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Topic Review
Biography
Topic Review
Equinox (Celestial Coordinates)
In astronomy, an equinox is either of two places on the celestial sphere at which the ecliptic intersects the celestial equator. Although there are two intersections of the ecliptic with the celestial equator, by convention, the equinox associated with the Sun's ascending node is used as the origin of celestial coordinate systems and referred to simply as "the equinox". In contrast to the common usage of spring/vernal and autumnal equinoxes, the celestial coordinate system equinox is a direction in space rather than a moment in time. In a cycle of about 25,800 years, the equinox moves westward with respect to the celestial sphere because of perturbing forces; therefore, in order to define a coordinate system, it is necessary to specify the date for which the equinox is chosen. This date should not be confused with the epoch. Astronomical objects show real movements such as orbital and proper motions, and the epoch defines the date for which the position of an object applies. Therefore, a complete specification of the coordinates for an astronomical object requires both the date of the equinox and of the epoch. The currently used standard equinox and epoch is J2000.0, which is January 1, 2000 at 12:00 TT. The prefix "J" indicates that it is a Julian epoch. The previous standard equinox and epoch was B1950.0, with the prefix "B" indicating it was a Besselian epoch. Before 1984 Besselian equinoxes and epochs were used. Since that time Julian equinoxes and epochs have been used.
689
02 Dec 2022
Topic Review
Equatorial Coordinate System
The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the centre of Earth, a fundamental plane consisting of the projection of Earth's equator onto the celestial sphere (forming the celestial equator), a primary direction towards the vernal equinox, and a right-handed convention. The origin at the centre of Earth means the coordinates are geocentric, that is, as seen from the centre of Earth as if it were transparent. The fundamental plane and the primary direction mean that the coordinate system, while aligned with Earth's equator and pole, does not rotate with the Earth, but remains relatively fixed against the background stars. A right-handed convention means that coordinates increase northward from and eastward around the fundamental plane.
1.3K
16 Nov 2022
Topic Review
Epoch
In astronomy, an epoch or reference epoch is a moment in time used as a reference point for some time-varying astronomical quantity. It is useful for the celestial coordinates or orbital elements of a celestial body, as they are subject to perturbations and vary with time. These time-varying astronomical quantities might include, for example, the mean longitude or mean anomaly of a body, the node of its orbit relative to a reference plane, the direction of the apogee or aphelion of its orbit, or the size of the major axis of its orbit. The main use of astronomical quantities specified in this way is to calculate other relevant parameters of motion, in order to predict future positions and velocities. The applied tools of the disciplines of celestial mechanics or its subfield orbital mechanics (for predicting orbital paths and positions for bodies in motion under the gravitational effects of other bodies) can be used to generate an ephemeris, a table of values giving the positions and velocities of astronomical objects in the sky at a given time or times. Astronomical quantities can be specified in any of several ways, for example, as a polynomial function of the time-interval, with an epoch as a temporal point of origin (this is a common current way of using an epoch). Alternatively, the time-varying astronomical quantity can be expressed as a constant, equal to the measure that it had at the epoch, leaving its variation over time to be specified in some other way—for example, by a table, as was common during the 17th and 18th centuries. The word epoch was often used in a different way in older astronomical literature, e.g. during the 18th century, in connection with astronomical tables. At that time, it was customary to denote as "epochs", not the standard date and time of origin for time-varying astronomical quantities, but rather the values at that date and time of those time-varying quantities themselves. In accordance with that alternative historical usage, an expression such as 'correcting the epochs' would refer to the adjustment, usually by a small amount, of the values of the tabulated astronomical quantities applicable to a fixed standard date and time of reference (and not, as might be expected from current usage, to a change from one date and time of reference to a different date and time).
644
22 Nov 2022
Topic Review
EnVision (Spacecraft)
EnVision is a proposed orbital mission to Venus that would perform high-resolution radar mapping and atmospheric studies. The mission would help scientists understand the relationships between its geological activity and the atmosphere, and it would investigate why Venus and Earth took such different evolutionary paths. The mission is studied by ESA in collaboration with NASA, with the potential sharing of responsibilities currently under assessment.
347
27 Oct 2022
Topic Review
Peer Reviewed
Entropy
The concept of entropy constitutes, together with energy, a cornerstone of contemporary physics and related areas. It was originally introduced by Clausius in 1865 along abstract lines focusing on thermodynamical irreversibility of macroscopic physical processes. In the next decade, Boltzmann made the genius connection—further developed by Gibbs—of the entropy with the microscopic world, which led to the formulation of a new and impressively successful physical theory, thereafter named statistical mechanics. The extension to quantum mechanical systems was formalized by von Neumann in 1927, and the connections with the theory of communications and, more widely, with the theory of information were respectively introduced by Shannon in 1948 and Jaynes in 1957. Since then, over fifty new entropic functionals emerged in the scientific and technological literature. The most popular among them are the additive Renyi one introduced in 1961, and the nonadditive one introduced in 1988 as a basis for the generalization of the Boltzmann–Gibbs and related equilibrium and nonequilibrium theories, focusing on natural, artificial and social complex systems. Along such lines, theoretical, experimental, observational and computational efforts, and their connections to nonlinear dynamical systems and the theory of probabilities, are currently under progress. Illustrative applications, in physics and elsewhere, of these recent developments are briefly described in the present synopsis.
3.4K
07 May 2022
Topic Review
Entrance Length
In fluid dynamics, the entrance length is the distance a flow travels after entering a pipe before the flow becomes fully developed. Entrance length refers to the length of the entry region, the area following the pipe entrance where effects originating from the interior wall of the pipe propagate into the flow as an expanding boundary layer. When the boundary layer expands to fill the entire pipe, the developing flow becomes a fully developed flow, where flow characteristics no longer change with increased distance along the pipe. Many different entrance lengths exist to describe a variety of flow conditions. Hydrodynamic entrance length describes the formation of a velocity profile caused by viscous forces propagating from the pipe wall. Thermal entrance length describes the formation of a temperature profile. Awareness of entrance length may be necessary for the effective placement of instrumentation, such as fluid flow meters.
910
17 Oct 2022
Topic Review
Englert–Greenberger–Yasin Duality Relation
The Englert–Greenberger–Yasin duality relation, often called the Englert–Greenberger relation, relates the visibility, [math]\displaystyle{ V }[/math], of interference fringes with the definiteness, or distinguishability, [math]\displaystyle{ D }[/math], of the photons' paths in quantum optics. As an inequality: Although it is treated as a single relation, it actually involves two separate relations, which mathematically look very similar. The first relationship was first experimentally shown by Greenberger and Yasin in 1988. It was later theoretically derived by Jaeger, Shimony, and Vaidman in 1995. This relation involves correctly guessing which of the two paths the particle would have taken, based on the initial preparation. Here [math]\displaystyle{ D }[/math] can be called the predictability, and is sometimes denoted by [math]\displaystyle{ P }[/math]. A year later Englert, in 1996, apparently unaware of this result, derived a related relation which dealt with knowledge of the two paths using an apparatus. Here [math]\displaystyle{ D }[/math] is called the distinguishability. The significance of the relation is that it expresses quantitatively the complementarity of wave and particle viewpoints in double slit experiments. The complementarity principle in quantum mechanics, formulated by Niels Bohr, says that the wave and particle aspects of quantum objects cannot be observed at the same time. The Englert–Greenberger relation makes this more precise; an experiment can yield partial information about the wave and particle aspects of a photon simultaneously, but the more information a particular experiment gives about one, the less it will give about the other. The distinguishability [math]\displaystyle{ D }[/math] which expresses the degree of probability with which path of the particle is known, is a measure of the particle information, while the visibility of the fringes [math]\displaystyle{ V }[/math] is a measure of the wave information. The relation shows that they are inversely related, as one goes up, the other goes down.
443
18 Oct 2022
Topic Review
Energy Budgets in Growing Cities
Energy rate density is a useful metric to track the evolution of energy budgets, which help facilitate how well or badly human society trends toward winning or losing. The fates of nations and their cities are unknown, their success is not assured. Those nations and cities with rising per-capita energy usage while developing and those that are nearly flat while already developed seem destined to endure; those with falling energy usage seem likely to fail.
427
17 Nov 2022
Topic Review
Energy Accidents
Energy resources bring with them great social and economic promise, providing financial growth for communities and energy services for local economies. However, the infrastructure which delivers energy services can break down in an energy accident, sometimes causing much damage, and energy fatalities can occur, and with many systems often deaths will happen even when the systems are working as intended. Historically, coal mining has been the most dangerous energy activity and the list of historical coal mining disasters is a long one. Underground mining hazards include suffocation, gas poisoning, roof collapse and gas explosions. Open cut mining hazards are principally mine wall failures and vehicle collisions. In the US alone, more than 100,000 coal miners have been killed in accidents over the past century, with more than 3,200 dying in 1907 alone. According to Benjamin K. Sovacool, 279 major energy accidents occurred from 1907 to 2007 and they caused 182,156 deaths with $41 billion in property damages, with these figures not including deaths from smaller accidents. However, by far the greatest energy fatalities that result from energy generation by humanity, is the creation of air pollution. The most lethal of which, particulate matter, which is primarily generated from the burning of fossil fuels and biomass is (counting outdoor air pollution effects only) estimated to cause 2.1 million deaths annually.
452
10 Nov 2022
Topic Review
Endeavour House
Endeavour House is the Suffolk County Council headquarters located in Ipswich, Suffolk, England. Endeavour House was originally owned by the American-based energy and electric company, TXU Corporation. Located on Russell Road Ipswich, England ; Endeavour House was constructed between 2001 and 2003 by Bovis Lend Lease (now Lend Lease Project Management & Construction) contractors as well as M&S contractors and was designed by TTSP architects. The building had an original construction cost of £28 million (Suffolk County Council paid £16.75 million for the building) and is one of the most energy efficient office buildings in Europe.
667
17 Oct 2022
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