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 through 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 geographical pole and the place. We may call these two circles the astronomical and geographical meridians. The relationship between the two meridians may be illustrated by a figure which I leave to the reader to draw for himself. 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 meridian. The Eulerian nutation of an absolutely rigid earth would be such that I would describe a circle round C in a period of 306 days; if we allow the earth to yield elastically to centrifugal force the period of the circular motion is augmented and observation shows that the period of 306 days becomes one of about 430 days; lastly it appears that in actuality the simple circular motion is perturbed by other inequalities.