Despite the fact that Newcomb in 1895 used the mean solar second between 1750 and 1892 as an independent unit of time, he had already in 1876 suspected that the Earth was a poor time-keeper.
Euler in 1765 suggested a free nutation http://en.wikipedia.org/wiki/Earth%27s_rotation Precession: Secondly, the position of the Earth in its orbit around the Sun at the solstices, equinoxes, or other time defined relative to the seasons, slowly changes.[2] For example, suppose that the Earth's orbital position is marked at the summer solstice, when the Earth's axial tilt is pointing directly towards the Sun. One full orbit later, when the Sun has returned to the same apparent position relative to the background stars, the Earth's axial tilt is not now directly towards the Sun: because of the effects of precession, it is a little way "beyond" this. In other words, the solstice occurred a little earlier in the orbit. Thus, the tropical year, measuring the cycle of seasons (for example, the time from solstice to solstice, or equinox to equinox), is about 20 minutes shorter than the sidereal year, which is measured by the Sun's apparent position relative to the stars. Note that 20 minutes per year is approximately equivalent to one year per 25,772 years, so after one full cycle of 25,772 years the positions of the seasons relative to the orbit are "back where they started". (In actuality, other effects also slowly change the shape and orientation of the Earth's orbit, and these, in combination with precession, create various cycles of differing periods; see also Milankovitch cycles. The magnitude of the Earth's tilt, as opposed to merely its orientation, also changes slowly over time, but this effect is not attributed directly to precession.) For identical reasons, the apparent position of the Sun relative to the backdrop of the stars at some seasonally fixed time, say the vernal equinox, slowly regresses a full 360° through all twelve traditional constellations of the zodiac, at the rate of about 50.3 seconds of arc per year (approximately 360 degrees divided by 25,772), or 1 degree every 71.6 years. http://ivs.nict.go.jp/mirror/publications/gm2004/chao/ A Search for the Earth's ‘Nearly Diurnal Free Wobble’ 1. M. G. Rochester1, 2. O. G. Jensen2, 3. D. E. Smylie2 Article first published online: 2 APR 2007 DOI: 10.1111/j.1365-246X.1974.tb04127.x The existence of the Earth's liquid core, besides affecting the period of the Chandler wobble, gives rise to a second free wobble mode, with period short of a sidereal day by a factor of the order of the ellipticity of the coremantle boundary. Seen from space this nearly diurnal geographic motion of the pole should appear as a nutation of the Earth's rotation axis, with a period theoretically calculated to be about 460 days, and (as pointed out by Toomre) an amplitude correspondingly 460 times larger than that of the associated wobble. The curious history of the ‘nearly diurnal wobble’ problem is reviewed, and the relevant dynamics derived and described in terms of Poinsot's scheme. A special method developed for harmonic analysis of non-equispaced data, applied to a 20-year record of observations with Loomis Polar Telescope of Yale University Observatory, sets an upper limit of 0"·12–0"·26 on the amplitude of any free nutation in obliquity in the frequency range 0–4 cpy, i.e. an upper limit of 0"·0003–0"·0006 on the amplitude of the nearly diurnal free wobbble. This supports Toomre's contention that earlier reports of observations of this wobble, with amplitudes in the range 0"·01–0"·02, cannot be correct. The instantaneous axis of rotation of a solid body cannot remain rigorously fixed inside the body if it oscillates in space, and vice-versa. The Poinsot movement consists in cones rolling on each other without slipping, the contact line being the rotation axis, which describes one cone in space and another inside the body. Usually, we call precession and nutation the motions of the Earth's rotation axis in the space reference frame. The polar motion (or "wobble") is the associated movement related to the Earth, modifying the latitude at a given place. The nutations forced by the Moon, the Sun and the planets are generally much more important than the free nutations. Among these free oscillations, there is the Free Core Nutation (FCN) and the "Free Inner Core Nutation"(FICN). The Free Core Nutation (FCN) is a mode related to non-alignment of the rotation axis of the core and of the mantle. It has a long period (of 432 days) in the celestial frame and is a retrograde mode. The physical parameters involved are the flattening of the Core Mantle Boundary (CMB) and the deformation of the CMB induced by the dynamic pressure. This mode is also called the Nearly Diurnal Free Wobble (NDFW, period of about 1 day), if observed in the terrestrial reference frame. The lunar-solar attraction is responsible not only for the orbital motion, but also for the nearly periodic tidal motion of the Earth. The tidal force tends to deform the Earth to a prolate ellipsoid aligned with the Earth-Moon and Earth-Sun axis. The most obvious phenomenon, which represents the Earth's response to the luni-solar tidal force, is the ocean tide, but also there are deformations of the solid Earth, simply called Earth tides. There are additionally induced variations of the Earth's orientation in space (nutations). Observations of the Earth's responses to tidal forces supply important constraints to understand the Earth's interior. In particular, the variation of the observed response as a function of the frequency in the tidal band tells us something about the interior of our planet that seismology cannot provide. This variation with frequency is caused by resonant behaviour of the Earth near 1 cycle per day, due to a normal mode of the rotating Earth. This mode is the Nearly Diurnal Free Wobble (NDFW, period of about 1 day, period = 1 / [- (1 + (1 / 432))] day), if observed in the terrestrial reference frame, and the Free Core Nutation (FCN, period = 432 days), if referenced in the space frame. Observing the NDFW / FCN is thus very useful to measure the CMB flattening and to obtain information about the dissipation effect at this interface. Fortunately, the eigenfrequency of the NDFW is located within the tidal band and is thus sensitive to the tidal forcing for the diurnal tides. In the space frame, the FCN is measured by the Very Long Baseline Interferometry (VLBI). It is worthy to note that the observation of the NDFW / FCN is a non-seismic proof of the fluidity of the outer core. http://hpiers.obspm.fr/eop-pc/: March 2010 : The Chile Earthquake of February 27th, 2010 of magnitude 8.8 has caused no DETECTABLE effect on the rotation pole and rotation rate. Independently of Earthquake, rotation pole moves of some mm up to several cm per day because of continuous atmospheric, oceanic and hydrologic mass transport. Till now the effect of Earthquake remains a theoretical matter. According to the seismic models, the recent Chile event will have the effect of disturbing the rotation pole by 8 cm in some months, small amount in comparison with the path a few meters it will achieve during this period. By handling atmospheric, oceanic and hydrologic data, this path will be modelled with a mean error of 50 cm, so that the tiny seismic effect will probably remain undiscernible. The Chile earthquake, would also have decrease the length of day, as high as 2 microseconds, that is to say below the current error on this quantity (10 microseconds), and much lower than the daily variation sometimes coming to 50 microseconds, and mainly caused by winds. Alone a mega-seism, such that of Chile in 1960, could cause a visible effect with the modern geodetic techniques.