7,011 research outputs found
Measuring the eccentricity of the Earth orbit with a nail and a piece of plywood
I describe how to obtain a rather good experimental determination of the
eccentricity of the Earth orbit, as well as the obliquity of the Earth rotation
axis, by measuring, over the course of a year, the elevation of the Sun as a
function of time during a day. With a very simple "instrument" consisting of an
elementary sundial, first-year students can carry out an appealing measurement
programme, learn important concepts in experimental physics, see concrete
applications of kinematics and changes of reference frames, and benefit from a
hands-on introduction to astronomy.Comment: 12 pages, 6 figure
Importance of an Astrophysical Perspective for Textbook Relativity
The importance of a teaching a clear definition of the ``observer'' in
special relativity is highlighted using a simple astrophysical example from the
exciting current research area of ``Gamma-Ray Burst'' astrophysics. The example
shows that a source moving relativistically toward a single observer at rest
exhibits a time ``contraction'' rather than a ``dilation'' because the light
travel time between the source and observer decreases with time. Astrophysical
applications of special relativity complement idealized examples with real
applications and very effectively exemplify the role of a finite light travel
time.Comment: 5 pages TeX, European Journal of Physics, in pres
Delineation of the forest-tundra ecotone using texture-based classification of satellite imagery
© 2020, © 2020 Informa UK Limited, trading as Taylor & Francis Group. The transition zone between the boreal forest and Arctic tundra, the forest-tundra ecotone (FTE), is an area of high ecological and climatological significance. Despite its importance, a globally consistent high spatial resolution mapping is lacking. Accurate mapping of the FTE requires the use of satellite remote sensing data. Here we use the Landsat Vegetation Continuous Fields (VCF) product and reference point data to derive the location and characteristics of the FTE. An image texture-based supervised classification scheme is developed based on a study area in Central Eurasia to statistically exploit the spatial patterns of the transition zone. Texture statistics for the VCF image are derived from the grey-level co-occurrence matrix (GLCM) based on which the study area is classified into forest, tundra, and FTEs. Adaptive parameterization is implemented to achieve optimal classification performance in the study area. This method is further applied to six additional study areas around the circumarctic region to test its adaptability. In all study areas, this method achieves better FTE delineation results than previously reported methods, showing better classification accuracies (average of 0.826) and more realistic and complete representation of the FTE as shown by visual examination. This shows the universal applicability of the method and it is potential to be used to achieve more detailed and accurate circumarctic mapping of the FTE, which could serve as the basis of time series analysis of FTE positions, eventually contributing to a better understanding of the inter-relations between climate change and shifts in sub-arctic vegetation.Grant no. 260400/E10 and 244557/RI, Research Council of Norwa
Seventy years of sex education in Health Education Journal: a critical review
This paper examines key debates and perspectives on sex education in Health Education Journal (HEJ), from the date of the journalâs first publication in March 1943 to the present day. Matters relating to sexuality and sexual health are revealed to be integral to HEJâs history. First published as Health and Empire (1921 â 1942), a key purpose of the journal since its inception has been to share information on venereal disease and its prevention within the UK and across the former British Empire. From 1943 to the present day, discussions on sex education in the newly-christened HEJ both reflect and respond to evolving socio-cultural attitudes towards sexuality in the UK. Changing definitions of sex education across the decades are examined, from the prevention of venereal disease and moral decline in war-time Britain in the 1940s, to a range of responses to sexual liberation in the 1960s and 1970s; from a focus on preventing sexually-transmitted infections, teenage pregnancy and HIV in the 1980s, to the provision of sexual health services alongside sex education in the 2000s. Over the past 70 years, a shift from prevention of pre-marital sexual activity to the management of its outcomes is apparent; however, while these changes over time are notable, perhaps the most striking findings of this review are the continuities in arguments for and against the discussion of sexual issues. After more than 70 years of debate, it would seem that there is little consensus concerning motivations for and the content of sex education
Constraining Alternate Models of Black Holes: Type I X-ray Bursts on Accreting Fermion-Fermion and Boson-Fermion Stars
The existence of black holes remains open to doubt until other conceivable
options are excluded. With this motivation, we consider a model of a compact
star in which most of the mass consists of dark particles of some kind, and a
small fraction of the mass is in the form of ordinary nucleonic gas. The gas
does not interact with the dark matter other than via gravity, but collects at
the center as a separate fermionic fluid component. Depending on whether the
dark mass is made of fermions or bosons, the objects may be called
fermion-fermion stars or boson-fermion stars, respectively. For appropriate
choices of the mass of the dark matter particles, these objects are viable
models of black hole candidates in X-ray binaries. We consider models with a
dark mass of 10 solar masses and a range of gas mass from 10^{-6} to nearly one
solar mass, and analyse the bursting properties of the models when they accrete
gas. We show that all the models would experience thermonuclear Type I X-ray
bursts at appropriate mass accretion rates. Since no Type I bursts have been
reported from black hole candidates, the models are ruled out. The case for
identifying black hole candidates in X-ray binaries as true black holes is thus
strengthened.Comment: 29 pages, 7 figures, to appear in The Astrophysical Journa
Research in Progress: Evaluating the Role of Risk Models in Information Assurance
Traditionally, organizations have approached the protection of valuable assets from a risk management perspective and applied a variety of models to protect the organization from losses. The intangible nature of information and the unknown threats to such information assets have given rise to questions concerning whether traditional approaches to risk management are sufficient for the domain of information assurance. This paper discusses the issues of applying traditional risk models to the domain of information assurance and proposes a focus group approach to determine what approaches to risk management are actually being used in practice and whether traditional risk management models are appropriate for information assurance
Semi-Analytical Models for Lensing by Dark Halos: I. Splitting Angles
We use the semi-analytical approach to analyze gravitational lensing of
quasars by dark halos in various cold dark matter (CDM) cosmologies, in order
to determine the sensitivity of the prediction probabilities of images
separations to the input assumptions regarding halos and cosmologies. The mass
function of dark halos is assumed to be given by the Press-Schechter function.
The mass density profile of dark halos is alternatively taken to be the
singular isothermal sphere (SIS), the Navarro-Frenk-White (NFW) profile, or the
generalized NFW profile. The cosmologies include: the Einstein-de Sitter model
(SCDM), the open model (OCDM), and the flat \Lambda-model (LCDM). As expected,
we find that the lensing probability is extremely sensitive to the mass density
profile of dark halos, and somewhat less so to the mean mass density in the
universe, and the amplitude of primordial fluctuations. NFW halos are very much
less effective in producing multiple images than SIS halos. However, none of
these models can completely explain the current observations: the SIS models
predict too many large splitting lenses, while the NFW models predict too few
small splitting lenses. This indicates that there must be at least two
populations of halos in the universe. A combination of SIS and NFW halos can
reasonably reproduce the current observations if we choose the mass for the
transition from SIS to NFW to be ~ 10^{13} solar masses. Additionally, there is
a tendency for CDM models to have too much power on small scales, i.e. too much
mass concentration; and it appears that the cures proposed for other apparent
difficulties of CDM would help here as well, an example being the warm dark
matter (WDM) variant which is shown to produce large splitting lenses fewer
than the corresponding CDM model by one order of magnitude.Comment: 46 pages, including 13 figures. Revised version with significant
improvemen
The Expected Rate of Gamma-Ray Burst Afterglows In Supernova Searches
We predict the rate at which Gamma-Ray Burst (GRB) afterglows should be
detected in supernova searches as a function of limiting flux. Although GRB
afterglows are rarer than supernovae, they are detectable at greater distances
because of their higher intrinsic luminosity. Assuming that GRBs trace the
cosmic star formation history and that every GRB gives rise to a bright
afterglow, we find that the average detection rate of supernovae and afterglows
should be comparable at limiting magnitudes brighter than K=18. The actual rate
of afterglows is expected to be somewhat lower since only a fraction of all
gamma-ray selected GRBs were observed to have associated afterglows. However,
the rate could also be higher if the initial gamma-ray emission from GRB
sources is more beamed than their late afterglow emission. Hence, current and
future supernova searches can place strong constraints on the afterglow
appearance fraction and the initial beaming angle of GRB sources.Comment: 13 pages, submitted to ApJ
Quasi-thermal Comptonization and gamma-ray bursts
Quasi-thermal Comptonization in internal shocks formed between relativistic
shells can account for the high energy emission of gamma-ray bursts. This is in
fact the dominant cooling mechanism if the typical energy of the emitting
particles is achieved either through the balance between heating and cooling or
as a result of electron-positron pair production. Both processes yield sub or
mildly relativistic energies. In this case the synchrotron spectrum is
self-absorbed, providing the seed soft photons for the Comptonization process,
whose spectrum is flat [F(v) ~ const], ending either in an exponential cutoff
or a Wien peak, depending on the scattering optical depth of the emitting
particles. Self-consistent particle energy and optical depth are estimated and
found in agreement with the observed spectra.Comment: 10 pages, ApJ Letters, accepted for publicatio
Dynamics of Cortical Degeneration Over a Decade in Huntington's Disease
BACKGROUND: Characterizing changing brain structure in neurodegeneration is fundamental to understanding longterm effects of pathology and ultimately providing therapeutic targets. It is well established that Huntingtonâs disease
(HD) gene carriers undergo progressive brain changes during the course of disease, yet the long-term trajectory of
cortical atrophy is not well defined. Given that genetic therapies currently tested in HD are primarily expected to
target the cortex, understanding atrophy across this region is essential.
METHODS: Capitalizing on a unique longitudinal dataset with a minimum of 3 and maximum of 7 brain scans from 49
HD gene carriers and 49 age-matched control subjects, we implemented a novel dynamical systems approach to
infer patterns of regional neurodegeneration over 10 years. We use Bayesian hierarchical modeling to map
participant- and group-level trajectories of atrophy spatially and temporally, additionally relating atrophy to the
genetic marker of HD (CAG-repeat length) and motor and cognitive symptoms.
RESULTS: We show, for the first time, that neurodegenerative changes exhibit complex temporal dynamics with
substantial regional variation around the point of clinical diagnosis. Although widespread group differences were seen
across the cortex, the occipital and parietal regions undergo the greatest rate of cortical atrophy. We have established
links between atrophy and genetic markers of HD while demonstrating that specific cortical changes predict decline in
motor and cognitive performance.
CONCLUSIONS: HD gene carriers display regional variability in the spatial pattern of cortical atrophy, which relates to
genetic factors and motor and cognitive symptoms. Our findings indicate a complex pattern of neuronal loss, which
enables greater characterization of HD progression
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