Time Zones

Theory

• Earth rotates on axis once every 24 hours. Therefore there are 24 time zones, with 1 hour difference between them.

• 360° in 24 hours = 15° of longitude in 1 hour.• Every place within a time zone has the same

time, referred to as its standard time.• (Developed by Canadian Engineer Sir Sandford

Fleming)

Prime Meridian

• Where to start? The meridian (line of longitude) that runs through the Royal Observatory in Greenwich, England as the prime, or zero, meridian. (Get it?)

• The prime meridian is the center of a time zone that extends 7.5° on either side, called universal time (UT) and the standard times in other zones are compared to it.

Ahead or Behind UT?

• Since the earth rotates from west to east, time zones that are east of the prime meridian have local times that are ahead of UT. Time zones that are west of the prime meridian have local times that are behind UT.

• Ex. If the sun is directly above the prime meridian, the local time in the city of Berlin, which is one time zone east of UT, is 1pm

• In Ottawa, it would be 7am (5 time zones west of the prime meridian)

Modifying Time Zones

• Countries may modify the shape of the time zones and the standard times they use (for political reasons).

• Ex. China has one standard time for the whole country despite the fact that it covers more than 60° of longitude.

• Canada has six standard time zones. The boundaries of the zones, however, do not follow exactly the meridians of longitude as discussed above.

Reasons for Modifying

• All of a province in one.• Do not pass through a city.• Location (ex. St. John’s, Newfoundland, at

52.5°W is located between the third and fourth time zones, west of the prime meridian so its time is 3.5 hours behind UT.

International Date Line

• When sailors first sailed around the world, they returned home either a day ahead or a day behind those people who stayed.

• Runs between the North and South Pole, and generally follows the 180° line of longitude through the Pacific Ocean (but zigzags to avoid land masses and islands that belong to the same country)

To Add or Subtract?

• If you cross moving westward, you add a day. If you cross moving eastward, you lose a day.

Daylight-Saving Time

• Change time according to the season.• Summer=Daylight-Saving Time• Used to extend daylight hours into the evening

when most people are awake, thereby cutting down on energy use since lights don’t have to be turned on until later in the day.

• Begins 2nd Sunday in March @2am. Ends 1st Sunday in November @2am.

Let’s Practice…