Great Circles

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Great Circles
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Great Circles

• Divides Earth into two equal parts
• The shortest route between two points is part of
  a great circle
• Infinite number
• Equator
• Any pair of longitudes opposite one another
• Those that dont conform to the graticule
• Ex. Great Circle connecting Capetown Sydney

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Global Time
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Time of Day Globally Speaking

• Earth rotates around its axis
• Solar Day
• one rotation with respect to the sun
• At any instant of time
• along a given longitude it is noon
• sun is highest in the sky
• 90 degrees eastward it is 6PM
• 90 degrees westward it is 6AM
• 180 degrees away it is midnight

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How time changes globally

• Every 15 degrees in longitude 1 hour
• 360 degrees / 24 hrs 15 degrees per hour
• time is later going eastward earlier westward
• ex. Noon in OKC later in New York earlier in LA
• Local Time
• noon is defined at time sun is highest in the sky
• changes at rate of 4 minutes per degree of
  longitude
• worked great until mid 1800s
• advent of rapid transportation- railroads
• Standard Time
• local time of closest standard meridian

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Time Zones

• In theory- 24 zones
• one for each hour of day
• 24 standard meridians
• starts at Prime Meridian
• proceeds in increments of 15 degrees east west
• each 15 degrees wide
• extent is 7 1/2 degrees east west of each
  standard meridian

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Changing Dates

• International Date Line (180th meridian)
• day later crossing westward earlier eastward
• western 1/2 of its time zone is first to have a
  new day (i.e., new date)
• New day has to start somewhere.

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Changing Dates, continued

• When does a new day (date) begin?
• Midnight (12AM)
• going eastward, if you cross a time zone and it
  is midnight (or up to 1AM), then it is a day
  later.
• Crossing a time zone westward, if is 11PM up to
  midnight, it is a day earlier.
• Ex. New Years Eve of the new millenium
• starts just west of IDL, and progresses westward
• NYC 12AM Jan 1, 2000 OKC 11PM Dec 31, 1999

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Daylight Savings Time

• Noon clock time is actually 11AM solar time at
  time zones standard meridian
• one extra hour of daylight in evening
• why do this?
• Principally saves on energy costs
• why not have it all year?
• In winter, wish to have daylight when going to
  work or school
• changing your clock
• Fall back to standard time
• Spring forward to daylight savings time

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The Reality of Time Zones

• The true size shape of Time Zones vary from the
  theoretical 15-degree widths
• political reasons
• individual countries
• large countries- mostly subdivided
• economic reasons
• be in same time zone as you do business

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Keeping Time

• by Time Zone Designation
• ex. CST CDT PST EST
• by Greenwich Mean Time (GMT)
• current standard time at Prime Meridian
• CST is six hours earlier than GMT
• by Zulu Time (Z)
• same as GMT, except 24-hour clock is used
• ex. 100 PM GMT 1300Z

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Reading Lab Manual Exercise 21Homework