Time Zones Map: By TimeZonesBoy [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or Public domain], via Wikimedia Commons

What is a Time Zone? A time zone is a region that has a standard time throughout it There are 24 times zones throughout the world Map: By TimeZonesBoy [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or Public domain], via Wikimedia Commons

Why do We Need Standard Time? Before the late 1800s, towns and cities would set their own times based on the sun Due to the Earth’s rotation, each location’s time was different Pictured is a sundial – they were used to determine times in towns and cities across the world before standard times were adopted Image: By Pearson Scott Foresman [Public domain], via Wikimedia Commons

Why do We Need Standard Time? When railroads and telecommunications began to improve, it became difficult for each location to have its own time Until the 1880s, most towns in the United States had their own local time, usually based on “high noon,” the time in the town when the sun was at the highest point in the sky Table: "Comparative-time-table" by Dinsmore (1857) - Dinsmore’s American Railroad and Steam Navigation Guide and Route-Book.. Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Comparative-time-table.jpg#mediaviewer/File:Comparative-time-table.jpg Table showing different times across the U.S. in 1857

Time Zones In 1884, the international standard time system was established The prime meridian was also established passing through Greenwich, England at 0° longitude The prime meridian was established at Greenwich, England since England was a major shipping country and already had several ships using a meridian at Greenwich and because there was a well-respected observatory in Greenwich at the time Image: http://commons.wikimedia.org/wiki/File:Primemeridian.jpg

Time Zones By the year 1900, almost everywhere on Earth used some form of standard time Image: http://commons.wikimedia.org/wiki/File:Primemeridian.jpg

Time Zones Since there are 360° of longitude around the Earth and 24 time zones, each time zone is approximately 15° of longitude wide Image: By Pearson Scott Foresman [Public domain], via Wikimedia Commons

Time Zones The prime meridian is at 0° longitude The international date line, where new days begin, is at 180° longitude Map: "TimeZones" by http://liftoff.msfc.nasa.gov/Academy/Rocket_Sci/clocks/TimeZones.GIF, as seen on http://liftoff.msfc.nasa.gov/Academy/Rocket_Sci/clocks/time-gmt.html. Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:TimeZones.GIF#mediaviewer/File:TimeZones.GIF

Time Zones Time zone boundaries are irregular primarily due to political reasons – often it is helpful for a country to be completely within one time zone Map: "TimeZones" by http://liftoff.msfc.nasa.gov/Academy/Rocket_Sci/clocks/TimeZones.GIF, as seen on http://liftoff.msfc.nasa.gov/Academy/Rocket_Sci/clocks/time-gmt.html. Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:TimeZones.GIF#mediaviewer/File:TimeZones.GIF

Russia’s different time zones Some countries are so large that they span multiple time zones Notable examples include: The United States Russia Brazil Map: PlatonPskov [CC BY-SA 2.5 (http://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons Russia’s different time zones

Time Zones Other large countries use single time zones that are larger than 15° of latitude Notable examples include: China India Map: "IST-CIA-TZ" by Nichalp, Ambuj.Saxena, Cpm0813 at en.wikipedia. - Source: en:Image:Timezones optimized.png Originally from en.wikipedia; description page is/was here.. Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:IST-CIA-TZ.png#mediaviewer/File:IST-CIA-TZ.png

Time Zones Many countries use daylight savings time (DST) during part of the year Typically during DST, clocks are advanced one hour forward in the spring and adjusted back one hour in the fall One way many people remember how to adjust clocks during daylight savings time is by thinking of it as “spring” forward and “fall” back Map: "DaylightSaving-World-Subdivisions" by Paul Eggert - based on Image:BlankMap-World-Subdivisions.PNG, plus the data in the tz database, plus data in the maps on the INMS's Time Zones & Daylight Saving Time page.. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:DaylightSaving-World-Subdivisions.png#mediaviewer/File:DaylightSaving-World-Subdivisions.png Blue = currently uses DST, orange = once used DST but not currently, red = has not used DST

Time Zones There are six time zones in the United States Four time zones are in the continental United States: Eastern Central Mountain Pacific Map: "National-atlas-timezones-2006". Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:National-atlas-timezones-2006.gif#mediaviewer/File:National-atlas-timezones-2006.gif The other two time zones are: Alaska time Hawaii time

Determining Times If you know your local time, you can determine what time it is across the world Map: "TimeZones" by http://liftoff.msfc.nasa.gov/Academy/Rocket_Sci/clocks/TimeZones.GIF, as seen on http://liftoff.msfc.nasa.gov/Academy/Rocket_Sci/clocks/time-gmt.html. Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:TimeZones.GIF#mediaviewer/File:TimeZones.GIF

3pm4pm 5pm  6pm Determining Times As you move east, add one hour to your time for each time zone you move through, for example: 3pm4pm 5pm  6pm Map: "National-atlas-timezones-2006". Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:National-atlas-timezones-2006.gif#mediaviewer/File:National-atlas-timezones-2006.gif

10am11am12pm 1pm Determining Times Conversely, as you move west, subtract one hour for each time zone, for example: 10am11am12pm 1pm Map: "National-atlas-timezones-2006". Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:National-atlas-timezones-2006.gif#mediaviewer/File:National-atlas-timezones-2006.gif

Practice Questions If it is 9am in California, what time is it in Colorado? (10am) If it is 6pm in Iowa, what time is it in New York? (7pm) If it is 3pm in Louisiana, what time is it in Maine? (4pm) If it is noon in Georgia, what time is it in Arkansas? (11am) If it is 6am in Ohio, what time is it in Wyoming? (4am) If it is 10pm in Nevada, what time is it in Illinois? (12am/midnight) If it is 2pm in Michigan, what time is it in Alabama (1pm) If it is 8am in Montana, what time is it in South Carolina? (10am) If it is midnight in West Virginia, what time is it in the state of Washington? (9pm) If it is 11am in New Mexico, what time is it in Massachusetts? (1pm) Most of Arizona does not use daylight savings time Map: "National-atlas-timezones-2006". Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:National-atlas-timezones-2006.gif#mediaviewer/File:National-atlas-timezones-2006.gif