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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.
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  • 01 Nov 2022
Topic Review
AIDA (Mission)
The Asteroid Impact and Deflection Assessment (AIDA) mission is a proposed pair of space probes which will study and demonstrate the kinetic effects of crashing an impactor spacecraft into an asteroid moon. The mission is intended to test and validate impact models of whether a spacecraft could successfully deflect an asteroid on a collision course with Earth. The original plan called for a European spacecraft, the Asteroid Impact Mission (AIM), to operate in synergy with a large NASA impactor called Double Asteroid Redirection Test (DART) and observe the immediate effects of the impact. AIM was cancelled in 2016 when Germany was unable to fund its portion, and after some backlash within ESA, AIM was replaced in 2018 with a smaller spacecraft called Hera that will launch five years after DART to orbit and study the crater on the asteroid. Hera will also deploy Europe's first CubeSats in deep space for close-up asteroid surveying: Juventas and Milani. DART is currently planned to impact in 2022 in the small moon of asteroid 65803 Didymos, while Hera will arrive at Didymos in 2027, five years after DART's impact.
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  • 01 Nov 2022
Topic Review
Methyl Iodide
Methyl iodide, also called iodomethane, and commonly abbreviated "MeI", is the chemical compound with the formula CH3I. It is a dense, colorless, volatile liquid. In terms of chemical structure, it is related to methane by replacement of one hydrogen atom by an atom of iodine. It is naturally emitted by rice plantations in small amounts. It is also produced in vast quantities estimated to be greater than 214,000 tons annually by algae and kelp in the world's temperate oceans, and in lesser amounts on land by terrestrial fungi and bacteria. It is used in organic synthesis as a source of methyl groups.
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  • 01 Nov 2022
Topic Review
Cyanophyceae
Cyanobacteria (/saɪˌænoʊbækˈtɪəri.ə/), also known as Cyanophyta, are a phylum of Gram-negative bacteria that obtain energy via photosynthesis. The name cyanobacteria refers to their color (from grc κυανός (kuanós) 'blue'), which similarly forms the basis of cyanobacteria's common name, blue-green algae.[note 1] They appear to have originated in a freshwater or terrestrial environment. Sericytochromatia, the proposed name of the paraphyletic and most basal group, is the ancestor of both the non-photosynthetic group Melainabacteria and the photosynthetic cyanobacteria, also called Oxyphotobacteria. Cyanobacteria use photosynthetic pigments, such as carotenoids, phycobilins, and various forms of chlorophyll, which absorb energy from light. Unlike heterotrophic prokaryotes, cyanobacteria have internal membranes. These are flattened sacs called thylakoids where photosynthesis is performed. Phototrophic eukaryotes such as green plants perform photosynthesis in plastids that are thought to have their ancestry in cyanobacteria, acquired long ago via a process called endosymbiosis. These endosymbiotic cyanobacteria in eukaryotes then evolved and differentiated into specialized organelles such as chloroplasts, chromoplasts, etioplasts, and leucoplasts, collectively known as plastids. Cyanobacteria are the first organisms known to have produced oxygen. By producing and releasing oxygen as a byproduct of photosynthesis, cyanobacteria are thought to have converted the early oxygen-poor, reducing atmosphere into an oxidizing one, causing the Great Oxidation Event and the "rusting of the Earth", which dramatically changed the composition of the Earth's life forms. The cyanobacteria Synechocystis and Cyanothece are important model organisms with potential applications in biotechnology for bioethanol production, food colorings, as a source of human and animal food, dietary supplements and raw materials. Cyanobacteria produce a range of toxins known as cyanotoxins that can pose a danger to humans and animals.
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  • 01 Nov 2022
Topic Review
Battle of Orgreave
The Battle of Orgreave was a violent confrontation on 18 June 1984 between pickets and officers of the South Yorkshire Police (SYP) and other police forces, including the Metropolitan Police, at a British Steel Corporation (BSC) coking plant at Orgreave, in Rotherham, South Yorkshire, England. It was a pivotal event in the 1984–1985 UK miners' strike, and one of the most violent clashes in British industrial history. Journalist Alastair Stewart has characterised it as "a defining and ghastly moment" that "changed, forever, the conduct of industrial relations and how this country functions as an economy and as a democracy". Most media reports at the time depicted it as "an act of self-defence by police who had come under attack". In 2015, the Independent Police Complaints Commission (IPCC) reported that there was "evidence of excessive violence by police officers, a false narrative from police exaggerating violence by miners, perjury by officers giving evidence to prosecute the arrested men, and an apparent cover-up of that perjury by senior officers". Historian Tristram Hunt has described the confrontation as "almost medieval in its choreography ... at various stages a siege, a battle, a chase, a rout and, finally, a brutal example of legalised state violence". 71 picketers were charged with riot and 24 with violent disorder. At the time, riot was punishable by life imprisonment. The trials collapsed when the evidence given by the police was deemed "unreliable". Gareth Peirce, who acted as solicitor for some of the pickets, said that the charge of riot had been used "to make a public example of people, as a device to assist in breaking the strike", while Michael Mansfield called it "the worst example of a mass frame-up in this country this century". In June 1991, the SYP paid £425,000 in compensation to 39 miners for assault, wrongful arrest, unlawful detention and malicious prosecution. Following the 2016 inquest verdict into the 1989 Hillsborough disaster, previously censored documents suggesting links between the actions of senior SYP officers at both incidents were published. This led to renewed calls for a public inquiry to be held into the actions of the police at Orgreave. In October 2016, in an Oral Answer to a Question in the House of Commons, a written ministerial statement to the House of Commons and Lords, and in a letter to the Orgreave Truth and Justice Campaign (OTJC), Home Secretary Amber Rudd announced there would be no statutory inquiry or independent review. In 2016, Alan Billings, the South Yorkshire Police and Crime Commissioner, admitted that the SYP had been "dangerously close to being used as an instrument of state". Despite the police evidence subsequently being deemed unreliable in court, there still exists a body of opinion that the police at Orgreave "were upholding the law in the face of intimidation from thousands of strikers".
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  • 01 Nov 2022
Topic Review
Autism and LGBT Identities
Although it is not known exactly what causes one to be on the autism spectrum or what causes one's sexual orientation or gender identity, a few limited studies into autistic people have shown higher rates of LGBT identities and feelings among autistic people than the general population.
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  • 01 Nov 2022
Topic Review
Laser Video Display
Laser color television (laser TV), or laser color video display utilizes two or more individually modulated optical (laser) rays of different colors to produce a combined spot that is scanned and projected across the image plane by a polygon-mirror system or less effectively by optoelectronic means to produce a color-television display. The systems work either by scanning the entire picture a dot at a time and modulating the laser directly at high frequency, much like the electron beams in a cathode ray tube, or by optically spreading and then modulating the laser and scanning a line at a time, the line itself being modulated in much the same way as with digital light processing (DLP). The special case of one ray reduces the system to a monochrome display as, for example, in black and white television. This principle applies to a direct view display as well as to a (front or rear) laser projector system. Laser TV technology began to appear in the 1990s. In the 21st century the rapid development and maturity of semiconductor lasers and other technologies gave it new advantages.
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  • 01 Nov 2022
Topic Review
High Resolution Wide Swath SAR Imaging
High Resolution Wide Swath (HRWS) imaging is an important branch in Synthetic aperture radar (SAR) imaging, a remote sensing technique capable of providing high resolution images independent of weather conditions and sunlight illumination. This makes SAR very attractive for the systematic observation of dynamic processes on the Earth's surface, which is useful for environmental monitoring, earth resource mapping and military systems. SAR technology has provided terrain structural information to geologists for mineral exploration, oil spill boundaries on water to environmentalists, sea state and ice hazard maps to navigators, and intelligence, surveillance, reconnaissance and detecting information to military operations. Conventional SAR systems are limited in that a wide swath can only be achieved at the expense of a degraded azimuth resolution. Since wide coverage swaths and high resolution output are both important, this poses challenges and contradicting requirements on the design of space-borne SAR systems and related new algorithms.
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  • 01 Nov 2022
Topic Review
Anatomy of the Human Nose
The visible part of the human nose is the protruding part of the face that bears the nostrils. The shape of the nose is determined by the ethmoid bone and the nasal septum, which consists mostly of cartilage and which separates the nostrils. On average the nose of a male is larger than that of a female. The nasal root is the top of the nose, forming an indentation at the suture where the nasal bones meet the nasal part of the frontal bone. The anterior nasal spine is the thin projection of bone at the midline on the lower nasal margin, holding the cartilaginous center of the nose. Adult humans have nasal hair in the nostrils.
  • 1.8K
  • 01 Nov 2022
Topic Review
Years of Living Dangerously
Years of Living Dangerously is an American documentary television series focusing on global warming. The first season was broadcast in the US in 2014 on Showtime. It won an Emmy Award as Outstanding Documentary or Nonfiction Series. The second season aired on the National Geographic Channel in 2016. Executive producers included James Cameron, Arnold Schwarzenegger, and series creators Joel Bach and David Gelber (formerly of 60 Minutes). Joseph Romm and Heidi Cullen were the chief science advisors. The weekly episodes featured celebrity hosts with a history of environmental activism and well-known journalists with a background in environmental reportage. These "correspondents" traveled the world and throughout the U.S. to interview experts and ordinary people affected by, and seeking solutions to, the effects of global warming. They acted as reporters and proxies for the audience, asking questions to find out people's opinions and to discover the scientific evidence. The final episode of season one featured an interview of President Barack Obama. Episodes explored the effects of rising sea levels, historic droughts and flooding, water scarcity, ocean acidification, deforestation and the rapidly increasing extinction rate of species, but also focused on "solutions that individuals, communities, companies and even governments can use to address worldwide climate change", including cheaper solar and wind energy, advancing battery technology and electric cars. Hosts included Cameron, Schwarzenegger, Harrison Ford, Ian Somerhalder, America Ferrera, David Letterman, Gisele Bündchen, Jack Black, Matt Damon, Jessica Alba, Sigourney Weaver, and various other actors and journalists. Schwarzenegger reflected on how the series tries to make the issue of climate change resonate with the public: "I think the environmental movement only can be successful if we are simple and clear and make it a human story. We will tell human stories in this project. The scientists would never get the kind of attention that someone in show business gets." Cameron elaborated: "We didn’t use our celebrities as talking head experts, because they’re not climate experts. They were concerned, intelligent, curious citizens who were out to find answers. They were functioning as journalists." Newsweek said that the celebrity hosts "lend sparks to an issue that sends most viewers for the exits".
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  • 01 Nov 2022
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