Time Zones

Time Zones
Everyone on the planet wants the sun to be at its highest point in the sky (crossing the meridian) at noon. If there were just one time zone, this would be impossible because the Earth rotates 15 degrees every hour. The idea behind multiple time zones is to divide the world into 24 15-degree slices and set the clocks accordingly in each zone. All of the people in a given zone set their clocks the same way, and each zone is one hour different from the next.

In the continental United States there are four time zones (click here for a map): Eastern, Central, Mountain and Pacific. When it is noon in the Eastern time zone, it is 11 a.m. in the Central time zone, 10 a.m. in the Mountain time zone and 9 a.m. in the Pacific time zone.

All time zones are measured from a starting point centered at England's Greenwich Observatory. This point is known as the Greenwich Meridian or the Prime Meridian. Time at the Greenwich Meridian is known as Greenwich Mean Time (GMT) or Universal Time. The Eastern time zone in the United States is designated as GMT minus five hours. When it is noon in the Eastern time zone, it is 5 p.m. at the Greenwich Observatory. The International Date Line (IDL) is located on the opposite side of the planet from the Greenwich Observatory.

Why is the Greenwich Observatory such a big deal? A bunch of astronomers declared the Greenwich Observatory to be the prime meridian at an 1884 conference. What's funny is that the observatory moved to Sussex in the 1950s, but the original site remains the prime meridian.

Daylight-saving Time
During World War I, many countries started adjusting their clocks during part of the year. The idea was to try to adjust daylight hours in the summer to more closely match the hours that people are awake. During World War I, the goal was to conserve fuel by lowering the need for artificial light.

The United States and several other countries still use some variation on this system. In the United States, traditionally, daylight-saving time has started on the first Sunday in April and ended on the last Sunday in October. However, the Energy Policy Act of 2005 mandated a change to the observed dates. Starting in 2007 and going forward, DST will now begin at 2 a.m. on the second Sunday of March and will end at 2 a.m. on the first Sunday in November. Here are the start and stop dates through 2015:

Daylight Savings Time through 2015
Year Start Stop
2007
March 11

November 4
2008
March 9

November 2
2009
March 8

November 1
2010
March 14

November 7
2011
March 13

November 6
2012
March 11

November 4
2013
March 10

November 3
2014
March 9

November 2
2015
March 8

November 1

To observe DST, clocks are advanced one hour in the spring and moved back one hour in the fall ("spring forward, fall back" is a phrase many people use to remember this). You lose an hour in the spring and get it back in the fall.

During the winter, the United States is on standard time. During the summer, the United States is on daylight-saving time. Even though it's an act of Congress, some states (like Arizona) ignore it and don't have daylight-saving time. They are on standard time all year.