the astronomical and geographical meridians. The rela- 
tionship between the two meridians may be illustrated 
by a figure which I leave to the reader to draw for him- 
self. If C be the geographical pole, I the instantaneous 
axis and P the place of observation, then IP is the 
astronomical meridian and CP the geographical meri- 
dian. The Eulerian nutation of an absolutely rigid Earth 
would be such that I would describe à circle round C in 
a period of 506 days; if we allow the Earth to yield elasti- 
cally to centrifugal force the period of the circular motion 
is augmented and observation shows that the period of 
506 days becomes one of about 430 days; lastly it 
appears that in actuality the simple cireular motion is 
perturbed by other inequalities. In the present discussion 
