187 research outputs found
The discovery circumstances of Earth-approaching asteroids
The discovery circumstances are analyzed for all Earth-approaching asteroids detected in the last twenty-four years. In particular, topocentric angular velocities, opposition distance, geocentric and heliocentric distances, phase angle, and lunar phase at discover were calculated in an effort to separate any selection effects between chance and purposeful (i.e., as the result of a systematic search) discoveries. Another motivation was the possibility of discerning useful clues how to search more efficiently for such objects. There are 60 minor planets in the sample. The principal result is that the discovery of Earth-approaching asteroids is dominated by serendipity. Therefore, searching for them at the current relatively bright limits at less than a very high rate seems pointless
Gamma ray burst astrometry 2: Numerical tests
Since the announcement of the discovery of sources of gamma ray radiation in 1973, many more reports of such bursts have been published. Numerous artificial satellites have been equipped with gamma ray detectors including GRO. Unfortunately, almost no progress has been made in identifying the sources of this high energy radiation. Only one visible counterpart is known. It is suspected that this is a consequence of the methods currently used to define gamma ray burst source 'error boxes'. An alternative procedure was proposed in 1988 by Taff. Herein, Monte Carlo simulations are reported of the efficacy of this technique using realistic burst timing uncertainties and satellite location errors as well as a variety of satellite constellations. Since these are controlled numerical experiments, the dependence is examined of the statistics of the errors in the deduced burst wavefront normal as a function of the timing inconsistencies, detector location standard deviations, and especially the number and distribution of the detectors. The results clearly show that an arc minute prediction of a unique burst location is routinely obtainable once there are at least two interplanetary detectors
FINANCIAL PERFORMANCE OF SELECTED GRASS-BASED FARMING SYSTEMS IN THE NORTHERN TALLGRASS PRAIRIE REGION
We develop a series of new grass-based budgets for use in FINPACK, a standard farm finance analysis tool. The new budgets are then applied to an evaluation of plausible grass-based systems on three farms in the Northern Tallgrass Prairie region of Minnesota and North Dakota. The farms are used for illustration purposes only. We find no grass-based alternative that financially outperforms current non-grass operations. Nor do any score particularly well under a lender credit rating process that we adapted for this study. Our results suggest caution before one accepts claims that grass-based systems are uniformly feasible and financially desirable. There undoubtedly are individual situations in which such alternatives make financial sense, but their use may have to be accompanied by income supplements from external sources in order to further wide-spread adoption." (p.3)Agricultural Finance,
Osculating orbits in Schwarzschild spacetime, with an application to extreme mass-ratio inspirals
We present a method to integrate the equations of motion that govern bound,
accelerated orbits in Schwarzschild spacetime. At each instant the true
worldline is assumed to lie tangent to a reference geodesic, called an
osculating orbit, such that the worldline evolves smoothly from one such
geodesic to the next. Because a geodesic is uniquely identified by a set of
constant orbital elements, the transition between osculating orbits corresponds
to an evolution of the elements. In this paper we derive the evolution
equations for a convenient set of orbital elements, assuming that the force
acts only within the orbital plane; this is the only restriction that we impose
on the formalism, and we do not assume that the force must be small. As an
application of our method, we analyze the relative motion of two massive
bodies, assuming that one body is much smaller than the other. Using the hybrid
Schwarzschild/post-Newtonian equations of motion formulated by Kidder, Will,
and Wiseman, we treat the unperturbed motion as geodesic in a Schwarzschild
spacetime whose mass parameter is equal to the system's total mass. The force
then consists of terms that depend on the system's reduced mass. We highlight
the importance of conservative terms in this force, which cause significant
long-term changes in the time-dependence and phase of the relative orbit. From
our results we infer some general limitations of the radiative approximation to
the gravitational self-force, which uses only the dissipative terms in the
force.Comment: 18 pages, 6 figures, final version to be published in Physical Review
Occupational engagement: some assumptions to inform occupational therapy
Occupational engagement has been conflated with other concepts, such as participation, active occupational performance, and therapeutic engagement. This critical review will discuss occupational engagement as a unique concept that describes a form of involvement in doing that does not require performance and foregrounds the subjective-affective and cognitive experiences of doing. We present some attributes of occupational engagement and the implications for the lack of clarity of this concept within the literature. The difference between occupational engagement and therapeutic engagement is discussed by comparing the foundational tenets of the Model of Human Occupation (MOHO) and the Canadian Model of Occupational Performance and Engagement (CMOP-E). Through one illustrative narrative, we discuss how occupational engagement can be understood as a phenomenon that is not performance-dependent and has different levels of engagement. To conclude, we point out some assumptions about occupational engagement that can inform occupational therapy research and practice
The Use of Mentored Inquiry Communities to Foster Scholarship of Teaching and Learning (SoTL): Outcomes of the American Occupational Therapy Association SoTL Program (2007-2017)
Scholarship of teaching and learning (SoTL) is recognized in Boyer’s model, along with discovery, integration, and application, as one of four essential scholarships and is the systematic study of teaching and learning. As such, in 2007, the American Occupational Therapy Foundation (AOTF), and later the American Occupational Therapy Association (AOTA), supported the development of a SoTL mentorship program in an effort to foster SoTL within the discipline of occupational therapy. This mixed-methods study describes AOTA SoTL Program outcomes across ten years, 2007-2017, gathered via survey of past program mentee (n = 50) and mentor (n = 12) participants. Results suggest that mentored inquiry communities are a useful approach to fostering SoTL collaborations. In the future, the AOTA SoTL Program and similar disciplinary-specific SoTL programs should consider additional strategies for supporting mentee participants to completion with their projects
The Large Quasar Reference Frame (LQRF) - an optical representation of the ICRS
The large number and all-sky distribution of quasars from different surveys,
along with their presence in large, deep astrometric catalogs,enables the
building of an optical materialization of the ICRS following its defining
principles. Namely: that it is kinematically non-rotating with respect to the
ensemble of distant extragalactic objects; aligned with the mean equator and
dynamical equinox of J2000; and realized by a list of adopted coordinates of
extragalatic sources. Starting from the updated and presumably complete LQAC
list of QSOs, the initial optical positions of those quasars are found in the
USNO B1.0 and GSC2.3 catalogs, and from the SDSS DR5. The initial positions are
next placed onto UCAC2-based reference frames, following by an alignment with
the ICRF, to which were added the most precise sources from the VLBA calibrator
list and the VLA calibrator list - when reliable optical counterparts exist.
Finally, the LQRF axes are inspected through spherical harmonics, contemplating
to define right ascension, declination and magnitude terms. The LQRF contains
J2000 referred equatorial coordinates for 100,165 quasars, well represented
across the sky, from -83.5 to +88.5 degrees in declination, and with 10 arcmin
being the average distance between adjacent elements. The global alignment with
the ICRF is 1.5 mas, and the individual position accuracies are represented by
a Poisson distribution that peaks at 139 mas in right ascension and 130 mas in
declination. It is complemented by redshift and photometry information from the
LQAC. The LQRF is designed to be an astrometric frame, but it is also the basis
for the GAIA mission initial quasars' list, and can be used as a test bench for
quasars' space distribution and luminosity function studies.Comment: 23 pages, 23 figures, 6 tables Accepted for publication by Astronomy
& Astrophysics, on 25 May 200
Tidal torques. A critical review of some techniques
We point out that the MacDonald formula for body-tide torques is valid only
in the zeroth order of e/Q, while its time-average is valid in the first order.
So the formula cannot be used for analysis in higher orders of e/Q. This
necessitates corrections in the theory of tidal despinning and libration
damping.
We prove that when the inclination is low and phase lags are linear in
frequency, the Kaula series is equivalent to a corrected version of the
MacDonald method. The correction to MacDonald's approach would be to set the
phase lag of the integral bulge proportional to the instantaneous frequency.
The equivalence of descriptions gets violated by a nonlinear
frequency-dependence of the lag.
We explain that both the MacDonald- and Darwin-torque-based derivations of
the popular formula for the tidal despinning rate are limited to low
inclinations and to the phase lags being linear in frequency. The
Darwin-torque-based derivation, though, is general enough to accommodate both a
finite inclination and the actual rheology.
Although rheologies with Q scaling as the frequency to a positive power make
the torque diverge at a zero frequency, this reveals not the impossible nature
of the rheology, but a flaw in mathematics, i.e., a common misassumption that
damping merely provides lags to the terms of the Fourier series for the tidal
potential. A hydrodynamical treatment (Darwin 1879) had demonstrated that the
magnitudes of the terms, too, get changed. Reinstating of this detail tames the
infinities and rehabilitates the "impossible" scaling law (which happens to be
the actual law the terrestrial planets obey at low frequencies).Comment: arXiv admin note: sections 4 and 9 of this paper contain substantial
text overlap with arXiv:0712.105
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