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Toroidal Planet
A toroidal planet is a hypothetical type of terrestrial planet, telluric exoplanet with a Torus, toroidal or doughnut shape. While no firm theoretical understanding as to how toroidal planets could Planetary formation, form naturally is necessarily known, the shape itself is potentially wiktionary:quasistable, quasistable, and is analogous to the physical parameters of a speculatively constructible megastructure in self-suspension, such as a Dyson sphere, ringworld, Stanford torus or Bishop Ring (habitat), Bishop Ring. Physical description At sufficiently large enough scales, rigid matter such as the typical silicate-ferrous composition of rocky planets Lane–Emden equation, behaves fluidly, and satisfies the condition for evaluating the mechanics of toroidal self-gravitating fluid bodies in context. A rotating mass in the form of a torus allows an effective balance between the gravitational attraction and the force due to centrifugal acceleration, when the angular momentum is ad ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Meridian (geography)
In geography and geodesy, a meridian is the locus connecting points of equal longitude, which is the angle (in degrees or other units) east or west of a given prime meridian (currently, the IERS Reference Meridian). In other words, it is a coordinate line for longitudes, a line of longitude. The position of a point along the meridian at a given longitude is given by its latitude, measured in angular degrees north or south of the Equator. On a Mercator projection or on a Gall-Peters projection, each meridian is perpendicular to all circles of latitude. Assuming a spherical Earth, a meridian is a great semicircle on Earth's surface. Adopting instead a spheroidal or ellipsoid model of Earth, the meridian is half of a north-south great ellipse. The length of a meridian is twice the length of an Earth quadrant, equal to on a modern ellipsoid ( WGS 84). Pre-Greenwich The first prime meridian was set by Eratosthenes in 200 BC. This prime meridian was used to provide mea ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Synestia
A synestia is a hypothesized rapidly spinning doughnut-shaped mass of vaporized rock. The term was coined in 2017 by Sarah T. Stewart-Mukhopadhyay, taken from Hestia, goddess of the hearth, combined with ''syn-'' meaning together. In computer simulations of giant impacts of rotating objects, a synestia can form if the total angular momentum is greater than the co-rotational limit. Beyond the co-rotational limit, the velocity at the equator of a body would exceed the orbital velocity. In the case of a ''synestia'', the result is an inner region rotating at a single rate with a loosely connected torus orbiting beyond it. Synestias also have differences in the mantles, both thermally and in their composition, from previous terrestrial evolution models due partially to a lower interior pressure. Composition A synestia is composed of three primary components: the innermost area called the corotating region, a middle area called the transition region, and the area farthest out, known ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Poisson's Equation
Poisson's equation is an elliptic partial differential equation of broad utility in theoretical physics. For example, the solution to Poisson's equation is the potential field caused by a given electric charge or mass density distribution; with the potential field known, one can then calculate the corresponding electrostatic or gravitational (force) field. It is a generalization of Laplace's equation, which is also frequently seen in physics. The equation is named after French mathematician and physicist Siméon Denis Poisson who published it in 1823. Statement of the equation Poisson's equation is \Delta\varphi = f, where \Delta is the Laplace operator, and f and \varphi are real or complex-valued functions on a manifold. Usually, f is given, and \varphi is sought. When the manifold is Euclidean space, the Laplace operator is often denoted as , and so Poisson's equation is frequently written as \nabla^2 \varphi = f. In three-dimensional Cartesian coordinates, it takes the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hubble Volume
In cosmology, a Hubble volume (named for the astronomer Edwin Hubble) or Hubble sphere, subluminal sphere, causal sphere and sphere of causality is a spherical region of the observable universe surrounding an observer beyond which objects recede from that observer at a rate greater than the speed of light due to the expansion of the universe. The Hubble volume is approximately equal to 1031 cubic light years (or about 1079 cubic meters). The proper radius of a Hubble sphere (known as the Hubble radius or the Hubble length) is c/H_0, where c is the speed of light and H_0 is the Hubble constant. The surface of a Hubble sphere is called the ''microphysical horizon'', the ''Hubble surface'', or the ''Hubble limit''. More generally, the term ''Hubble volume'' can be applied to any region of space with a volume of order (c/H_0)^3. However, the term is also frequently (but mistakenly) used as a synonym for the observable universe; the latter is larger than the Hubble volume.For a di ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Particle Horizon
The particle horizon (also called the cosmological horizon, the comoving horizon (in Scott Dodelson's text), or the cosmic light horizon) is the maximum distance from which light from particles could have traveled to the observer in the age of the universe. Much like the concept of a terrestrial horizon, it represents the boundary between the observable and the unobservable regions of the universe, so its distance at the present epoch defines the size of the observable universe. Due to the expansion of the universe, it is not simply the age of the universe times the speed of light (approximately 13.8 billion light-years), but rather the speed of light times the conformal time. The existence, properties, and significance of a cosmological horizon depend on the particular cosmological model. Kinematic model The particle horizon is a distance in a comoving coordinate system, a system that has the expansion of the universe built-in. The expansion is defined by a (dimensionless ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Giant-impact Hypothesis
The giant-impact hypothesis, sometimes called the Theia Impact, is an astrogeology hypothesis for the formation of the Moon first proposed in 1946 by Canadian geologist Reginald Daly. The hypothesis suggests that the Early Earth collided with a Mars-sized protoplanet of the same orbit approximately 4.5 billion years ago in the early Hadean eon (about 20 to 100 million years after the Solar System coalesced), and the ejecta of the impact event later accreted to form the Moon. The impactor planet is sometimes called Theia, named after the mythical Greek Titan who was the mother of Selene, the goddess of the Moon. Analysis of lunar rocks published in a 2016 report suggests that the impact might have been a direct hit, causing a fragmentation and thorough mixing of both parent bodies. The giant-impact hypothesis is currently the favored hypothesis for lunar formation among astronomers. Evidence that supports this hypothesis includes: * The Moon's orbit has a sim ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sarah T
Sarah (born Sarai) is a Patriarchs (Bible)#Matriarchs, biblical matriarch, prophet, and major figure in Abrahamic religions. While different Abrahamic faiths portray her differently, Judaism, Christianity, and Islam all depict her character similarly, as that of a Piety, pious woman, renowned for her hospitality and beauty, the wife of Abraham, and the mother of Isaac. Sarah has her Calendar of saints, feast day on 1 September in the Catholic Church, 19 August in the Coptic Orthodox Church, 20 January in the Lutheran Church–Missouri Synod, LCMS, and 12 and 20 December in the Eastern Orthodox Church. In the Hebrew Bible Family According to Book of Genesis 20:12, in conversation with the Philistines, Philistine king Abimelech, Abimelech of Gerar, Abraham describes Sarah as both his wife and his half-sister ("my father's daughter, but not my mother's"). Such unions were later explicitly banned in the Book of Leviticus (). However, some commentators identify her as Iscah (Genesis ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Synestia
A synestia is a hypothesized rapidly spinning doughnut-shaped mass of vaporized rock. The term was coined in 2017 by Sarah T. Stewart-Mukhopadhyay, taken from Hestia, goddess of the hearth, combined with ''syn-'' meaning together. In computer simulations of giant impacts of rotating objects, a synestia can form if the total angular momentum is greater than the co-rotational limit. Beyond the co-rotational limit, the velocity at the equator of a body would exceed the orbital velocity. In the case of a ''synestia'', the result is an inner region rotating at a single rate with a loosely connected torus orbiting beyond it. Synestias also have differences in the mantles, both thermally and in their composition, from previous terrestrial evolution models due partially to a lower interior pressure. Composition A synestia is composed of three primary components: the innermost area called the corotating region, a middle area called the transition region, and the area farthest out, known ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Protoplanetary Disk
A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may not be considered an accretion disk; while the two are similar, an accretion disk is hotter and spins much faster; it is also found on black holes, not stars. This process should not be confused with the accretion process thought to build up the planets themselves. Externally illuminated photo-evaporating protoplanetary disks are called proplyds. Formation Protostars form from molecular clouds consisting primarily of molecular hydrogen. When a portion of a molecular cloud reaches a critical size, mass, or density, it begins to collapse under its own gravity. As this collapsing cloud, called a solar nebula, becomes denser, random gas motions originally present in the cloud average out in favor of the direction of the nebula's net angular momentum. Conservation of angular momentum causes the r ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Isostatic Equilibrium
Isostasy (Greek ''ísos'' 'equal', ''stásis'' 'standstill') or isostatic equilibrium is the state of gravitational equilibrium between Earth's crust (or lithosphere) and mantle such that the crust "floats" at an elevation that depends on its thickness and density. This concept is invoked to explain how different topographic heights can exist at Earth's surface. Although originally defined in terms of continental crust and mantle, it has subsequently been interpreted in terms of lithosphere and asthenosphere, particularly with respect to oceanic island volcanoes, such as the Hawaiian Islands. Although Earth is a dynamic system that responds to loads in many different ways, isostasy describes the important limiting case in which crust and mantle are in static equilibrium. Certain areas (such as the Himalayas and other convergent margins) are not in isostatic equilibrium and are not well described by isostatic models. The general term ''isostasy'' was coined in 1882 by the Amer ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Tectonic Plates
Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed during the first decades of the 20th century. Plate tectonics came to be accepted by geoscientists after seafloor spreading was validated in the mid-to-late 1960s. The processes that result in plates and shape Earth's crust are called ''tectonics''. Tectonic plates also occur in other planets and moons. Earth's lithosphere, the rigid outer shell of the planet including the crust and upper mantle, is fractured into seven or eight major plates (depending on how they are defined) and many minor plates or "platelets". Where the plates meet, their relative motion determines the type of plate boundary (or fault): , , or . The relative movement of the plates typically ranges from zero to 10 cm annually. Faults tend to be geologically active, ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
