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At least 199 records · Page 11Linked to original sources
PubMed1984

Terrestrial passage theory of the moon illusion.

Theories of the celestial, or moon, illusion have neglected geometric characteristics of movement along and above the surface of the earth. The illusion occurs because the characteristics of terrestrial passage are attributed to celestial passage. In terrestrial passage, the visual angle subtended by an object changes discriminably as an essentially invariant function of elevation above the horizon. In celestial passage, by contrast, change in visual angle is indiscriminable at all elevations. If a terrestrial object gains altitude, its angular subtense fails to follow the expansion projected for an orbital course: Angular diminution or constancy is equivalent to distancing. On the basis of terrestrial projections, a similar failure of celestial objects in successive elevations is also equivalent to distancing. The illusion occurs because of retinal image constancy, not--as traditionally stated--despite it.

Astronomical Phenomena↗
PubMed2002

The confinement of Neptune's ring arcs by the moon Galatea.

Neptune has five narrow ring arcs, spanning about 40 degrees in longitude, which are apparently confined against the rapid azimuthal and radial spreading that normally results from inter-particle collisions. A gravitational resonance based on the vertical motion of the nearby neptunian moon Galatea was proposed to explain the trapping of the ring particles into a sequence of arcs. But recent observations have indicated that the arcs are away from the resonance, leaving their stability again unexplained. Here we report that a resonance based on Galatea's eccentricity is responsible for the angular confinement of the arcs. The mass of the arcs affects the precession of Galatea's eccentric orbit, which will enable a mass estimate from future observations of Galatea's eccentricity.

Journal Article↗
PubMed2006

A giant impact origin for Pluto's small moons and satellite multiplicity in the Kuiper belt.

The two newly discovered satellites of Pluto (P1 and P2) have masses that are small compared to both Pluto and Charon-that is, between 5 x 10(-4) and 1 x 10(-5) of Pluto's mass, and between 5 x 10(-3) and 1 x 10(-4) of Charon's mass. This discovery, combined with the constraints on the absence of more distant satellites of Pluto, reveal that Pluto and its moons comprise an unusual, highly compact, quadruple system. These facts naturally raise the question of how this puzzling satellite system came to be. Here we show that P1 and P2's proximity to Pluto and Charon, the fact that P1 and P2 are on near-circular orbits in the same plane as Pluto's large satellite Charon, along with their apparent locations in or near high-order mean-motion resonances, all probably result from their being constructed from collisional ejecta that originated from the Pluto-Charon formation event. We also argue that dust-ice rings of variable optical depths form sporadically in the Pluto system, and that rich satellite systems may be found--perhaps frequently--around other large Kuiper belt objects.

Journal Article↗
PubMed1976

Did Ptolemy understand the moon illusion?

Ptolemy is often wrongly credited with an explanation of the moon illusion based on the size-distance invariance principle. This paper elucidates the two Ptolemaic accounts: one in the Almagest, based on atmospheric refraction, and the other in the Optics, based on the difficulty of looking upwards. It is the latter passage which has been thought to refer to size-distance invariance, but it is more probable that it refers to the idea that the visual rays are diminished by the force of gravity (i.e. that the retinal image is reduced in size). Alhazen was probably the first author to explain the illusion by the size-distance invariance principle, and Roger Bacon the first to explain the enlarged apparent distance of the horizon by the presence of intervening objects. Della Porta was the first to credit Ptolemy with these explanations, and this mistake was repeated by many subsequent authors.

Astronomy↗
PubMed1997

Visualization of Freezing Behaviors in Leaf and Flower Buds of Full-Moon Maple by Nuclear Magnetic Resonance Microscopy.

1H-Nuclear magnetic resonance (NMR) microscopy was used to study the freezing behavior of wintering buds of full-moon maple (Acer japonicum Thunb.). The images obtained predominantly reflected the density of mobile (i.e. non-ice) protons from unfrozen water. A comparison of NMR images taken at different subfreezing temperatures revealed which tissues produced high- and low-temperature exotherms in differential thermal analyses. In leaf and lower buds of A. japonicum, the scales and stem bark tissues were already frozen by -7[deg]C, but the primordial inflorescence and terminal primordial shoots remained supercooled at -14[deg]C, and the lateral primordial shoots were unfrozen even at -21[deg]C. The freezing of these supercooled tissues was associated with their loss of viability. The size of the supercooled primordial shoots and inflorescences was gradually reduced with decreasing temperature, indicating extraorgan freezing in these tissues. During this process the formation of dark regions beneath the primordia and subsequent gradual darkening in the basal part of supercooled primordia were visible. As the lateral shoot primordia were cooled, the unfrozen area was considerably reduced. Since the lateral primordia remained viable down to -40[deg]C, with no detectable low-temperature exotherms, they probably underwent type I extraorgan freezing. Deep supercooling in the xylem was clearly imaged. NMR microscopy is a powerful tool for noninvasively visualizing harmonized freezing behaviors in complex plant organs.

Journal Article↗
PubMed2005

Hf-W chronometry of lunar metals and the age and early differentiation of the Moon.

The use of hafnium-tungsten chronometry to date the Moon is hampered by cosmogenic tungsten-182 production mainly by neutron capture of tantalum-181 at the lunar surface. We report tungsten isotope data for lunar metals, which contain no 181Ta-derived cosmogenic 182W. The data reveal differences in indigenous 182W/184W of lunar mantle reservoirs, indicating crystallization of the lunar magma ocean 4.527 +/- 0.010 billion years ago. This age is consistent with the giant impact hypothesis and defines the completion of the major stage of Earth's accretion.

Journal Article↗
PubMed2007

Highly siderophile element constraints on accretion and differentiation of the Earth-Moon system.

A new combined rhenium-osmium- and platinum-group element data set for basalts from the Moon establishes that the basalts have uniformly low abundances of highly siderophile elements. The data set indicates a lunar mantle with long-term, chondritic, highly siderophile element ratios, but with absolute abundances that are over 20 times lower than those in Earth's mantle. The results are consistent with silicate-metal equilibrium during a giant impact and core formation in both bodies, followed by post-core-formation late accretion that replenished their mantles with highly siderophile elements. The lunar mantle experienced late accretion that was similar in composition to that of Earth but volumetrically less than (approximately 0.02% lunar mass) and terminated earlier than for Earth.

Journal Article↗
PubMed1996

Late Proterozoic and Paleozoic Tides, Retreat of the Moon, and Rotation of the Earth

The tidal rhythmites in the Proterozoic Big Cottonwood Formation (Utah, United States), the Neoproterozoic Elatina Formation of the Flinders Range (southern Australia), and the Lower Pennsylvanian Pottsville Formation (Alabama, United States) and Mansfield Formation (Indiana, United States) indicate that the rate of retreat of the lunar orbit is dxi/dt approximately k2 sin(2delta) (where xi is the Earth-moon radius vector, k2 is the tidal Love number, and delta is the tidal lag angle) and that this rate has been approximately constant since the late Precambrian. When the contribution to tidal friction from the sun is taken into account, these data imply that the length of the terrestrial day 900 million years ago was approximately18 hours.

Journal Article↗
PubMed1998

Improved gravity field of the moon from lunar prospector

An improved gravity model from Doppler tracking of the Lunar Prospector (LP) spacecraft reveals three new large mass concentrations (mascons) on the nearside of the moon beneath the impact basins Mare Humboltianum, Mendel-Ryberg, and Schiller-Zucchius, where the latter basin has no visible mare fill. Although there is no direct measurement of the lunar farside gravity, LP partially resolves four mascons in the large farside basins of Hertzsprung, Coulomb-Sarton, Freundlich-Sharonov, and Mare Moscoviense. The center of each of these basins contains a gravity maximum relative to the surrounding basin. The improved normalized polar moment of inertia (0.3932 +/- 0.0002) is consistent with an iron core with a radius of 220 to 450 kilometers.

Journal Article↗
PubMed1998

Major compositional units of the moon: lunar prospector thermal and fast neutrons

Global maps of thermal and fast neutron fluxes from the moon suggest three end-member compositional units. A high thermal and low fast neutron flux unit correlates with the lunar highlands and is consistent with feldspathic rocks. The South Pole-Aitken basin and a strip that surrounds the nearside maria have intermediate thermal and fast neutron flux levels, consistent with more mafic rocks. There appears to be a smooth transition between the most mafic and feldspathic compositions, which correspond to low and high surface altitudes, respectively. The maria show low thermal and high fast neutron fluxes, consistent with basaltic rocks.

Journal Article↗
PubMed2002

Speciation and phylogeography in the cosmopolitan marine moon jelly, Aurelia sp.

BACKGROUND: The cosmopolitan moon jelly Aurelia is characterized by high degrees of morphological and ecological plasticity, and subsequently by an unclear taxonomic status. The latter has been revised repeatedly over the last century, dividing the genus Aurelia in as many as 12 or as little as two species. We used molecular data and phenotypic traits to unravel speciation processes and phylogeographic patterns in Aurelia. RESULTS: Mitochondrial and nuclear DNA data (16S and ITS-1/5.8S rDNA) from 66 world-wide sampled specimens reveal star-like tree topologies, unambiguously differentiating 7 (mtDNA) and 8 (ncDNA) genetic entities with sequence divergences ranging from 7.8 to 14% (mtDNA) and 5 to 32% (ncDNA), respectively. Phylogenetic patterns strongly suggest historic speciation events and the reconstruction of at least 7 different species within Aurelia. Both genetic divergences and life history traits showed associations to environmental factors, suggesting ecological differentiation forced by divergent selection. Hybridization and introgression between Aurelia lineages likely occurred due to secondary contacts, which, however, did not disrupt the unambiguousness of genetic separation. CONCLUSIONS: Our findings recommend Aurelia as a model system for using the combined power of organismic, ecological, and molecular data to unravel speciation processes in cosmopolitan marine organisms.

Animals↗
PubMed1992

Anisotropic perception of visual angle: implications for the horizontal-vertical illusion, overconstancy of size, and the moon illusion.

Three experiments investigated anisotropic perception of visual angle outdoors. In Experiment 1, scales for vertical and horizontal visual angles ranging from 20 degrees to 80 degrees were constructed with the method of angle production (in which the subject reproduced a visual angle with a protractor) and the method of distance production (in which the subject produced a visual angle by adjusting viewing distance). In Experiment 2, scales for vertical and horizontal visual angles of 5 degrees-30 degrees were constructed with the method of angle production and were compared with scales for orientation in the frontal plane. In Experiment 3, vertical and horizontal visual angles of 3 degrees-80 degrees were judged with the method of verbal estimation. The main results of the experiments were as follows: (1) The obtained angles for visual angle are described by a quadratic equation, theta' = a + b theta + c theta 2 (where theta is the visual angle; theta', the obtained angle; a, b, and c, constants). (2) The linear coefficient b is larger than unity and is steeper for vertical direction than for horizontal direction. (3) The quadratic coefficient c is generally smaller than zero and is negatively larger for vertical direction than for horizontal direction. And (4) the obtained angle for visual angle is larger than that for orientation. From these results, it was possible to predict the horizontal-vertical illusion, over-constancy of size, and the moon illusion.

Adult↗
PubMed2001

Challenges and approaches to the robotic detection of enantioenrichment on Saturn's moon, Titan.

Saturn's moon, Titan, is one of the most interesting locations for organic chemistry in the solar system. As a possible follow up to the Cassini-Huygens mission, which will conduct the first in situ analysis of the Titan atmosphere in 2004, studies are underway for missions to explore in detail the chemical composition of Titan's atmosphere, surface, and oceans. In order to seek out and explore complex chemical systems that may represent steps on the pathway to life, instruments for detection of enantioenrichment should be included on such a mission. The formidable challenges of robotic measurement of enantioenrichment on Titan are summarized, and experimental approaches to this problem are reviewed. In addition, some speculations are offered concerning locations on Titan where complex fractionation of organic materials may have occurred.

Journal Article↗