Time

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Time

• Michelle Houck
• April 28, 2008, 415pm EST

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What is time?

• Standard by which we measure
• One of the fundamental units of physics
• Broken into seconds, minutes, hours, days, weeks,
  months, years, decades, centuries, etc.

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What is a second?

• 1 second 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

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Mean Solar Time

• We use the angle of the sun to tell time
• 1 rotation of the Earth is one day, broken into
  24 hours
• Abbreviated UT1

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Greenwich Mean Time

• Based at Royal Observatory, Greenwich, London
• Mean solar time uses the angle of the sun to
  declare time, 1 rotation of the Earth 24 hours
• Prime Meridian, all world times are based on this
  time
• Locations east are hours and locations west are
  hours from GMT

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Time Zones

• Divisions of Earth with hourly displacements from
  GMT
• Could divide the earth into 24 equally sized
  wedges, but political and geographical boundaries
  make some time zones irregular.

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(No Transcript)
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Daylight Savings Time

• Some time zones will move an hour ahead or an
  hour back twice a year to allow for the hours of
  sunlight to be at the most convenient part of the
  day

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Coordinated Universal Time

• Abbreviated UTC
• Very precise time standard kept using the average
  of 300 atomic clocks around the world

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Leap Year and Leap Second

• To adjust for slight differences in the solar
  time and standard time, we periodically add to
  our time.
• Once every 4 years we add one day (February 29)
  to the year
• When The difference between UTC and UT1 becomes
  greater than 0.6s, a leap second is announced,
  then either at the end of June or the end of
  December, one second is added between 235959
  and 000000 of the next day

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How does my computer know what time it is?

• The computer has an internal battery so it can
  keep track of time even when its turned off
• If you have a continuous internet connection, the
  computer will automatically synchronize with an
  internet time server regularly
• You can force the computer to synchronize with a
  time server
• Can also set the computer to automatically adjust
  for daylight savings time, and other similar
  changes

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What time is it?

• http//www.time.gov/timezone.cgi?Eastern/d/-5/java