A light flashing approximately once per second

The second (SI unit symbol: s) is the International System of Units (SI) base unit of time^[1] and also a unit of time in other systems (abbreviated s or sec^[2]). Between 1000 (when al-Biruni used seconds) and 1960 the second was defined as 1/86,400 of a mean solar day (that definition still applies in some astronomical and legal contexts).^[3][4] Between 1960 and 1967, it was defined in terms of the period of the Earth's orbit around the Sun in 1900,^[5] but it is now defined more precisely in atomic terms. Seconds may be measured using mechanical, electric or atomic clocks.

However, 19th- and 20th-century astronomical observations revealed that this average time is lengthening and thus the sun–earth motion is no longer considered a suitable basis for definition. With the advent of atomic clocks, it became feasible to define the second based on fundamental properties of nature. Since 1967, the second has been defined to be:

the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.^[1]

SI prefixes are frequently combined with the word second to denote subdivisions of the second, e.g., the millisecond (one thousandth of a second), the microsecond (one millionth of a second), and the nanosecond (one billionth of a second). Though SI prefixes may also be used to form multiples of the second such as kilosecond (one thousand seconds), such units are rarely used in practice. The more common larger non-SI units of time are not formed by powers of ten; instead, the second is multiplied by 60 to form a minute, which is multiplied by 60 to form an hour, which is multiplied by 24 to form a day.

The second is also the base unit of time in the centimetre-gram-second, metre-kilogram-second, metre-tonne-second, and foot-pound-second systems of units.

International second[link]

Under the International System of Units (via the International Committee for Weights and Measures, or CIPM), since 1967 the second has been defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.^[1] In 1997 CIPM added that the periods would be defined for a caesium atom at rest, and approaching the theoretical temperature of absolute zero, and in 1999, it included corrections from ambient radiation.^[1]

This definition refers to a caesium atom at rest at a temperature of 0 K (absolute zero). Absolute zero implies no movement, and therefore zero external radiation effects (i.e., zero local electric and magnetic fields). The second thus defined is consistent with the ephemeris second, which was based on astronomical measurements. (See History below.)

The realization of the standard second is described briefly in a special publication from the National Institute of Standards and Technology,^[6] and in detail by the National Research Council of Canada.^[7]

Equivalence to other units of time[link]

1 international second is equal to:

• 1/60 minute (but see also leap second)
• 1/3,600 hour
• 1/86,400 day (IAU system of units)
• 1/31,557,600 Julian year (IAU system of units)

History[link]

Before mechanical clocks[link]

The Egyptians subdivided daytime and nighttime into twelve hours each since at least 2000 BC, hence the seasonal variation of their hours. The Hellenistic astronomers Hipparchus (c. 150 BC) and Ptolemy (c. AD 150) subdivided the day sexagesimally and also used a mean hour (¹⁄₂₄ day), simple fractions of an hour (¹⁄₄, ²⁄₃, etc.) and time-degrees (¹⁄₃₆₀ day or four modern minutes), but not modern minutes or seconds.^[8]

The day was subdivided sexagesimally, that is by ¹⁄₆₀, by ¹⁄₆₀ of that, by ¹⁄₆₀ of that, etc., to at least six places after the sexagesimal point (a precision of better than 2 microseconds) by the Babylonians after 300 BC. For example, six fractional sexagesimal places of a day was used in their specification of the length of the year, although they were unable to measure such a small fraction of a day in real time. As another example, they specified that the mean synodic month was 29;31,50,8,20 days (four fractional sexagesimal positions), which was repeated by Hipparchus and Ptolemy sexagesimally, and is currently the mean synodic month of the Hebrew calendar, though restated as 29 days 12 hours 793 halakim (where 1 hour = 1080 halakim).^[9] The Babylonians did not use the hour, but did use a double-hour lasting 120 modern minutes, a time-degree lasting four modern minutes, and a barleycorn lasting 3¹⁄₃ modern seconds (the helek of the modern Hebrew calendar),^[10] but did not sexagesimally subdivide these smaller units of time. No sexagesimal unit of the day was ever used as an independent unit of time.

In 1000, the Persian scholar al-Biruni gave the times of the new moons of specific weeks as a number of days, hours, minutes, seconds, thirds, and fourths after noon Sunday.^[4] In 1267, the medieval scientist Roger Bacon stated the times of full moons as a number of hours, minutes, seconds, thirds, and fourths (horae, minuta, secunda, tertia, and quarta) after noon on specified calendar dates.^[11] Although a third for ¹⁄₆₀ of a second remains in some languages, for example Polish (tercja) and Turkish (salise), the modern second is subdivided decimally.

Seconds measured by mechanical clocks[link]

The earliest clocks to display seconds appeared during the last half of the 16th century. The earliest spring-driven timepiece with a second hand which marked seconds is an unsigned clock depicting Orpheus in the Fremersdorf collection, dated between 1560 and 1570.^{[12]:417–418[13]} During the 3rd quarter of the 16th century, Taqi al-Din built a clock with marks every 1/5 minute.^[14] In 1579, Jost Bürgi built a clock for William of Hesse that marked seconds.^[12]:105 In 1581, Tycho Brahe redesigned clocks that displayed minutes at his observatory so they also displayed seconds. However, they were not yet accurate enough for seconds. In 1587, Tycho complained that his four clocks disagreed by plus or minus four seconds.^[12]:104

The second first became accurately measurable with the development of pendulum clocks keeping mean time (as opposed to the apparent time displayed by sundials). In 1644, Marin Mersenne calculated that a pendulum with a length of 39.1 inches (0.994 m) would have a period at standard gravity of precisely two seconds, one second for a swing forward and one second for the return swing, enabling such a pendulum to tick in precise seconds.

In 1670, London clockmaker William Clement added this seconds pendulum to the original pendulum clock of Christian Huygens.^[15] From 1670 to 1680, Clement made many improvements to his clock and introduced the longcase or grandfather clock to the public. This clock used an anchor escapement mechanism with a seconds pendulum to display seconds in a small subdial. This mechanism required less power, caused less friction and was accurate enough to measure seconds reliably as one-sixtieth of a minute than the older verge escapement. Within a few years, most British precision clockmakers were producing longcase clocks and other clockmakers soon followed. Thus the second could now be reliably measured.

Modern measurements[link]

In 1956 the second was defined in terms of the period of revolution of the Earth around the Sun for a particular epoch, because by then it had become recognized that the Earth's rotation on its own axis was not sufficiently uniform as a standard of time. The Earth's motion was described in Newcomb's Tables of the Sun (1895), which provide a formula estimating the motion of the Sun relative to the epoch 1900 based on astronomical observations made between 1750 and 1892.^[5] The second thus defined is

the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.^[5]

This definition was ratified by the Eleventh General Conference on Weights and Measures in 1960. The tropical year in the definition was not measured, but calculated from a formula describing a mean tropical year that decreased linearly over time, hence the curious reference to a specific instantaneous tropical year. This definition of the second was in conformity with the ephemeris time scale adopted by the IAU in 1952,^[16] defined as the measure of time that brings the observed positions of the celestial bodies into accord with the Newtonian dynamical theories of their motion (those accepted for use during most of the 20th century being Newcomb's Tables of the Sun, used from 1900 through 1983, and Brown's Tables of the Moon, used from 1923 through 1983).^[5]

With the development of the atomic clock, it was decided to use atomic clocks as the basis of the definition of the second, rather than the revolution of the Earth around the Sun.

Following several years of work, Louis Essen from the National Physical Laboratory (Teddington, England) and William Markowitz from the United States Naval Observatory (USNO) determined the relationship between the hyperfine transition frequency of the caesium atom and the ephemeris second.^[5][17] Using a common-view measurement method based on the received signals from radio station WWV,^[18] they determined the orbital motion of the Moon about the Earth, from which the apparent motion of the Sun could be inferred, in terms of time as measured by an atomic clock. They found that the second of ephemeris time (ET) had the duration of 9,192,631,770 ± 20 cycles of the chosen caesium frequency.^[17] As a result, in 1967 the Thirteenth General Conference on Weights and Measures defined the second of atomic time in the International System of Units as

FOCS 1, a continuous cold caesium fountain atomic clock in Switzerland, started operating in 2004 at an uncertainty of one second in 30 million years.

the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.^[5]

This SI second, referred to atomic time, was later verified to be in agreement, within 1 part in 10¹⁰, with the second of ephemeris time as determined from lunar observations.^[19] (Nevertheless, this SI second was already, when adopted, a little shorter than the then-current value of the second of mean solar time.^[20][21])

During the 1970s it was realized that gravitational time dilation caused the second produced by each atomic clock to differ depending on its altitude. A uniform second was produced by correcting the output of each atomic clock to mean sea level (the rotating geoid), lengthening the second by about 1×10⁻¹⁰. This correction was applied at the beginning of 1977 and formalized in 1980. In relativistic terms, the SI second is defined as the proper time on the rotating geoid.^[22]

The definition of the second was later refined at the 1997 meeting of the BIPM to include the statement

This definition refers to a caesium atom at rest at a temperature of 0 K.

The revised definition seems to imply that the ideal atomic clock contains a single caesium atom at rest emitting a single frequency. In practice, however, the definition means that high-precision realizations of the second should compensate for the effects of the ambient temperature (black-body radiation) within which atomic clocks operate, and extrapolate accordingly to the value of the second at a temperature of absolute zero.

Today, the atomic clock operating in the microwave region is challenged by atomic clocks operating in the optical region. To quote Ludlow et al.,^[23] “In recent years, optical atomic clocks have become increasingly competitive in performance with their microwave counterparts. The overall accuracy of single trapped ion based optical standards closely approaches that of the state-of-the-art caesium fountain standards. Large ensembles of ultracold alkaline earth atoms have provided impressive clock stability for short averaging times, surpassing that of single-ion based systems. So far, interrogation of neutral atom based optical standards has been carried out primarily in free space, unavoidably including atomic motional effects that typically limit the overall system accuracy. An alternative approach is to explore the ultranarrow optical transitions of atoms held in an optical lattice. The atoms are tightly localized so that Doppler and photon-recoil related effects on the transition frequency are eliminated.”

The NRC attaches a "relative uncertainty" of 2.5×10⁻¹¹ (limited by day-to-day and device-to-device reproducibility) to their atomic clock based upon the ¹²⁷I₂ molecule, and is advocating use of an ⁸⁸Sr ion trap instead (relative uncertainty due to linewidth of 2.2×10⁻¹⁵). See magneto-optical trap and "Trapped ion optical frequency standards". National Physical Laboratory. http://www.npl.co.uk/server.php?show=ConWebDoc.1086.  Such uncertainties rival that of the NIST F-1 caesium atomic clock in the microwave region, estimated as a few parts in 10¹⁶ averaged over a day.^[24][25]

SI multiples[link]

SI prefixes are commonly used to measure time less than a second, but rarely for multiples of a second (which is known as metric time). Instead, the non-SI units minutes, hours, days, Julian years, Julian centuries, and Julian millennia are used.

Other current definitions[link]

For specialized purposes, a second may be used as a unit of time in time scales where the precise length differs slightly from the SI definition. One such time scale is UT1, a form of universal time. McCarthy and Seidelmann refrain from stating that the SI second is the legal standard for timekeeping throughout the world, saying only that "over the years UTC [which ticks SI seconds] has become either the basis for legal time of many countries, or accepted as the de facto basis for standard civil time".^[26]

See also[link]

• Atomic clock
• Becquerel
• Coordinated Universal Time
• Hertz
• International Atomic Time
• International System of Units
• Leap second
• Magneto-optical trap
• Time standard

References[link]

External links[link]

Look up second in Wiktionary, the free dictionary.

Wikimedia Commons has media related to: Second

• National Physical Laboratory: Trapped ion optical frequency standards
• High-accuracy strontium ion optical clock; National Physical Laboratory (2005)
• National Research Council of Canada: Optical frequency standard based on a single trapped ion
• NIST: Definition of the second; notice the cesium atom must be in its ground state at 0 K
• Official BIPM definition of the second
• Seconds and leap seconds by the USNO
• The leap second: its history and possible future
• What is a Cesium atom clock?

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Saif Ali Khan
Khan at the 2011 Cannes Film Festival.
Born	(1970-08-16) 16 August 1970 (age 41) New Delhi, India
Occupation	Actor, Producer
Years active	1992–present
Spouse	Amrita Singh (m. 1991–2004) «start: (1991)–end+1: (2005)»"Marriage: Amrita Singh to Saif Ali Khan" Location: (linkback:http://en-wiki.pop.wn.com/index.php/Saif_Ali_Khan) (divorced)
Partner	Kareena Kapoor (2007-present)
Children	2

Saif Ali Khan (pronounced [ˈsɛːf əˈli ˈxaːn]; born 16 August 1970) is an Indian actor known for his work in Bollywood films. He is the son of the cricket player and last titular Nawab of Pataudi, late Mansoor Ali Khan Pataudi, and actress Sharmila Tagore, a great-grandniece of poet Rabindranath Tagore.^[1]

Having made his acting debut in 1992 with Parampara, Khan had his first success with the 1994 films Main Khiladi Tu Anari and Yeh Dillagi. After going through several years of decline throughout the 1990s, he rose into prominence with his performance in Dil Chahta Hai (2001), which marked his professional turning point.^[2] His work in Kal Ho Naa Ho (2003) won him the Filmfare Award for Best Supporting Actor, and he was awarded the National Film Award for Best Actor for his performance in Hum Tum (2004).

Khan subsequently had further mainstream success with films like Salaam Namaste (2005) and Race (2008) and starred in critically acclaimed projects such as Parineeta (2005), Being Cyrus and Omkara (2006).^[3] In 2009, Khan branched out into film production with his company Illuminati Films' first film Love Aaj Kal, which became a box-office success. These accomplishments have established him as one of the leading actors of Hindi cinema.^[4] He was awarded the Padma Shri civilian award by the Government of India in 2010.^[5]

Khan was married to actress Amrita Singh from 1991 to 2004, having two children together. His sister, Soha Ali Khan, is also a film actress.

Life[link]

Saif Ali Khan has Muslim ancestry through Nawabs of Pataudi lineage from his father's side and Hindu Bengali Tagore ancestry from his mother's side.^[6][7] His paternal grandfather, Iftikhar Ali Khan Pataudi, was the Nawab of Pataudi as well as an accomplished cricketer who played for England and then for India as the captain of the team.^[7] His paternal grandmother was Sajida Sultan, Begum of Bhopal and his great-uncle was Pakistani general Nawabzada Sher Ali Khan Pataudi. His father was Mansoor Ali Khan Pataudi, a former captain of the Indian cricket team and the ninth Nawab of Pataudi until the abolition of titles by India in 1971.^[6][7] Following the death of his father, Saif Ali Khan was proclaimed the Nawab of Pataudi by villagers of Pataudi State in deference to tradition, although the title carries no official or legal significance.^[8]

Khan has two sisters Saba Ali Khan and Soha Ali Khan. Khan's mother is Sharmila Tagore, a Bengali Indian film actress and a member of Bengal's Tagore family. She is also the head of the Indian film censorship board^[9] and the grand-niece of Nobel laureate Rabindranath Tagore.^[1] Tagore converted to Islam from Hinduism after her marriage to Khan's father, and changed her name to Begum Ayesha Sultana.^[7] Khan spent his childhood in a Muslim atmosphere, reading the Quran, with his grandmother being the centre of his religious education. Khan says that "Religion played a major role in my upbringing."^[6] Khan also states that his family is "cool or rather massively open–minded about religious matters." For example, he acknowledges that his daughter is born into a Muslim family, however he states that he will not decide her religion for her.^[6]

Initially Khan studied at the Lawrence School Sanawar^[10] and later attended Lockers Park School in Hertfordshire. He followed in his father's footsteps by attending Winchester College, an independent school for boys in the UK. Khan can speak his native languages of Hindi and Bengali fluently, as well as English.^[11]

Acting career[link]

Early work, until 2000[link]

In 1993, he won the Filmfare Best Male Debut Award for Aashiq Awara. His breakthrough role came in 1994 with Yeh Dillagi starring opposite Akshay Kumar and Kajol which was his first major hit.^[12] Khan went on to star in many films but they failed at the box office. He only had success with multi-starrer hits like Main Khiladi Tu Anari (1994), Imtihaan (1995),^[13] Kachche Dhaage (1999) and Hum Saath-Saath Hain: We Stand United (1999).^[14] After some box office failures he had a hit with the film Kya Kehna (2000).^[15]

Turning point, 2001-03[link]

His acting career, more or less, went through a relative slump until 2001, when he was cast in Farhan Akhtar's National Film Award-winning Dil Chahta Hai, co-starring Aamir Khan and Akshaye Khanna.^[16] Depicting the contemporary routine life of Indian affluent youth, it is set in modern-day urban Mumbai and focuses on a major period of transition in the lives of three young friends. Despite receiving a positive critical reception, it was only a moderate box office success in India;^[17] it performed well in the big cities but failed in the rural areas, which was attributed by critics to the urban-oriented lifestyle depicted in the movie.^[18] He received much acclaim for his role as Sameer with critic Taran Adarsh calling it his career-best performance.^[19]

He has since had many box office successes, starting with Nikhil Advani's drama Kal Ho Na Ho (2003), co-starring alongside Shahrukh Khan and Preity Zinta. The film, set in New York, starred Saif as Rohit, Zinta's best friend, who later realises that he loves her.^[20] The film was the second top-grossing film of the year, and Khan's performance was praised by critics. He won the Filmfare Best Supporting Actor Award, as well as other awards in that category in other award ceremonies.

Solo success and critical acclaim, 2004–08[link]

To date, Khan's films have often featured him as the supporting cast, though following the success of Dil Chahta Hai and Kal Ho Naa Ho, producers began signing him on for the main lead.^[21] In early 2004, Khan was approached by Sriram Raghavan to star as the globe-trotting bachelor, Karan Singh Rathod, in his directorial debut, Ek Hasina Thi. The film tells the story of a young woman (played by Urmila Matondkar) who meets with Khan's character, and is subsequently arrested for having links with the underworld. When Khan was initially offered the project, he was unable to do it due to his busy schedule.^[21] However, when the director approached him several months later, he agreed to do the film.^[21] Upon release, the film was critically embraced, with Khan's performing earning praise. The Deccan Herald reported, "From suave loverboy to self-serving skunk, he (Khan) moves with such brilliance that he leaves you breathless [...] He holds his own in a movie that is completely Urmila's."^[22] For his performance, Khan received nominations at the Star Screen, Zee Cine and IIFA ceremonies.

He followed it with the leading role in Kunal Kohli's romantic comedy Hum Tum. Featured opposite Rani Mukerji, the film follows the encounters of the two main characters until they, after several years and various meetings, become friends and finally fall in love. The film was received favourably by critics and became one of the year's most successful films, with a domestic box office of 210 million (US$4.19 million).^[23] Khan played the role of Karan Kapoor, a young cartoonist and womaniser whose relationship with Rhea Prakash (played by Mukerji) changes his perception of women and life. Rediff.com wrote about his performance: "Saif reprises his urbane self from Dil Chahta Hai and Kal Ho Naa Ho, peppering it with occasional fits of introspection and angst, and marking himself as an actor whose time has come."^[24] He won the Filmfare Award for Best Performance in a Comic Role and was conferred the coveted National Film Award for Best Actor in 2005 among much controversy.^[25] It marked the beginning of his work with Yash Raj Films, one of the largest production houses in Bollywood.

In 2005, Khan appeared in two movies. His first release was Pradeep Sarkar's Parineeta (2005), an adaptation of Sarat Chandra Chattopadhyay's 1914 Bengali novella by the same name.^[26] Prior to being cast, the film's producer, Vidhu Vinod Chopra was initially reluctant in casting Khan as the male protagonist because he felt that "I could not manage it".^[27] After much deliberation, Chopra was persuaded by Sarkar who felt that Khan was perfect for the role.^[28] The film garnered critical acclaim upon release and Khan's portrayal of Shekhar Rai earned him nominations for Best Actor at a number of award ceremonies. Derek Elley from Variety wrote, "Khan, who has gradually been developing away from light comedy, again shows smarts as a substantial actor."^[26] His next release was Siddharth Anand's comedy-drama Salaam Namaste opposite Preity Zinta. Produced by Yash Raj Films, it was the first Indian feature to be filmed entirely in Australia and went on to become the year's highest-grossing Bollywood production outside of India, earning 570 million (US$11.37 million) internationally.^[29] Khan played the role of the egotistic, laid-back chef, Nikhil Arora. According to Khalid Mohamed, "Salaam Namaste is unthinkable without Saif Ali Khan. He rescues several untidily written scenes with his neat wit and that flustered 'how-did-I-get-into-this-soup?' facial expression."^[30]

He next played the protagonist in the English language art film, Being Cyrus (2006), co-starring alongside Naseeruddin Shah and Dimple Kapadia. Directed by debutant Homi Adajania, the psychological drama revolves around a dysfunctional Parsi family with who Khan's character moves into. The film received predominantly positive reviews, and Khan was particularly praised.^[31] Later in the year, he portrayed the character of Iago in Omkara, the Indian adaptation of William Shakespeare's Othello. Directed by Vishal Bhardwaj, the film is a tragedy of sexual jealousy set against the backdrop of the political system in Uttar Pradesh.^[32] The film premiered at the 2006 Cannes Film Festival and was also selected for screening at the Cairo International Film Festival.^[32][33] Omkara was embraced by critics and Khan went on to receive major acclaim earning the awards for Best Actor in a Negative Role at the Filmfare, Star Screen, Zee Cine and IIFA ceremonies; his performance was later included in the 2010 issue of the "Top 80 Iconic Performances" by Filmfare.^[34] Variety described his performance as "powerhouse" and further wrote, "It is Khan's movie through and through, in a performance of rugged, contained malevolence which trades on his previous screen persona as a likable best friend as well as his stint as the manipulative outsider in Being Cyrus. It is smart casting, superbly realized."^[35]