Precession Of Equinoxes Research Articles

Rotation/precession of NH 3 groups in Hofmann clathrates

Vibrational spectra, crystallography and rotational spectra of the Hofmann clathrate Ni(NH 3) 2Ni(CN) 4 · 2C 6D 6 are all consistent with the displacement of the N atom during the free rotation of the NH 3 groups. Molecular dynamics calculations using a slightly modified universal force field reproduce the measured inelastic neutron scattering spectrum and crystal structure. An examination of the H and N atom trajectories from a simulation show that the NH 3 rotation is best described as a rigid rotor in which the H3 triangle turns around the NiN axis whilst the NiN axis precesses round the crystallographic z-direction.

On the precession constant: Values and constraints on the dynamical ellipticity; link with Oppolzer terms and tilt-over-mode

The luni-solar precession, derived by theoretical considerations from the precession of the equator, is one of the most important parameters for computing not only precession but also nutations, due to its relation to the dynamical flattening. In this paper, we review the numerical values of this parameter, from the geodynamical point of view as well as the astronomical point of view, from the observational point of view as well as from the theoretical point of view. In particular, we point out a difference of about 1 percent between the global Earth dynamical flattening derived from the astronomical observations and the values derived from the different geophysical computations. The nutation amplitudes depend on the Earth dynamical flattening and this dependence is amplified by a resonance at an important normal mode, the Tilt-Over-Mode (TOM). Since the astronomical point of view as well as the geophysical one are confronted, we also take the opportunity to make the link between the TOM and the expressions of the nutations of the different axes which, in turn, are related with one another by the Oppolzer terms. Both, the Oppolzer terms and the TOM originate from a reference frame tilt effect. In writing the link between the nutational motions of the different axes, and so, in writing the Oppolzer terms, we also make the link with the precessional motion.

The motion of a spinning object in a higher-dimensional spacetime

We solve exactly the equations of 5D general relativity for the case of a spinning test object moving in a circular orbit in a Schwarzschild-like spacetime augmented by one extra dimension. All of the classical tests give their standard results, but the spin axis precesses at a different rate in 4D and 5D. This suggests that the dimensionality of the world could be tested using a gyroscope in Earth orbit.

Effects of precession uncertainties on planetary ephemerides

Several methods of data analysis applied recently to precise astrometric observations give evidence for a correction of the luni-solar precession (ψ) of the order of Δψ = −3 milliarcseconds per year (Williams et al., 1994). When studying the motion of bodies of the planetary system from photographic observations, an inertial stellar reference frame is required. Thus, any imperfection of the luni-solar precession would have repercussions on the determination of the orbit dynamics of the celestial body under investigation.

Observation of liquid motion in a precessing oblate spheroid

We summarize experimental results for transient flow patterns in a liquid-filled oblate spheroid, and compare the observed flows with existing geodynamo theories and evidence. Flow patterns are traced from a state near inertial rest through spin-up at a constant cavity spin rate, to forced precession where the cavity spin axis precesses at a constant rate about an inertially fixed direction displaced from the cavity spin axis by a constant half-coning angle. Space- and time-dependent flow with various instabilities are recorded for precession-induced boundary layer and deep interior flow velocities. Slow precession induces cylindrical shear layers and mostly closed meridional convection in the bulk of the core, the same features as found in buoyancy-driven convection models and in a ‘weak Taylor state’ solution for a numerical αω dynamo with small non-zero viscous coupling. Observed precessional flows exhibit significant departures from geostrophic equilibrium.

Experiments on precessing flows in the Earth's liquid core

Experiments simulating flow in the Earth's liquid core induced by luni-solar precession of the solid mantle indicate, to a first approximation, that the core behaves like a rigidized fluid sphere spinning slower than the mantle and with its spin axis lagging the mantle spin axis in precession. Secondary flow patterns are always present. At low precession rates the fluid sphere is subdivided into a set of cylinders coaxial with the fluid spin axis, the cylinders rotating alternately at slightly faster and slower rates relative to the net retrograde motion of the fluid as a whole. Slow non-axisymmetric columnar wave patterns develop between the differentially rotating cylinders. Axial flows between the spheroidal cavity boundary and the interior are observed. Fluid motion becomes turbulent only at precession rates large enough to cause the fluid spin axis to align nearly with the precession axis. There is no evidence that the Earth's liquid spin axis direction departs more than a fraction of a degree from geographic north. Our observations suggest precession induces a complex variety of laminar flows, including slowly varying and/or periodic patterns, in the Earth's liquid core.

Determination of Precession and Nutation from Lunar Laser Ranging Analysis

AbstractFrom the analysis of 24 yr of lunar laser ranges, the correction to the IAU luni-solar precession constant is found to be −3.2 ± 0.3 mas/yr for a total value of 50.3846˝/yr at J2000. The size of the 18.6 yr nutation of the pole needs to be increased by about 2 mas over the 1980 IAU theory.

Observations of Luni-Solar Precession

The observations of dψ and dε used in this analysis were taken from the combination solution of the IERS Sub-Bureau for Rapid Service and Precition (McCarthy and Luzum 1991a). Besides corrections to the coefficients determined from VLBI, additional estimates can be derived from lunar laser ranging (LLR) observations (Williams et al. 1991; Whipple 1993). Table ?? shows a comparison of the estimates of the change in longitude and obliquity derived in this analysis with corresponding terms from analyses by other authors using different observations. The theoretical value of obliquity by Williams (1994) is also included for comparison. There are significant unexplained discrepancies among the rate estimates. Difference in the methods of the analyses such as the procesure for the estimation of the nutation coefficients and correlations within the solutions probably account for the greater part of these discrepancies. In considering the adoption of changes in the IAU model for nutation, it is important to recall that changes must also be made in the precession constant. Introduction of changes in nutation without corresponding changes in precession will not improve the agreement between observations and theory.

A note on orbital control of equator-pole heat fluxes

We note that orbital (Milankovitch) variations, in particular the precession of the equinoxes, can lead to profound variations in the flux of heat from the tropics to higher latitudes. The mechanism involves changing the intensity of the Hadley circulation by varying the maximum displacement from the equator of the zonally averaged surface temperature maximum in summer. The precession of the equinoxes causes this quantity to vary by more than a factor of 2. The intensity of the Hadley circulation has a major influence on the heat fluxes in the winter hemisphere. Summer heat fluxes are generally small. Although the precession cycle is characterized by periods in the neighborhood of 20000 years, the variations are modulated by the eccentricity whose variation is dominated by periods in the neighborhood of 100000 years and 400000 years. We show how the fact that both small and large heat fluxes lead to low snowfall (and, hence, small glacial accumulation) causes the demodulation of the heat flux leading to dominant eccentricity periods in the resulting glaciation.

Precession motion around intermediate inertia axis: Molecules and deformed nuclei.

It is shown that an orbiting particle stabilizes the intermediate moment of inertia axis of an asymmetric top due to the anisotropic interaction. The region of stabilization, the classical precessional motion around the intermediate axis, and its quantization have been found. The predicted phenomenon may be observed by studying the lowest levels of rotational multiplets in the one-electron Rydberg states of asymmetric top molecules and odd triaxial nuclei. The dip and the specific set of inversion doublets in rotational spectra provide the signature of an intermediate axis precession.

Obliquity‐oblateness feedback: Are climatically sensitive values of obliquity dynamically unstable?

A new model is presented for feedback between rotational and climatic variations, operative on time scales of 104–107 years. Due to the combined effect of planetary perturbations to the Earth's orbit plane and luni‐solar torques on the oblate figure of the Earth, the obliquity varies by ∼1° on a 4·104 year time scale. Associated changes in the seasonal and latitudinal pattern of incident solar radiation cause major glaciations. Mass transport from the oceans to the polar ice sheets during these glaciations can change the gravitational oblateness of the Earth by amounts approaching 1%. As the rate of spin axis precession is directly proportional to the oblateness, the climatically forced mass transport can by dynamically significant. A simple parameterization of us coupled orbital‐rotational‐climatic system suggests that there is a strong tendency for the system to evolve away from climatically sensitive values of the obliquity. This may explain the mid‐Pleistocene transition from an obliquity dominated regime to the present regime in which most climatic variability is concentrated at longer (105 year) periods.

The Martian mean moment-of-inertia and the size of the Mars' core

The mean moment-of-inertia ratio,I/MR2, of Mars cannot be derived from its precessional constant because the exact value of the Martian axial precession is unknown presently. Using the known geodetic parameters of Mars as the constrained condition, we constructed nine Martian internal structure models (see Table II). We can then estimate the nonhydrostatic components of the principal moment-of-inertia for these models. The interplanetary comparison suggests that the reasonable range of the mean moment-of-inertia ratio,I/MR2, of Mars is 0.350 ∼ 0.360, and the range of the corresponding radius of Mars' core is 1520 ∼ 1850 km. The two parameterically simple models recommended in this paper (see Table IV) can be used for reference in the future theoretical researches.

The impact of climate change, tectonism and hydrology on the formation of Carboniferous tropical intermontane mires: the Springhill coalfield, Cumberland Basin, Nova Scotia

Both climate and tectonism are influential in determining basin and mire hydrology. In the rapidly subsiding intermontane basins of Euramerica which formed in response to the Variscan-Hercynian-Alleghenian orogeny, however, the record of climate change has been muted and may therefore go unrecognized. The complex interplay of climate and tectonism in a tropical continental setting is recorded in the > 4 km thick Westphalian A-B fill of the Cumberland Basin. Thick peat accumulation was favoured at the southern basin margin where rheotrophic forested mires 4–9 km wide flourished between basin-axis rivers and coalesced alluvial fans derived from the Cobequid highlands. As a result of their geomorphic setting the thicker coals (⩽ 4.3 m) exhibit distinct areal zones: piedmont, inner mire and riverine. Evidence that the forest swamps were nourished both by rainfall and by groundwater discharge from alluvial fans includes the stratigraphic affinity of thick coals and basin-margin conglomerates, interfingering of distal fan sheetflood and mire deposits, petrographic evidence of elevated pH (partial gelification of tissues), and predominance of the arboreous lycopsids Lepidodendron and Paralycopodites. Whereas such groundwater supply is crucial to maintenance of modern wetlands in drier periods, a subhumid climate is implied. Coals at Joggins, further removed from groundwater discharge at the basin margin, are less well developed. A 600-m thick “coal window” within the fining and reddening upward basin-fill sequence at Springhill is inferred to represent a coincidence of optimal groundwater supply and net subsidence. The ultimate decline of peat accumulation in the basin may reflect decrease in catchment area and fan size and loss of rainshadow as the Cobequid Massif was denuded and the basin filled. The coal window [ca. (6−9) × 10 5 yr] is punctuated by non-marine cyclothems of coal, grey sideritic mudrock and multistorey sandstone that share similar time frames with astronomic cycles of axial precession [ca. (1−4) × 10 4 yr]. The stress of related climate change is inferred to have caused peat accumulation to fall into disequilibrium with sediment flux and basin subsidence. The Late Quaternary history of certain African mires illustrates the dramatic impact of climate change (especially temperature and seasonality) and orographic effects on tropical continental peat accumulation. Aspects of the groundwater-recharge model may be applicable to the Westphalian-Stephanian intermontane mires of Spain and Mesozoic coal basins of northern China.

Late Cretaceous Precessional Cycles in Double Time: A Warm-Earth Milankovitch Response

Late Cretaceous climatic cycles are reflected in lithological and magnetic variations in carbonate sediments from South Atlantic Deep-Sea Drilling Project site 516F at a paleolatitude of roughly 30 degrees S. Magnetic susceptibility cycles 20 to 60 centimeters in length appear to be controlled by the precession of the equinoxes. Cyclicity is particularly robust within a 24-meter interval in the lower Campanian, where overtone spectral peaks are observed as well as secondary susceptibility maxima within individual precession cycles. One model for this behavior is that sedimentation in the narrow Cretaceous South Atlantic was controlled by equatorial climate dynamics, with the precessional insolation signal rectified by the large land masses surrounding the ocean basin.

OBSERVED AND PREDICTED DATA IN RADIO ASTROMETRIC OBSERVATIONS

ABSTRACT Atmospheric effects are the main source of erros in astrometry by connected-element radio interferometry. An improvement to this situation can be obtained by taking account of the changing atmospheric states. In this work it is shown that the monitoring of near-ground-level gradients of the radio refractivity index parameters would be of use when analyzing the differences between observed and predicted data, on which the astrometric results are based. A discussion of the mechanisms by which the atmospheric effects disturb the radio astrometric results is presented. Practical experiments are reported, which support the obtained conclusions. The raw data from these measurements are appended. The analysis is further extended to ionospheric effects. The expressions for polar motion and annual aberration are also thoroughly considered. We have then been able to analyze the almost 15 years of 5 GHz positional results from the MRAO/5km Telescope. The details of each observation are listed and appended. The analysis of this long series of results has enabled an accurate determination of the main terms of the luni-solar precession and of the nutation. The work related in this dissertation indicates that a degree of optimization of the positional measurements can be achieved by monitoring the atmospheric gradients. It also indicates that the mean MRAO/5km Telescope astrometric results are consistent to a very high level; this is substantiated by the precessional and nutational corrections obtained.

Doppler imaging based on radar target precession

A unique form of target motion reminiscent of the precession of a rotating top is investigated. Images are generated with the radar looking either along the precession axis or orthogonally to it. Features of the two methods are discussed and demonstrated by imaging simulated point target scenes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Frequency variations of the Earth's obliquity and the 100-kyr ice-age cycles

VARIATIONS in the Earth's orbital parameters modulate the seasonal distribution of solar radiation and thereby induce changes in the Earth's climate1. Periodicities in the geological climate record with cycles of 100, 41 and 23 kyr have been linked with changes in eccentricity, obliquity and precession of the equinoxes, respectively2,3. But although the eccentricity does vary with a 100-kyr period, the effect on the incoming solar radiation is rather weak relative to the signals from obliquity and precession variations. The 100-kyr signal in the climate record should therefore be of negligible intensity, yet it is observed instead to dominate the record. Internal, nonlinear processes within the climate system have been proposed to account for this fact4–14. In contrast, I show here that variations in the frequency of the obliquity cycle can give rise to strong 100-kyr forcing of climate.

Analysis of orbital perturbations acting on objects in orbits near geosynchronous Earth orbit

Orbital evolution has been numerically simulated for objects started in geosynchronous Earth orbit (GEO) or in orbits near GEO, during a project to study potential orbital debris problems in this region. Perturbations simulated include nonspherical terms in the Earth's geopotential field, lunar and solar gravity, and solar radiation pressure. Objects simulated include large satellites, for which solar radiation pressure is insignificant, and small particles (a few microns in diameter), for which solar radiation pressure is an important force. Results for large satellites are largely in agreement with previous GEO studies that used classical perturbation techniques; orbital evolution studies were extended to possible storage orbits slightly above or below GEO. One notes that while the orbit planes of GEO satellites initially placed in equatorial orbits precess, so that those orbits reach inclinations of 14° to 15° to the equator, a “stable plane” exists inclined approximately 7.3° to the equator. The orbit planes of GEO satellites placed in such a stable plane orbit experience very little precession, remaining always within 1.2° of their initial orientation. Solar radiation pressure generates two major effects on small particles. One is an orbital eccentricity oscillation anticipated from previous research. The other is an oscillation in orbital inclination. This orbital inclination pattern is due to a precession of the small particle's orbital angular momentum vector about an axis offset from the Earth's polar axis. The magnitude of the precession axis offset angle depends on the particle's cross‐sectional area to mass ratio. The rate for this precession differs greatly from the precession rate predicted in a previous study using perturbation techniques. This difference points up the inadequacy of those perturbation techniques for orbits with large eccentricities. For one sequence of runs with small particles, Poynting‐Robertson drag was added to the simulation in order to slowly reduce the orbital semimajor axis and probe for possible orbital period resonances near the GEO distance. A significant resonance was found at the geosynchronous distance, where small grains are trapped into a 1:1 resonance with the Earth's daily rotation.

Corrections to the luni-solar precession derived from a compilation catalogue of extragalactic radio sources

Observation catalogues of extragalactic radio sources obtained by Very long Baseline Interferometry during the last decade agree in the mean to a few milliarcseconds (mas). Within this range the position differences show constant, linear and periodic offsets. To reduce the influence of individual catalogue properties the construction of a compilation catalogue seems to be the appropriate procedure. In some detail the compilation method is described providing simultaneous adjustment of source positions and catalogue corrections. The compilation catalogue consists of 40 objects having positional errors of 0.2 mas in right ascension (RA) and 0.3 mas in declination (Dec). Comparing this catalogue with the IERS Celestial Reference Frame compiled by means of other precepts yields weighted root-mean-square differences of 0.7 mas in RA and 1.3 mas in Dec. Finally, the terms of general precession in RA and Dec are included in the adjustment process giving estimates of the correction to the luni-solar precession between −1 and −3 mas/yr, the latter figure applying when some early data are added.

A geodynamo powered by luni-solar precession

Abstract Controlled spin and precession of liquid filled oblate spheroidal cavities in laboratory experiments that simulate luni-solar forced precession of the mantle and a simplified analytical liquid flow model, indicate Earth core flows approximated as spin about a core axis that lags behind the precessing spin axis of the mantle by a small angle (∼0·01°). Core spin is slower than mantle spin by an amount consistent with westward drift of the magnetic field (∼0·2°/yr). Theoretical and experimental evidence argue against inertia waves being effective in preventing such motion. This precessional flow model is consistent with geodynamo requirements indicating a kinematic over a magnetohydrodynamic dynamo in that viscous core-mantle coupling may dominate magnetic coupling, with only negligible topographic coupling. While the model appears to provide geodynamo requirements, it does not preclude simultaneous density gradient convective mechanisms.