Global Time and Different Time Scales

Global Time

There’s an old Canadian joke that goes, “Repent! The world will end at midnight!—or, 12:30 a.m. in Newfoundland.” It’s humorous because independent-minded Newfoundlanders use a time zone that is a half hour ahead of the other Canadian maritime provinces. It highlights the fact that one single instant across the world—no matter how cataclysmic—is simultaneously labeled by different times in different local places.

Humans long ago decided to divide the solar day into 24 units, called hours, and devised clocks to keep track of hours in groups of 12. Yet, different regions set their clocks differently—when it is 10:03 a.m. in New York, it is 9:03 a.m. in Chicago, 8:03 a.m. in Denver, and 7:03  a.m. in Los Angeles. These times differ by exactly one hour. How did this system come about? How does it work?

Even in today’s advanced age, our global time system is oriented to the Sun. Think for a moment about the Sun moving across the sky. In the morning, the Sun is low on the eastern horizon, and as the day progresses, it raises higher until at solar noon it reaches its highest point in the sky. If you check your watch at that moment, it will read a time somewhere near twelve o ’clock (noon). After solar noon, the Sun ’s elevation in the sky decreas-es. By late afternoon, the Sun hangs low in the sky, and at sunset it rests on the western horizon.

Imagine for a moment that you are in Chicago. The time is noon, and the Sun is at or near its highest point in the sky. You call a friend in New York and ask about the position of the Sun. Your friend will say that the Sun has already passed solar noon, its highest point, and is beginning its descent down. Meanwhile, a friend in Portland will report that the Sun is still work-ing its way up to its highest point. But a friend in Mobile, Alabama, will tell you that the time in Mobile is the same as in Chicago, and that the Sun is at about solar noon. How do we explain these different observations?

The difference in time makes sense because solar noon can only occur simultaneously in places with the same lon-gitude. Only one meridian can be directly under the Sun and experience solar noon at a given moment. Locations on meridians to the east of Chicago, like New York, have passed solar noon, and locations to the west of Chicago, like Portland, have not yet reached solar noon. Since Mobile and Chicago have nearly the same longitude, they experience solar noon at approximately the same time.

Since the Earth turns 360° in a 24-hour day, the rota-tion rate is 360° / 24 5 15°per hour. So 15° of longitude equates to one hour of time.

STANDARD TIME

We ’ve just seen that loca-tions with different longi-tudes experience solar noon at different times. But what would happen if each town or city set its clocks to read 12:00 at its own local solar noon? All cities and towns on different meridians would have different local time systems. With today ’s instantaneous global communica-tion, chaos would soon result.

Standard time simplifies the global timekeeping prob-lem. In the standard time system the globe is divided into time zones. People within a zone keep time according to a standard meridian that passes through their zone. Since the standard meridians are usually 15° apart, the difference in time between adjacent zones is normally one hour. In some geographic regions, however, the dif-ference is only one half hour.

The United States and its Caribbean possessions fall within seven time zones. Six zones Cover Canada. Their names and standard meridians of longi-tude are shown in Table:

WORLD TIME ZONES

According to our map of the world’s time zones, the country spanning the greatest num-ber of time zones is Russia. From east to west, Russia spans 11 zones, but groups them into 8 standard time zones. China covers 5 time zones but runs on a single national time using the standard meridian of Beijing.

A few countries, such as India and Iran, keep time using a meridian that is positioned midway between standard meridians, so that their clocks depart from those of their neighbors by 30 or 90 minutes. Some states or provinces within countries also keep time by 7½° meridians, such as the Canadian province of Newfoundland and the interior Australian states of South Australia and Northern Territory.

World time zones are often referred to by number to indicate the difference in hours between time in a zone and time in Greenwich. A number of –7, for example, indicates that local time is seven hours behind Greenwich time, while a 13 indicates that local time is three hours ahead of Greenwich time.

INTERNATIONAL DATE LINE

Take a world map or globe with 15° meridians. Start at the Greenwich 0° meridian and count along the 15° meridians in an eastward direction. You will find that the 180th meridian is number 12 and that the time at this meridian is, therefore, 12 hours later than Greenwich time. Counting in a similar manner westward from the Greenwich meridian, we find that the 180th meridian is again number 12 but that the time is 12 hours ear-lier than Greenwich time. We seem to have a paradox: How can the same meridian be both 12 hours ahead of Greenwich time and 12 hours behind it? The answer is that each side of this meridian is experiencing a different day.

Imagine that you are on the 180° meridian on June 26. At the exact instant of midnight, the same 24-hour calendar day covers the entire globe. Stepping east will place you in the very early morn-ing of June 26, while stepping west will place you very late in the evening of June 26. You are on the same calendar day on both sides of the meridian but 24 hours apart in time.

Doing the same experiment an hour later, at 1:00 a.m., stepping east you will find that you are in the early morning of June 26. But if you step west, you will find that midnight of June 26 has passed, and it is now the early morning of June 27. So on the west side of the 180th meridian, it is also 1:00 a.m. but it is one day later than on the east side. For this reason, the 180th merid-ian serves as the International Date Line. This means that if you travel westward across the date line, you must advance your calendar by one day. If traveling eastward, you set your calendar back by a day.

Air travelers on Pacific routes between North America and Asia cross the date line. For exam-ple, flying westward from Los Angeles to Sydney, Australia, you may depart on a Tuesday evening and arrive on a Thursday morn-ing after a flight that lasts only 14 hours. On an east-ward flight from Tokyo to San Francisco, you may actu-ally arrive the day before you take off, taking the date change into account!

Actually, the International Date Line does not fol-low the 180th meridian exactly. Like many time zone boundaries, it deviates from the meridian for practical reasons. It has a zigzag offset between Asia and North America, as well as an eastward offset in the South Pacific to keep clear of New Zealand and several island groups.

DAYLIGHT SAVING TIME

The United States and many other countries observe some form of daylight saving time , in which clocks are set ahead by an hour (sometimes two) for part of the year. Although it was once thought that adding daylight hours to the end of the workday would save electricity and reduce traffic accidents and crime, the evidence now shows that the primary effects are eco-nomic—allowing more retail shopping and recreation, for example. Although something of a mixed blessing, daylight saving time is now a part of normal life in most places.

In the United States, daylight saving time comes into effect on the second Sunday in March and is dis-continued on the first Sunday of November. Arizona (except the Navajo Nation), Puerto Rico, Hawaii, U.S. Virgin Islands, Guam, the Northern Mariana Islands, and American Samoa do not observe daylight saving time. Although many other nations observe daylight saving time, they do not always begin and end it on the same days of the year. In the European Union, daylight saving time is called summer time. It begins on the last Sunday in March and ends on the last Sunday in October.

PRECISE TIMEKEEPING

Since the 1950s, the most accurate time has been kept using atomic clocks, which are based on the frequency of microwave energy emission from atoms of the ele-ment cesium cooled to near absolute zero. These very accurate clocks keep time to better than one part in 1 trillion. Atomic time is a universal standard that is not related to the Earth’s rotation. Civil time sources use Coordinated Universal Time (UTC), which is derived from atomic time and provides a day of 86,400 seconds (24 hours) in length to match the Earth ’s mean rota-tion rate with respect to the Sun. Coordinated Universal Time is administered by the Bureau International del’Heure, located near Paris.

Our Earth is a much less precise timekeeper, exhibiting small changes in the angular velocity of its rotation on its axis and variations in the time it takes to complete one circuit around the Sun. As a result, constant adjustments to the timekeeping system are necessary.