Overview of Time Zones Share By Matt Rosenberg Prior to the late nineteenth century, time keeping was a purely local phenomenon. Each town would set their clocks to noon when the sun reached its zenith each day. A clockmaker or town clock would be the "official" time and the citizens would set their pocket watches and clocks to the time of the town - enterprising citizens would offer their services as mobile clock setters, carrying a watch with the accurate time to adjust the clocks in customer's homes on a weekly basis. Travel between cities meant having to change one's pocket watch upon arrival. However, once railroads began to operate and move people rapidly across great distances, time became much more critical. In the early years of the railroads, the schedules were very confusing because each stop was based on a different local time. The standardization of time was essential to efficient operation of railroads. In 1878, Canadian Sir Sanford Fleming proposed the system of worldwide time zones that we use today. continue reading below our video 10 Facts About the Titanic That You Don't Know He recommended that the world be divided into twenty-four time zones, each spaced 15 degrees of longitude apart.

Since the earth rotates once every 24 hours and there are 360 degrees of longitude, each hour the earth rotates one-twenty-fourth of a circle or 15 degrees of longitude. Sir Fleming's time zones were heralded as a brilliant solution to a chaotic problem worldwide. United States railroad companies began utilizing Fleming's standard time zones on November 18, 1883. In 1884 an International Prime Meridian Conference was held in Washington D.C. to standardize time and select the prime meridian . The conference selected the longitude of Greenwich, England as zero degrees longitude and established the 24 time zones based on the prime meridian. Although the time zones had been established, not all countries switched immediately. Though most U.S. states began to adhere to the Pacific, Mountain, Central, and Eastern time zones by 1895, Congress didn't make the use of these time zones mandatory until the Standard Time Act of 1918. Today, many countries operate on variations of the time zones proposed by Sir Fleming.

All of China (which should span five time zones) uses a single time zone - eight hours ahead of Coordinated Universal Time (known by the abbreviation UTC - based on the time zone running through Greenwich at 0 degrees longitude). Australia uses three time zones - its central time zone is a half-hour ahead of its designated time zone. Several countries in the Middle East and South Asia also utilize half-hour time zones. Since time zones are based on segments of longitude and lines of longitude narrow at the poles, scientists working at the North and South Poles simply use UTC time. Otherwise, Antarctica would be divided into 24 very thin time zones! The time zones of the United States are standardized by Congress and although the lines were drawn to avoid populated areas, sometimes they've been moved to avoid complication. There are nine time zones in the U.S. and its territories, they include Eastern, Central, Mountain, Pacific, Alaska, Hawaii-Aleutian, Samoa, Wake Island, and Guam. With the growth of the Internet and global communication and commerce, some have advocated a new worldwide time system.

Longitude, time zones and navigation, Our world, Investigating the World, SOSE: Geography Year 8, NSW | Online Education Home Schooling Skwirk Australia navigation Introduction Early sailors and navigators did not have world maps neatly marked with the longitude and latitude grid lines. The earliest sailors stayed within a certain distance of the coast so that certain landmarks could be viewed to mark the distance and direction of travel. Once new lands had begun to be charted, the stars could be used as a way to measure latitude but sailors still did not know their speed to know how far north, south, east or west they might be travelling. The nautical measurement of speed is the knot. A rope was tied to a piece of wood. At regular intervals knots were tied along the rope. One man would drop the rope overboard and count the number of knots that passed through his hand while someone else took the time. Today, nautical speed is still measured in knots. One knot is equal to one nautical mile (about 1.85 km). What is longitude? Lines of longitude are lines drawn vertically around the globe from north to south to measure the distance east or west of the Prime Meridian (0o).

These lines of longitude are known as meridians and all meet at the north pole and south pole. Longitude is measured in degrees and minutes. The Prime Meridian is the line of longitude that is measured at 0o. This line of longitude is also known as the Greenwich Meridian because it passes through Greenwich, England. On the other side of the world this line is measured at 180o and is known as the International Date Line. This meridian is bent so that is does not pass through any land. The area between 0o and 180o east of the Prime Meridian is known as the eastern hemisphere. On the western side of the Prime Meridian the area between 0o and 180o is known as the western hemisphere. Refer Image 1 World time zones The meridians around the globe divide the Earth into time zones. Time zones have been created because everyone on Earth does not experience night and day at the same time. The Earth is in continual rotation. Time zones allow the whole world to function together as one unit without disruption in transportation schedules. The Earth completes one full rotation every 24 hours. These hours represent one day according to the calendar.

Every hour the Earth turns 15o out of the complete 360o rotation. There are 24 time zones based on 15o segments of one hour. The time zones start at the Prime Meridian and go toward the east, advancing one hour for every 15o of longitude. The International Date Line at 180o represents the point at which 12 hours have passed of the total 24. This is the point at which a new day might start. If the time in Greenwich is (after) 12 noon on Tuesday, the time at the International Date Line would be (after) 12 midnight on Wednesday. For example, if it were 1:15am on Wednesday in New York, it would be 3:15pm on a Wednesday in Sydney and it would be 6:15am on Wednesday in London. If you fly from Australia to the United States you are 'adding' an extra day to your trip. Your arrival at the local day and time might be earlier than the local day and time of your departure. If you are flying from the United States to Australia, however, you will 'lose' a day.

If you leave the United States on a Monday, you will skip Tuesday and arrive in Australia on Wednesday. Time zone variations Even though the world is easily divided into 24 different time zones, each an hour later going towards the east, it is not always so simple. There are places in the world that set their clocks forward an hour in the spring and then back again in the autumn. This is called Daylight Saving Time. When the southern hemisphere is on Standard Time, the northern hemisphere is on Daylight Saving Time. Some places in the world do not change their clocks at all, such as Arizona in the United States. Other places only change the clock forward half an hour. In Australia, Queensland, and the Northern Territory do not change their clocks at all. The Northern Territory and South Australia are 30 minutes behind standard (non-daylight saving) Sydney time. Also remember that while the southern hemisphere is experiencing spring, the northern hemisphere is experiencing autumn. While Sydney is setting the clock ahead during the spring, Los Angeles is setting the clock back to standard time. Latitude + longitude = navigation Refer video A map or globe that shows the lines of latitude and the lines of longitude will look as though it has a grid placed on top of it.

This grid is a key element in finding any location in the world. Sailors use measurements of latitude and longitude to determine their location as they sail around the Earth. This is navigation. Latitude and longitude are both measured using degrees, minutes and seconds. Sixty seconds make up one minute (60" = 1'). Sixty minutes make up one degree (60' = 1o). Degrees are shown as north or south for measurements of latitude and east or west for measurements of longitude. Degrees are the most general measurements. Minutes and seconds are used to find more exact locations, such as a specific place within a large city. On a map, the lines of latitude and longitude are drawn on and marked with their measurements. The measurements in degrees usually appear at the edges of the map. Latitude (the horizontal lines) is given first. Longitude (the vertical lines) is given second. Together, these are called coordinates. Refer Image 2 If you are looking at a map and want to know the coordinates for Melbourne, there are several steps to take. First, locate Australia, then Victoria, then Melbourne on a map. Mark the location of Melbourne. Then draw a line or use a ruler to make a horizontal line to the edge of the map.

Estimate what the latitude would be using the given measurements. Write this measurement down in degrees north or south (oN or oS). This is the latitude. Finally, find the lines of longitude that are closest to Melbourne. Follow those to the top or bottom of the map. Estimate what the degree of longitude would be based on the given measurements. Write this down in degrees east or west (oE or oW). This is the longitude. For example, using the following coordinates, try to find these places on a map: South-eastern hemisphere Sydney, Australia: 34o 0' S / 151o 0' E North-eastern hemisphere Bangkok, Thailand: 13o 45' N / 100o 30' E South-western hemisphere Buenos Aires, Argentina: 34o 35' S / 58o 22' W North-western hemisphere Longitude, Prime Meridian, International Date Line; Latitude 34 North - Cities; Longitude Return to the page index. Return to the top of the page.

International Date Line The world is divided into 24 time zones, each time zone is 15° of longitude wide (360° divided by 24 hours – the approximate length of a day ). A new day begins at The International Date Line (IDL). The International Date Line is located at 180° line of longitude, exactly one-half way around the planet from Greenwich, England. Each Time Zone is measured relative to Greenwich, England . The military, meteorologist and the aviation industry refer to Greenwich Mean Time as Zulu time . As you travel from west to east, and travel from one time zone to the next, the time you will set your watch forward by one hour as you enter each new time zone. Tine zones to the east of Greenwich England are denoted by a positive number. Example, if it is noon in Greenwich, it will be 1:00 pm in the time zone to the east of Greenwich, or GMT + 1. The first time zone to the east is said to be +1, the next is +2 until you reach the time zone just west of the International Date Line at +12.

When you cross the International Date Line, you subtract one day and the time zone is now -12. The next time zone is -11. This pattern continues until we once again reach the prime meridian at time zone zero. As an example, London England is in time zone zero while Los Angeles, California is in time zone -8 . This means when it is 2 pm (or 14:00) in London, it is only 6:00 am in Los Angeles (14:00 - 8 = 6). If we traveled east from London to Wellington, New Zealand which is time zone +12 , then it will be 2 am the next day in Wellington, New Zealand (do you really want to see the math? OK 14 + 12 = 26, since 26 is greater than 24, then the time is 26 - 24 = 2 on the next day). The International Date Line makes detours around political boundaries because the date to the east of the line is one day earlier than that to the west of the line.

The line deviates to pass around the far east of Russia and various island groups in the Pacific because no country wants to have its citizens functioning on two different dates. Time zones also often follow political boundaries for the same reason. Some countries have adopted non-standard time zones, usually 30 minutes offset. Two countries that have adopted this standard are Afghanistan and India . This allows all locations within the country to have the same local time. Return to the page index. Return to the top of the page. For more information about the International Date Line, visit the following sites: Return to the page index. Return to the top of the page. Royal Observatory, Greenwich England The Royal Observatory located in Greenwich England is the oldest scientific institution in Britain, founded for navigational purposes in 1675 by Charles II . John Flamsteed , Astronomer Royal, was appointed to work at the Royal Observatory and to get the help of the heavens in finding longitude at sea by calculating " time " from the movement of the heavens. Flamsteed spent forty years plotting the stars in the sky. The Royal Observatory main contributions have been in navigation, timekeeping, determination of star positions, and almanac publication.

In 1767 it began publishing The Nautical Almanac, based on the time at the longitude of Greenwich; its popularity among navigators led in part to the Greenwich meridian's being made Earth's prime meridian and the starting point for international time zones in 1884. For more information about the Royal Observatory, Greenwich England, visit the following sites: Return to the page index. Return to the top of the page. Distance between lines of longitude The distance between each line of longitude changes as you move north or south. To visualize this think of the earth as a peeled orange with the lines between each wedge of the orange as the lines of longitude. At the equator (0° latitude), each degree of longitude is about 70 land miles apart (The circumference of the earth is 24,859.82 miles divided by 360 degrees equaling 69.06 miles per degree at the equator). At 34° north or south, each degree of longitude is 57.3436 miles. Close to the poles at 80° North or South, there are only 12 miles between each degree of longitude.

For more information about Calculating the distances between two places on earth, visit the following sites: Return to the page index. Return to the top of the page. John Harrison On October 22nd, 1707 a British fleet led by Sir Cloudesley Shovell struck the rocks of Gilstone Ledges on the Scilly Isles , about 2000 men were lost in the accident. This tragedy led the British Parliament to authorize the British Longitude Act in 1714. This created the Longitude Prize in the amount of 20,000 pounds reward (£10,000 for any method capable of determining a ship´s longitude within one degree, £15,000, within 40 minutes, and £20,000 within one half a degree) for anyone who could devise a practical method of determining longitude at sea. The estblishment of the Longitude Prize led to a race between men of education ( Astronomers ) and men that labored with their hands (mechanic's/Clock makers) to invent an accurate method of determining the time at sea.

Eventually John Harrison (1693-1776) designed and built the world's first successful chronometers . Between 1730 and 1759, John Harrison designed 4 chronometers designated H1 to H4. It was Harrison´s H4 watch that proved to be three times better than required to win longitude prize. For more information about John Harrison, visit the following sites: Lines of Longitude in Geography Longitude is the angular distance of any point on Earth measured east or west of a point on Earth's surface. Where Is Zero Degrees Longitude? Unlike latitude there is no easy point of reference such as the equator to be designated as zero degrees in the longitude system. To avoid confusion, the world's nations have agreed that the Prime Meridian , which passes through the Royal Observatory in Greenwich , England, will serve as that reference point and be designated as zero degrees. Because of this designation, longitude is measured in degrees west or east of the Prime Meridian. For example, 30°E, the line passing through eastern Africa, is an angular distance of 30° east of the Prime Meridian.

30°W, which is in the middle of the Atlantic Ocean, is an angular distance of 30° west of the Prime Meridian. There are 180 degrees east of the Prime Meridian and coordinates are sometimes given without the designation of "E" or east. When this is used, a positive value represents coordinates east of the Prime Meridian. continue reading below our video Latitude and Longitude There are also 180 degrees west of the Prime Meridian and when "W" or west is omitted in a coordinate a negative value such as -30° represents coordinates west of the Prime Meridian. The 180° line is neither east nor west and approximates the International Date Line . On a map ( diagram ), lines of longitude are the vertical lines running from the North Pole to the South Pole and are perpendicular to lines of latitude. Every line of longitude also crosses the equator. Because longitude lines are not parallel, they are known as meridians. Like parallels, meridians name the specific line and indicate the distance east or west of a 0° line.

Meridians converge at the poles and are farthest apart at the equator (about 69 miles (111 km) apart). Development and History of Longitude For centuries, mariners and explorers worked to determine their longitude in an effort to make navigation easier. Latitude was determined easily by observing the inclination of the sun or the position of known stars in the sky and calculating the angular distance from the horizon to them. Longitude could not be determined in this way because Earth's rotation constantly changes the position of stars and the sun. The first person to offer a method for measuring longitude was the explorer Amerigo Vespucci . In the late 1400s, he began measuring and comparing the positions of the moon and Mars with their predicted positions over several nights at the same time ( diagram ). In his measurements, Vespucci calculated the angle between his location, the moon and Mars. By doing this, Vespucci got a rough estimate of longitude. This method did not become widely used however because it relied on a specific astronomical event. Observers also needed to know the specific time and measure the moon and Mars' positions on a stable viewing platform- both of which were difficult to do at sea.

In the early 1600s, a new idea to measure longitude was developed when Galileo determined that it could be measured with two clocks. He said that any point on Earth took 24 hours to travel the full 360° rotation of Earth. He found that if you divide 360° by 24 hours, you find that a point on Earth travels 15° of longitude every hour. Therefore, with an accurate clock at sea, a comparison of two clocks would determine longitude. One clock would be at the home port and the other on the ship. The clock on the ship would need to be reset to local noon each day. The time difference would then indicate the longitudinal difference traveled as one hour represented a 15° change in longitude. Shortly thereafter, there were several attempts to make a clock that could accurately tell time on the unstable deck of a ship. In 1728, clockmaker John Harrison began working on the problem and in 1760, he produced the first marine chronometer called Number 4. In 1761, the chronometer was tested and determined to be accurate, officially making it possible to measure longitude on land and at sea.

Measuring Longitude Today Today, longitude is more accurately measured with atomic clocks and satellites. The Earth is still divided equally into 360° of longitude with 180° being east of the Prime Meridian and 180° west. Longitudinal coordinates are divided into degrees, minutes and seconds with 60 minutes making up a degree and 60 seconds comprising a minute. For example, Beijing, China's longitude is 116°23'30"E. The 116° indicates that it lies near the 116th meridian while the minutes and seconds indicate just how close it is to that line. The "E" indicates that it is that distance east of the Prime Meridian. Although less common, longitude can also be written in decimal degrees . Beijing's location in this format is 116.391°. In addition to the Prime Meridian, which is the 0° mark in today's longitudinal system, the International Date Line is also an important marker. It is the 180° meridian on the opposite side of the Earth and is where the eastern and western hemispheres meet. It also marks the place where each day officially begins.

At the International Date Line, the west side of the line is always one day ahead of the east side, no matter what time of day it is when the line is crossed. This is because the Earth rotates east on its axis. Longitude and Latitude Lines of longitude or meridians are the vertical lines running from the South Pole to the North Pole . Lines of latitude or parallels are the horizontal lines running from the west to the east. The two cross each other at perpendicular angles and when combined as a set of coordinates they are extremely accurate in locating places on the globe. They are so accurate that they can locate cities and even buildings to within inches. For example, the Taj Mahal, located in Agra, India, has a coordinate set of 27°10'29"N, 78°2'32"E. To view the longitude and latitude of other places, visit the collection of Locate Places Worldwide resources on this site. Geography Glossary Geography Glossary latitude lines Imaginary lines running horizontally around the globe. Also called parallels, latitude lines are equidistant from each other. Each degree of latitude is about 69 miles (110 km) apart.

Zero degrees (0°) latitude is the equator, the widest circumference of the globe. Latitude is measured from 0° to 90° north and 0° to 90° south—90° north is the North Pole and 90° south is the South Pole. longitude lines Imaginary lines, also called meridians, running vertically around the globe. Unlike latitude lines, longitude lines are not parallel. Meridians meet at the poles and are widest apart at the equator. Zero degrees longitude (0°) is called the prime meridian. The degrees of longitude run 180° east and 180° west from the prime meridian. geographic coordinates Latitude and longitude lines form an imaginary grid over the Earth's surface. By combining longitude and latitude measurements, any location on earth can be determined. The units of measurement for geographic coordinates are degrees (°), minutes ('), and seconds ("). Like a circle, the Earth has 360 degrees. Each degree is divided into 60 minutes, which in turn is divided into 60 seconds.

Latitude and longitude coordinates also include cardinal directions: north or south of the equator for latitude, and east or west of the prime meridian for longitude. The geographic coordinates of New York City, for example, are 40° N, 74° W, meaning that it is located 40 degrees north latitude and 74 degrees west longitude. Using minutes and seconds as well as degrees, the coordinates for New York would be 40°42'51" N, 74°0'23" W. (Latitude is always listed first.) A less common format for listing coordinates is in decimal degrees. The Tropic of Cancer, for example, can be expressed in degrees and minutes (23°30' N) or in decimal degrees (23.5° N). hemisphere A hemisphere is half the Earth's surface. The four hemispheres are the Northern and Southern hemispheres, divided by the equator (0° latitude), and the Eastern and Western hemispheres, divided by the prime meridian (0° longitude) and the International Date Line (180°). equator Zero degrees latitude.

The Sun is directly overhead the equator at noon on the two equinoxes (March and Sept. 20 or 21). The equator divides the globe into the Northern and Southern hemispheres. The equator appears halfway between the North and South poles, at the widest circumference of the globe. It is 24,901.55 miles (40,075.16 km) long. prime meridian Zero degrees longitude (0°). The prime meridian runs through the Royal Greenwich Observatory in Greenwich, England (the location was established in 1884 by international agreement). The prime meridian divides the globe into the Western and Eastern hemispheres. The Earth's time zones are measured from the prime meridian. The time at 0° is called Universal Time (UT) or Greenwich Mean Time (GMT). With the Greenwich meridian as the starting point, each 15° east and west marks a new time zone. The 24 time zones extend east and west around the globe for 180° to the International Date Line. When it is noon along the prime meridian, it is midnight along the International Date Line.

International Date Line Located at 180° longitude (180° E and 180° W are the same meridian). Regions to the east of the International Date Line are counted as being one calendar day earlier than the regions to the west. Although the International Date Line generally follows the 180° meridian (most of which lies in the Pacific Ocean), it does diverge in places. Since 180° runs through several countries, it would divide those countries not simply into two different time zones, but into two different calendar days. To avoid such unnecessary confusion, the date line dips and bends around countries to permit them to share the same time. Tropic of Cancer A line of latitude located at 23°30' north of the equator. The Sun is directly overhead the Tropic of Cancer on the summer solstice in the Northern Hemisphere (June 20 or 21). It marks the northernmost point of the tropics, which falls between the Tropic of Cancer and the Tropic of Capricorn. map projections Two-dimensional representations of the three-dimensional Earth. Because projections attempt to present the spherical Earth on a flat plane, they inevitably produce distortions.

Map projections are numerous and complex (e.g., there are a variety of cylindrical, conic, or azimuthal projections). Each projection has advantages and serves different purposes, and each produces different types of distortions in direction, distance, shape, and relative size of areas. One of the most famous projections is the Mercator, created by Geradus Mercator in 1569. It is a rectangular-shaped map in which all longitude and latitude lines are parallel and intersect at right angles (on a globe, meridians are not parallel, but grow narrower, eventually converging at the poles). Near the equator, the scale of the Mercator is accurate, but the farther one moves toward the poles, the greater the distortion—Antarctica in the far south and Greenland in the far north, for example, appear gigantic. The Mercator projection was used well into the 20th century, but has now been superseded by others, including the widely used Robinson projection. The Robinson projection is an elliptical-shaped map with a flat top and bottom. Developed in 1963 by Arthur H. Robinson, it is an orthophanic (“right appearing”) projection, which attempts to reflect the spherical appearance of the Earth.

The meridians, for example, are curved arcs, which gives the flat map a three-dimensional appearance. But to convey the likeness of a curved, three-dimensional globe, the Robinson projection must in fact distort shape, area, scale, and distance. The Albers, Lambert, Mollweide, and Winkel Tripel are some of the other commonly used map projections. Time Zone – what is it and why have them? A time zone is a region where the same standard time is used. Time Difference from UTC The local time within a time zone is defined by its offset (difference) from Coordinated Universal Time (UTC), the world's time standard. This offset is expressed as either UTC- or UTC+ and the number of hours and minutes. More Than 24 Time Zones If each time zone were one hour apart, there would be 24 in the world. However, the International Date Line , creates 3 more. In addtion, several time zones are only 30 and 45 minutes apart. This makes the total number of time zones worldwide much higher. Not the Same as Local Time The term time zone is often used instead of local time.

For instance, during Daylight Saving Time (DST), it is common to say "California and Arizona are now in same time zone". However, the correct thing to say would be: "California and Arizona now have the same local time". The reason is that California's local time during DST is UTC-7, but the standard time in California is minus one more hour: UTC-8. However, Arizona's local time is always UTC-7, because there's no DST in Arizona , and they remain on standard time all year. Local Time Zone Names Mountain Standard Time or Mountain Daylight Time or just Mountain Time? The formal name for Mountain Time in the US is either Mountain Standard Time or Mountain Daylight Time. PunchStock.com To confuse matters more, each time zone has many different local time zone names , usually linked to the geographical name of the country or region. The time zone names in the US are consistent: On the mainland, there are 5 time zones with 5 names and corresponding DST names. However, in other places in the world, the time zone names may be completely different, even though the UTC offset is the same.

For instance in Miami, Florida , the standard time zone is UTC-5, and it's called Eastern Standard Time – EST . In Havana, Cuba , the standard time zone is also UTC-5, but it's called Cuba Standard Time – CST . Military Time Zones There are also 25 military time zones which follow the 15 degrees longitudes. These are named according to the NATO phonetic alphabet: Alpha , Bravo , Charlie , etc. and are used in aviation, at sea, and in telecommunications. Identical Abbreviations Another point that can cause confusion, is that some time zone's names in totally different places have exactly the same abbreviation. For example: World Geography Glossary World Geography Glossary latitude lines Imaginary lines running horizontally around the globe. Also called parallels, latitude lines are equidistant from each other. Each degree of latitude is about 69 miles (110 km) apart. Zero degrees (0°) latitude is the equator, the widest circumference of the globe. Latitude is measured from 0° to 90° north and 0° to 90° south—90° north is the North Pole and 90° south is the South Pole.

longitude lines Imaginary lines, also called meridians, running vertically around the globe. Unlike latitude lines, longitude lines are not parallel. Meridians meet at the poles and are widest apart at the equator. Zero degrees longitude (0°) is called the prime meridian. The degrees of longitude run 180° east and 180° west from the prime meridian. geographic coordinates Latitude and longitude lines form an imaginary grid over the Earth's surface. By combining longitude and latitude measurements, any location on earth can be determined. The units of measurement for geographic coordinates are degrees (°), minutes ('), and seconds ("). Like a circle, the Earth has 360 degrees. Each degree is divided into 60 minutes, which in turn is divided into 60 seconds. Latitude and longitude coordinates also include cardinal directions: north or south of the equator for latitude, and east or west of the prime meridian for longitude. The geographic coordinates of New York City, for example, are 40° N, 74° W, meaning that it is located 40 degrees north latitude and 74 degrees west longitude.

Using minutes and seconds as well as degrees, the coordinates for New York would be 40°42'51" N, 74°0'23" W. (Latitude is always listed first.) A less common format for listing coordinates is in decimal degrees. The Tropic of Cancer, for example, can be expressed in degrees and minutes (23°30' N) or in decimal degrees (23.5° N). Theory that the earth's continents at one time were once part of a singular landmass. It is believed that the continents have spread out due to plate tectonics. hemisphere A hemisphere is half the Earth's surface. The four hemispheres are the Northern and Southern hemispheres, divided by the equator (0° latitude), and the Eastern and Western hemispheres, divided by the prime meridian (0° longitude) and the International Date Line (180°). equator Zero degrees latitude. The Sun is directly overhead the equator at noon on the two equinoxes (March and Sept. 20 or 21). The equator divides the globe into the Northern and Southern hemispheres.

The equator appears halfway between the North and South poles, at the widest circumference of the globe. It is 24,901.55 miles (40,075.16 km) long. prime meridian Zero degrees longitude (0°). The prime meridian runs through the Royal Greenwich Observatory in Greenwich, England (the location was established in 1884 by international agreement). The prime meridian divides the globe into the Western and Eastern hemispheres. The Earth's time zones are measured from the prime meridian. The time at 0° is called Universal Time (UT) or Greenwich Mean Time (GMT). With the Greenwich meridian as the starting point, each 15° east and west marks a new time zone. The 24 time zones extend east and west around the globe for 180° to the International Date Line. When it is noon along the prime meridian, it is midnight along the International Date Line. International Date Line Located at 180° longitude (180° E and 180° W are the same meridian).

Regions to the east of the International Date Line are counted as being one calendar day earlier than the regions to the west. Although the International Date Line generally follows the 180° meridian (most of which lies in the Pacific Ocean), it does diverge in places. Since 180° runs through several countries, it would divide those countries not simply into two different time zones, but into two different calendar days. To avoid such unnecessary confusion, the date line dips and bends around countries to permit them to share the same time. tropics Also referred to as the torrid zone or tropical zone, all the water and land of the earth between the Tropic of Cancer and the Tropic of Capricorn. The tropics experience at least one day per year in which the sun passes directly overhead. Tropic of Cancer A line of latitude located at 23°30' north of the equator. The Sun is directly overhead the Tropic of Cancer on the summer solstice in the Northern Hemisphere (June 20 or 21). It marks the northernmost point of the tropics, which falls between the Tropic of Cancer and the Tropic of Capricorn.

A directional device that is made of a magnetic needle that synchronizes with the north and south poles. climate maps Give general information about the climate and precipitation (rain and snow) of a region. Cartographers, or mapmakers, use colors to show different climate or precipitation zones. economic or resource maps Feature the type of natural resources or economic activity that dominates an area. Cartographers use symbols to show the locations of natural resources or economic activities. For example, oranges on a map of Florida tell you that oranges are grown there. physical maps Illustrate the physical features of an area, such as the mountains, rivers and lakes. The water is usually shown in blue. Colors are used to show relief-differences in land elevations. Green is typically used at lower elevations, and orange or brown indicate higher elevations. political maps Do not show physical features. Instead, they indicate state and national boundaries and capital and major cities. A capital city is usually marked with a star within a circle. road maps Show major-some minor highways-and roads, airports, railroad tracks, cities and other points of interest in an area. People use road maps to plan trips and for driving directions. topographic maps Include contour lines to show the shape and elevation of an area.

Lines that are close together indicate steep terrain, and lines that are far apart indicate flat terrain. map projections Two-dimensional representations of the three-dimensional Earth. Because projections attempt to present the spherical Earth on a flat plane, they inevitably produce distortions. Map projections are numerous and complex (e.g., there are a variety of cylindrical, conic, or azimuthal projections). Each projection has advantages and serves different purposes, and each produces different types of distortions in direction, distance, shape, and relative size of areas. One of the most famous projections is the Mercator, created by Geradus Mercator in 1569. It is a rectangular-shaped map in which all longitude and latitude lines are parallel and intersect at right angles (on a globe, meridians are not parallel, but grow narrower, eventually converging at the poles). Near the equator, the scale of the Mercator is accurate, but the farther one moves toward the poles, the greater the distortion—Antarctica in the far south and Greenland in the far north, for example, appear gigantic. The Mercator projection was used well into the 20th century, but has now been superseded by others, including the widely used Robinson projection.

The Robinson projection is an elliptical-shaped map with a flat top and bottom. Developed in 1963 by Arthur H. Robinson, it is an orthophanic (“right appearing”) projection, which attempts to reflect the spherical appearance of the Earth. The meridians, for example, are curved arcs, which gives the flat map a three-dimensional appearance. But to convey the likeness of a curved, three-dimensional globe, the Robinson projection must in fact distort shape, area, scale, and distance. The Albers, Lambert, Mollweide, and Winkel Tripel are some of the other commonly used map projections. Longitude and Time Zones Unit 2 : Activity 2 : The Terrestrial Coordinate System : Longitude and Time Zones : Earth Clock : Unit Exam Longitude and Time Zones As the earth turns, the sun appears to move across the sky. This apparent motion gives us day and night. A sundial uses the position of the sun in the sky to tell the time. The time measured by a sundial is solar time . Solar time is 12 noon when the sun is on the (observer's local) meridian . Note that we don't say "when the sun is directly overhead" -- why not?

Various Kinds of Time Sundials do not keep time at a uniform rate the way mechanical clocks do. If you set a sundial to be on time at one time of the year then it will run fast or slow at other times of the year. When people started depending more on mechanical clocks than sundials it became convenient to switch to using time that, on average, matched sundial time, but ran at a constant rate. This is mean solar time . The difference between mean solar time and solar time is called the " equation of time ." Here's something to remember: Mean Solar Time is directly related to longitude . Your longitude is equal to [(your Mean Solar Time) minus (Greenwich Mean Solar Time) ] x 15 degrees. You need to be sure to express the mean solar time in hours (and fractions of an hour). 1) There is a location on Earth that has a mean solar time 8 hours different than Greenwich's mean solar time. What is this location's longitude? It's impossible to tell without knowing exactly what time it is. It's 120 West There are two places, 120 East and 120 West. Iowa State University is located in Ames, Iowa.

There is a town nearby called Boone. Because they are located at different longitudes, the two towns have slightly different mean solar times. It would certainly be inconvenient if we still used mean solar time for each town! When railroads came in and allowed people to travel across the country quickly, mean solar time was replaced by zone time (or standard time) for everyone to use. With zone time we all agree to use the mean solar time of the nearest longitude divisible by 15 deg. Thus, for Central Standard Time used at Iowa State, we use the mean solar time for longitude 90 deg W. A quick side note: Standard time for longitude 0 deg is called Greenwich Mean Time or Universal Time, or just UT for short. Standard time for the time zone we are in, centered around 90 deg West, is Central Standard Time, or CST. 2) What is the time called in the zones centered at 75 deg W, 105 deg W, and 120 deg W? -------- The earth is the fundamental "clock" we use in defining time, because we are sensitive to the difference between day and night.

However, the earth is not a perfect clock and there are now atomic clocks that are more accurate than the earth. In order to keep the time displayed by these highly accurate clocks in agreement with the Earth's rotation we have to insert "leap seconds" at midnight on December 31, every now and then. However, for astronomical purposes it is useful to have a kind of time that isn't complicated by leap-seconds. This kind of time is called ephemeris time . Ephemeris time once agreed with Universal Time (UT) at the beginning of the 20th century; now they differ by about half a minute. In the United States, we usually use A.M. to mean the morning. "AM" is from "ante meridiem", meaning before the sun crosses the meridian, or before noon. P.M., post meridiem, helps, of course, to distinguish between morning and afternoon hours. In Europe and in the U.S. military, it is common to use a 24-hour clock , starting at midnight. So, 1 AM just after midnight, is 0100 hours in military language, while 1 P.M.

is 1300 hours and so on. Your computer and most digital watches have an option to display 24-hour time instead of the usual ante meridiem and post merdiem time. It is actually easier for calculations to use the 24-hour clock, so you will find 24 hour time scattered througout this course. 3) On a 24-hour clock, we might eat dinner at 1800. What time is this in terms of AM / PM? -------- Latitude and Longitude 8a. The Horizon Any location on Earth is described by two numbers--its latitude and its longitude. If a pilot or a ship's captain wants to specify position on a map, these are the "coordinates" they would use. Actually, these are two angles, measured in degrees, "minutes of arc" and "seconds of arc." These are denoted by the symbols ( °, ', " ) e.g. 35° 43' 9" means an angle of 35 degrees, 43 minutes and 9 seconds (do not confuse this with the notation (', ") for feet and inches!).

A degree contains 60 minutes of arc and a minute contains 60 seconds of arc--and you may omit the words "of arc" where the context makes it absolutely clear that these are not units of time. Calculations often represent angles by small letters of the Greek alphabet, and that way latitude will be represented by λ (lambda, Greek L), and longitude by φ (phi, Greek F). Here is how they are defined . PLEASE NOTE: Charts used in ocean navigation often use the OPPOSITE notation--λ for LONGITUDE and φ for LATITUDE. The convention followed here resembles the one used by mathematicians in 3 dimensions for spherical polar coordinates . Imagine the Earth was a transparent sphere (actually the shape is slightly oval; because of the Earth's rotation, its equator bulges out a little). Through the transparent Earth (drawing) we can see its equatorial plane, and its middle the point is O, the center of the Earth. To specify the latitude of some point P on the surface, draw the radius OP to that point. Then the elevation angle of that point above the equator is its latitude λ--northern latitude if north of the equator, southern (or negative) latitude if south of it.

Latitude The latitude angle lambda Imagine the Earth was a transparent sphere (actually the shape is slightly oval; because of the Earth's rotation, its equator bulges out a little). Through the transparent Earth (drawing) we can see its equatorial plane, and its middle the point is O, the center of the Earth. To specify the latitude of some point P on the surface, draw the radius OP to that point. Then the elevation angle of that point above the equator is its latitude λ--northern latitude if north of the equator, southern (or negative) latitude if south of it. [How can one define the angle between a line and a plane, you may well ask? After all, angles are usually measured between two lines! Good question. We must use the angle which completes it to 90 degrees, the one between the given line and one perpendicular to the plane. Here that would be the angle (90°-λ) between OP and the Earth's axis, known as the co-latitude of P.] Lines of latitude On a globe of the Earth, lines of latitude are circles of different size.

The longest is the equator, whose latitude is zero, while at the poles--at latitudes 90° north and 90° south (or -90°) the circles shrink to a point. Longitude On the globe, lines of constant longitude ("meridians") extend from pole to pole, like the segment boundaries on a peeled orange. Every meridian must cross the equator. Since the equator is a circle, we can divide it--like any circle--into 360 degrees, and the longitude φ of a point is then the marked value of that division where its meridian meets the equator. Longitude lines or "meridians" What that value is depends of course on where we begin to count--on where zero longitude is. For historical reasons, the meridian passing the old Royal Astronomical Observatory in Greenwich, England, is the one chosen as zero longitude. Located at the eastern edge of London, the British capital, the observatory is now a public museum and a brass band stretching across its yard marks the "prime meridian." Tourists often get photographed as they straddle it--one foot in the eastern hemisphere of the Earth, the other in the western hemisphere.

A lines of longitude is also called a meridian, derived from the Latin, from meri, a variation of "medius" which denotes "middle", and diem, meaning "day." The word once meant "noon", and times of the day before noon were known as "ante meridian", while times after it were "post meridian." Today's abbreviations a.m. and p.m. come from these terms, and the Sun at noon was said to be "passing meridian". All points on the same line of longitude experienced noon (and any other hour) at the same time and were therefore said to be on the same "meridian line", which became "meridian" for short . About time--Local and Universal Two important concepts, related to latitude and (especially) longitude are Local time (LT) and Universal time (UT) Local time is actually a measure of the position of the Sun relative to a locality. At 12 noon local time the Sun passes to the south and is furthest from the horizon (northern hemisphere). Somewhere around 6 am it rises, and around 6 pm it sets.

Local time is what you and I use to regulate our lives locally, our work times, meals and sleep-times. But suppose we wanted to time an astronomical event--e.g. the time when the 1987 supernova was first detected. For that we need a single agreed-on clock, marking time world-wide, not tied to our locality. That is universal time (UT), which can be defined (with some slight imprecision, no concern here) as the local time in Greenwich, England, at the zero meridian. Local Time (LT) and Time Zones Longitudes are measured from zero to 180° east and 180° west (or -180°), and both 180-degree longitudes share the same line, in the middle of the Pacific Ocean. As the Earth rotates around its axis, at any moment one line of longitude--"the noon meridian"--faces the Sun, and at that moment, it will be noon everywhere on it. After 24 hours the Earth has undergone a full rotation with respect to the Sun, and the same meridian again faces noon. Thus each hour the Earth rotates by 360/24 = 15 degrees.

When at your location the time is 12 noon, 15° to the east the time is 1 p.m., for that is the meridian which faced the Sun an hour ago. On the other hand, 15° to the west the time is 11 a.m., for in an hour's time, that meridian will face the Sun and experience noon. In the middle of the 19th century, each community across the US defined in this manner its own local time, by which the Sun, on the average, reached the farthest point from the horizon (for that day) at 12 oclock. However, travelers crossing the US by train had to re-adjust their watches at every city, and long distance telegraph operators had to coordinate their times. This confusion led railroad companies to adopt time zones, broad strips (about 15° wide) which observed the same local time, differing by 1 hour from neighboring zones, and the system was adopted by the nation as a whole. The continental US has 4 main time zones--eastern, central, mountain and western, plus several more for Alaska, the Aleut islands and Hawaii.

Canadian provinces east of Maine observe Atlantic time; you may find those zones outlined in your telephone book, on the map giving area codes. Other countries of the world have their own time zones; only Saudi Arabia uses local times, because of religious considerations. In addition, the clock is generally shifted one hour forward between April and October. This "daylight saving time" allows people to take advantage of earlier sunrises, without shifting their working hours. By rising earlier and retiring sooner, you make better use of the sunlight of the early morning, and you can enjoy sunlight one hour longer in late afternoon. The Date Line and Universal Time (UT) Suppose it is noon where you are and you proceed west--and suppose you could travel instantly to wherever you wanted. Fifteen degrees to the west the time is 11 a.m., 30 degrees to the west, 10 a.m., 45 degrees--9 a.m. and so on. Keeping this up, 180 degrees away one should reach midnight, and still further west, it is the previous day. This way, by the time we have covered 360 degrees and have come back to where we are, the time should be noon again--yesterday noon.

Hey--wait a minute! You cannot travel from today to the same time yesterday! We got into trouble because longitude determines only the hour of the day--not the date, which is determined separately. To avoid the sort of problem encountered above, the international date line has been established--most of it following the 180th meridian--where by common agreement, whenever we cross it the date advances one day (going west) or goes back one day (going east). That line passes the Bering Strait between Alaska and Siberia, which thus have different dates, but for most of its course it runs in mid-ocean and does not inconvenience any local time keeping. Astronomers, astronauts and people dealing with satellite data may need a time schedule which is the same everywhere, not tied to a locality or time zone. The Greenwich mean time, the astronomical time at Greenwich (averaged over the year) is generally used here. It is sometimes called Universal Time (UT). Right Ascension and Declination The globe of the heavens resembles the globe of the Earth, and positions on it are marked in a similar way, by a network of meridians stretching from pole to pole and of lines of latitude perpendicular to them, circling the sky.

To study some particular galaxy, an astronomer directs the telescope to its coordinates. On Earth, the equator is divided into 360 degrees, with the zero meridian passing Greenwich and with the longitude angle φ measured east or west of Greenwich, depending on where the corresponding meridian meets the equator. In the sky, the equator is also divided into 360 degrees, but the count begins at one of the two points where the equator cuts the ecliptic--the one which the Sun reaches around March 21. It is called the vernal equinox ("vernal" means related to spring) or sometimes the first point in Aries, because in ancient times, when first observed by the Greeks, it was in the zodiac constellation of Aries, the ram. It has since then moved, as is discussed in the later section on precession . The celestial globe, however, uses terms and notations which differ somewhat from those of the globe of the Earth. Meridians are marked by the angle α (alpha, Greek A), called right ascension, not longitude.

It is measured from the vernal equinox, but only eastward, and instead of going from 0 to 360 degrees, it is specified in hours and other divisions of time, each hour equal to 15 degrees. Similarly, where on Earth latitude goes from 90° north to 90° south (or -90°), astronomers prefer the co-latitude, the angle from the polar axis,equal to 0° at the north pole, 90° on the equator, and 180° at the south pole. It is called declination and is denoted by the letter δ (delta, Greek small D). The two angles (α, δ), used in specifying (for instance) the position of a star are jointly called its celestial coordinates. The next section tells how the stars, the Sun and accurate clocks allowed sailors to find their latitude and longitude. Further Exploring

Question: In Geography, one time zone usually represents how many degrees of longitude?
Answer: 15