CS240A: A Short History of Time

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CS240A A Short History of Time

• Carlo Zaniolo
• Department of Computer Science
• University of California, Los Angeles

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Calendars

• Thirty days hath September, April, June, and
  November All the rest have thirty-one
  Excepting February alone Which hath but
  twenty-eight, in fine, Till leap year gives it
  twenty-nine.
• Blue moon

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Lunar Calendars versus Solar ones

• Keeping track of time was a major challenge for
  early civilizations- an issue involving science,
  religion and magic.
• The problem is caused by the difficulty of
  synchronizing days, lunar cycles (between 29 and
  30 days) and solar years. But 30 12 360 while
  a solar year has between 365 and 366 days.
• The Roman calendar adopted in the western world
  is a solar calendar consisting of 12 months and
  365 days, with a leap year of 366 every four
  year.
• This is called Julian Calendar because it was set
  by Julius Caesar in 45 B.C. The month of July
  was named after him.

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The Gregorian Calendar

• The Julian calendar had too many leap years.
• Thus, in 1582 AD, Pope Gregory XIII (helped by
  astronomer Christopher Clavius) introduced the
  current calendar as follows
• 10 days were skipped in October 1582 Thursday,
  October 4, 1582-Julian was followed by Friday,
  October 15-Gregorian.
• Years ending with 00 must also be divisible by
  400 in order to have 29 days in February
• Because of Gregory's refinement of the leap year
  rule, our Western calendar has almost kept pace
  with the seasons.

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Slow Adoption

• Great Britain colonies adopted the Gregorian
  calendar in 1752---by then, eleven days had to be
  eliminated from the Julian calendar. (At the same
  time, they changed the beginning of their legal
  year from March 25 to January 1.)
• This is the calendar implemented in most DBMSs
• In Russia the switch was put into effect on
  January 1, 1918- after the October revolution
  that occurred on October 25, 1917 (well, on
  November 7, 1917)

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The Seven-Day Week

• Seven days in the Bible. Seven days also used in
  Egypt, Persia, India, Tibet, Burma and many other
  sites of civilizations.
• This is because the weekly cycle was set after
  the seven planets that in ancient times were
  seen to circle the Earth Sun, Moon, Mars,
  Mercury, Jupiter, Venus, Saturn
• Romance languages have keep those names, except
  for Sunday which was given a Christian name.
• The days of the week in English, are denoted with
  Anglo-Saxon names for those same seven heavenly
  bodies. Likewise in all Germanic languages (ditto
  for the names of their Gods).

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Keeping Time

• The time units were set very early Division of
  the day in 12 hours traces back to Egyptians.
• Subdividing hours and minutes into 60 was
  probably done to facilitate computations
• But the ancient instruments for measuring time
  were inaccurate.
• Sundials (shadow clocks) daytime clocks for
  sunny days. However, between different seasons,
  there can be a shift up to 15 minutes
• Water clocks and clepsydras for nights and rainy
  days.
• Keeping accurate time over several days was
  impossible.

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Modern Times

• In 1656, Christiaan Huygens, a Dutch scientist,
  made the first pendulum clock. The pendulum has a
  natural period of oscillation (Galileo
  Galilei). The error was reduced to less than 1
  minute a day.
• Spring-powered clocks were invented by Peter
  Henlein at the beginning of 1500
• Accurate spring clocks became critical for
  oceanic navigation (determining longitude). In
  1761 John Harrison won the British government's
  2,000,000 prize (in today's money) with a marine
  chronometer with a spring and balance wheel
  escapement that was accurate to one-fifth of a
  second a day.
• Today we have Quartz clocks, atomic clocks
  (based on cesium), and time-servers on the web.
• The Coordinated Universal Time (UTC) uses atomic
  time. But every year, the international
  commission can prescribe an adjustment of one or
  two seconds as needed to compensate for slowing
  or earths rotation (leap seconds).

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Time ZonesAlso see http//physics.nist.gov/GenInt
/Time/time.html

• The U.S.A. was divided in four time zones in
  1883. Until then, most cities relied upon their
  own local sun time, which changes by
  approximately one minute for every 12 miles
  traveled east or west. This became a major cause
  of train accidentsthus the time zones.
• The World's Time Zones were established in 1884.
• Greenwich, England, was designated the center of
  the zones. The international dateline was drawn
  to generally follow the 180 meridian in the
  Pacific Ocean
• Because of some countries, islands or states do
  not want to be divided into several zones, the
  zones' boundaries tend to wander considerably
  from straight north-south lines.
• Summer time not all states/countries
  switch---those that do, often switch on different
  days.

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On the web

• Gregorian Calendar (Introduced in 1582)
• Gregory XIII
• Christopher Clavius
• Inter Gravissima
• A slow transition (1752)
• Eastern Orthodox (1923)
• Longitude and the movie on John Harrison