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ΔT
In precise timekeeping, ΔT (Delta T, delta-T, deltaT, or DT) is a measure of the cumulative effect of the departure of the Earth's rotation period from the fixed-length day of atomic time. Formally it is the time difference obtained by subtracting Universal Time (UT, defined by the Earth's rotation) from Terrestrial Time (TT, independent of the Earth's rotation): ΔT = TT − UT. The value of ΔT for the start of 1902 was approximately zero; for 2002 it was about 64 seconds. So the Earth's rotations over that century took about 64 seconds longer than would be required for days of atomic time. As well as this long-term drift in the length of the day there are short-term fluctuations in the length of day (Δτ) which are dealt with separately.
  • 695
  • 01 Nov 2022
Topic Review
Zodiac
The zodiac is a belt-shaped region of the sky that extends approximately 8° north or south (as measured in celestial latitude) of the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year. The paths of the Moon and visible planets are within the belt of the zodiac. In Western astrology, and formerly astronomy, the zodiac is divided into twelve signs, each occupying 30° of celestial longitude and roughly corresponding to the following star constellations: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. These astrological signs form a celestial coordinate system, or more specifically an ecliptic coordinate system, which takes the ecliptic as the origin of latitude and the Sun's position at vernal equinox as the origin of longitude.
  • 7.4K
  • 31 Oct 2022
Topic Review
ZnO Nanostructures
Zinc oxide (ZnO) nanostructures are structures with at least one dimension on the nanometre scale, composed predominantly of zinc oxide. They may be combined with other composite substances to change the chemistry, structure or function of the nanostructures in order to be used in various technologies. Many different nanostructures can be synthesised from ZnO using relatively inexpensive and simple procedures. ZnO is a semiconductor material with a wide band gap energy of 3.3eV and has the potential to be widely used on the nanoscale. ZnO nanostructures have found uses in environmental, technological and biomedical purposes including ultrafast optical functions, dye-sensitised solar cells, lithium-ion batteries, biosensors, nanolasers and supercapacitors. Research is ongoing to synthesise more productive and successful nanostructures from ZnO and other composites. ZnO nanostructures is a rapidly growing research field, with over 5000 papers published during 2014-2019.
  • 479
  • 07 Nov 2022
Topic Review
Zisman Plot
The Zisman plot the graphical method of the Zisman theory or the Zisman method for characterizing the wettability of a solid surface , named for the American chemist and geophysicist, William Albert Zisman (1905–1986). It is a prominent Sessile drop technique used for characterizing liquid-surface interactions based on the contact angle of a single drop of liquid sitting on the solid surface.
  • 509
  • 24 Nov 2022
Topic Review
Zeta Ursae Majoris
Mizar is a 2nd magnitude star in the handle of the Big Dipper asterism in the constellation of Ursa Major. It has the Bayer designation ζ Ursae Majoris (Latinised as Zeta Ursae Majoris). It forms a well-known naked eye double star with the fainter star Alcor, and is itself a quadruple star system. The whole system lies about 83 light-years away from the Sun, as measured by the Hipparcos astrometry satellite, and is part of the Ursa Major Moving Group.
  • 618
  • 29 Sep 2022
Biography
Yury Romanenko
Yury Viktorovich Romanenko (Russian: Ю́рий Ви́кторович Романе́нко, Jurij Viktorovič Romanenko; born August 1, 1944) is a former Soviet cosmonaut, twice Hero of the Soviet Union (March 16, 1978 and September 26, 1980). Over his career, Yury Romanenko spent a total of 430 days 20 hours 21 minutes 30 seconds in space and 18 hours in space walks.[1] In 1987 he was a reside
  • 367
  • 07 Dec 2022
Biography
Yuri Raizer
Yuri Raizer was born in 1927 in Kharkov, Ukraine. He graduated from the Leningrad Polytechnic Institute in 1949, and became Doctor of Sciences (Physics and Mathematics) in 1959, full professor in 1968, and Honored Scientist of the Russian Federation in 2002. He was a chief researcher at the Institute for Problems in Mechanics of the Russian Academy of Sciences, and Honored Professor of the Mosco
  • 394
  • 29 Aug 2022
Biography
Yuri N. Gnedin
Prof. Dr. Yuri N. Gnedin was an outstanding astrophysicist whose scientific interests and expertise were extraordinarily wide. He was an expert in theoretical investigation of the polarized radiation transfer, generation of high-energy radiation in close binary star systems and galactic nuclei, and cyclotron lines in spectra of accreting neutron stars. Prof. Dr. Yuri N. Gnedin developed the pion
  • 634
  • 20 Dec 2022
Topic Review
Yield (Engineering)
In materials science and engineering, the yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Below the yield point, a material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible and is known as plastic deformation. The yield strength or yield stress is a material property and is the stress corresponding to the yield point at which the material begins to deform plastically. The yield strength is often used to determine the maximum allowable load in a mechanical component, since it represents the upper limit to forces that can be applied without producing permanent deformation. In some materials, such as aluminium, there is a gradual onset of non-linear behavior, making the precise yield point difficult to determine. In such a case, the offset yield point (or proof stress) is taken as the stress at which 0.2% plastic deformation occurs. Yielding is a gradual failure mode which is normally not catastrophic, unlike ultimate failure. In solid mechanics, the yield point can be specified in terms of the three-dimensional principal stresses ([math]\displaystyle{ \sigma_1, \sigma_2 , \sigma_3 }[/math]) with a yield surface or a yield criterion. A variety of yield criteria have been developed for different materials.
  • 2.5K
  • 17 Oct 2022
Topic Review
Yellow Supergiant Star
A yellow supergiant (YSG) is a star, generally of spectral type F or G, having a supergiant luminosity class (e.g. Ia or Ib). They are stars that have evolved away from the main sequence, expanding and becoming more luminous. Yellow supergiants are smaller than red supergiants; naked eye examples include Polaris. Many of them are variable stars, mostly pulsating Cepheids such as δ Cephei itself.
  • 1.4K
  • 03 Nov 2022
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