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The Earth in Space LATITUDE, LONGITUDE AND TIME LATITUDE A Series of imaginary lines, known as an earth grid, is A Series of imaginary lines, known as an earth grid, is drawn on maps and globes in order that places can easily be located. The imaginary lines called lines of latitude and longitude intersect one another at right angles as they run from north to south (longitude) and east to west (latitude). Latitude is the distance in degrees on the surface of the earth north or south of the equator. Since the earth is a sphere which rotates on an axis, two fixed points of reference are available. These points, called the North Pole and South Pole (geographic poles) are located where the axis intersects the surface of the earth. If we draw a circle around the earth half way between the geographic poles and perpendicular to the line joining them, we have established a third point of reference. This line, called the equator, is an east-west line. It represents the largest of a series of circles called parallels of latitude. The distance between any two parallels is always the same. As the parallels of latitude approach the poles, they become smaller and smaller circles, although all cover 360°. The north-south location of any point on the earth’s surface is called the latitude of that point. Latitude is measured in degrees. He earth is a sphere containing 360°. The distance from the equator to either of the poles is one-quarter of 360°; therefore, each quarter segment of the earth is 90°. The equator has a latitude of 0°; thus, both the north and south poles have latitudes of 90°. Compass directions are assigned to the parallels of latitude which lie above and below the equator. All parallels drawn between the equator and the North Pole are north latitudes. In the same way, all parallels of latitude drawn between the equator and the South Pole are called south latitudes. For example, a point in the Northern Hemisphere located halfway between the equator and the North Pole would have a latitude of 45° N. The latitude of New York City is approximately 41° N., which places it 41° north of the equator. The boundary of the United States is between 25° N and 50° N. Latitude Can Be Measured In Miles. Since the circle of latitudes are parallel, the distance between them never changes and can be measured. Each degree of latitude, whether near the equator or near the poles, is 70 miles wide. In order to be more exact in determining location, each degree of latitude is divided into 60 equal units called minutes. Each minute is further subdivided into 60 seconds. As we have learned, there are 70miles to one degree of latitude and, since each degree of latitude is 60 minutes wide, each minute is equal to 1 1/6 miles or 6,000 ft. In turn, each second of latitude is equal to 100 ft. This breakdown of distance along the Earth’s surface enables us to calculate distance in latitude and to express them in miles and feet. The following illustration will show you how you can calculate the distance between two points of latitude that lie on the same line of longitude. Example: What is the distance expressed in miles between a point on the equator and a point located 30° 12’ 03” N., with both points having the same longitude? Solution: Keeping in mind that each degree of latitude is 70 miles wide, we can calculate the number of miles represented by 30°. 70 x 30 = 2,100 miles 1. Each minute of latitude is 1 1/6 miles wide. Therefore, 12 minutes of latitude are 14 miles wide. 1 1/6 x 12 = 14 miles 2. If each second of latitude is 100 feet wide, 3 seconds of latitude is 300 feet wide. 3. By adding the number of miles in 30° of latitude to the number of miles in 12 minutes of latitude, we find the mileage in 30° 12’ of latitude. 2,100 – Number of miles in 30° of latitude 14 – Number of miles in 12 minutes of latitude ______ 2,114 Total number of miles in 30° 12’ of latitude 4. If we now add the number of feet in 3 seconds of latitude, we obtain the total distance between these two points to be 2,114 miles, 300 feet. 2,114 + 300 ft. = 2,114 mi., 300 ft. Determining Latitude. Being able to determine the latitude of a position is of great importance to a navigator. In most instances, the navigator uses celestial navigation; that is, he determines his position in relation to the positions of the heavenly bodies the positions of these objects appear to change as the navigator moves north or south of his present position. Polaris, the North Star, is often used by navigators to determine their latitude. Polaris is about 90° above the horizon at the North Pole (90° N). At the equator (0°), Polaris would have an altitude of 0°. Therefore, if an observer standing at the North Pole would change his position to 89° N, one degree away from the pole, Polaris would sink one degree lower in the sky. The angle that a heavenly body, such as the sun or Polaris, is above the horizon is called its altitude. An observer can determine the latitude by measuring the altitude of Polaris. The latitude of an observer is approximately equal to the number of degrees Polaris is above the horizon. The instrument used to determine the altitude of a star is called a sextant. Many other stars are used as reference points for determining latitude. However, corrections must be made since other stars do not lie directly over the North Pole as does Polaris. The Nautical Almanac contains charts (Astronomical Tables) which provide corrected data for determining latitude. These charts take into account the date (since the change in season affects a star’s position in the heavens) in the time of day that the observation is made. The navigator can also determine the latitude of his position by finding the altitude of the sun. This is known as “shooting the sun”. Again, he must consult the astronomical tables to obtain the correct values for his latitude readings. LONGITUDE Meridians are semi-circles drawn around the earth from the North to the South Pole. Like parallels, they are measured in degrees. These imaginary lines lie at right angles to the parallel lines of latitude. The position of a place on the earth in relation to a particular meridian is called longitude. While the equator provides us with a natural starting point and is designated as 0o latitude, there is no natural reference line for longitude. A meridian called the prime meridian passes through Greenwich, England. Since meridians are drawn east ans west of the prime meridian, they can be drawn only halfway around the earth in each direction. Thus, the meridian which lies directly opposite the prime meridian is 180o. This is called the International Dateline. Chicago which lies about halfway between the prime meridian and the International Dateline and is located west of the prime meridian, has a longitude of about 90oW. All meridians converge at the poles. The width of each meridian, therefore, changes as the latitude of a point on the meridian changes, the width becoming narrower as the latitude increase. For this reason, no standards or constants of distance can be used in calculating longitude. Finding Longitude. All points on a given meridian have the same time. Thus, if it is 12 noon at a point on the 75o meridian, it is 12 noon at ANY point on that meridian. A day on earth—24 hours—is defined as the time it takes the earth to make one complete rotation on its axis. The earth must rotate at a rate of 1o every 4 minutes or 15o per hour to rotate through 360o in 24 hours. Therefore, there is a difference of one hour for every 15o of longitude. You know that the sun rises in the east and sets in the west. For this reason a place in the east will receive the sun before a place in the west. Therefore, a traveler moving westward must turn his watch back one hour for every 15o of longitude traveled. How can this information be used to determine longitude? The prime meridian is used as a reference point in determining longitude. Navigators use an instrument called a chronometer to tell them Greenwich time. The chronometer is a special clock with numbers ranging from 1 to 24. Numbers 1 through 12 represent the time from 1am to 12 noon. Numbers 13 through 24 represent the hours from 1:00pm to 12:00 midnight. Therefore, it the chronometer reads 15 o’clock Greenwich time, it is 3pm in Greenwich, England. The Following illustrations should help you understand how a knowledge of Greenwich time helps a navigator determine his longitude. Example 1. Suppose the time in Greenwich according to your chronometer is 8pm and your local time according to your wristwatch is 10pm. What is your longitude? (a) The time difference between your position and that of Greenwich, England, is two hours. 10pm – Local time. -- 8pm -- Greenwich time 2 hour difference in time (b) We know that for every hour of time there are 15º of longitude. A difference of two hours would indicate the passage of 30º of longitude. Therefore, you must be located on a meridian of 30 º away from Greenwich, England (0º longitude). (c) Would you be located at 30ºW or 30ºE? Since your local time is later than Greenwich time, you must be located to the east of Greenwich. Your longitude, therefore, must be 30º E. Example 2. If you were located ion New York City (75oW) and your local time is 5am, what time would the chronometer read? (a) Since you are located on the 75oW meridian and Greenwich, England is at 0o, there is a five-hour time difference. (b) Because you are west of Greenwich, Greenwich time must be later than your local time. Therefore, you must add five hours to your local time to find Greenwich time. (c) Thus, the chronometer would read 10am. Example 3. What time would it be in Greenwich, England, if the hands on the chronometer indicated 18 o’clock. (a) Since the numbers 1 through 12 indicate ante meridian (am) time, the higher numbers 13 through 24 must indicate post meridian (pm) time. (b) All you need do to find Greenwich time, if the chronometer reads greater than 12 noon, is to subtract 12 from that number. (c) If your chronometer reads 18 and you subtract 12, then Greenwich time must be 6pm. Other Methods for Determining Position. Heavy clouds may make it impossible to find latitude of a position; therefore, other methods must be used. One method is called dead reckoning. IN this method, the navigator carefully records the speed of his craft and the direction of travel. Knowing the point of departure, the speed he is traveling, wind or current movements, and the time moved in a particular direction, the navigator can locate his position on a map. Although plotting of positions on a map by dead reckoning is not easy and the results are not as accurate as those obtained by other navigational methods, dead reckoning is often used in bad weather. A very effective way of determining position is the long range navigation (Loran) method of electronic navigation. Although it is an accurate way to determine location in all kinds of weather, there are still many areas of the world beyond the range at which radio signals can be received. In its operation, radio signals are sent out from pairs of radio stations. Aircraft and ships equipped with special radio receiving equipment can pick up the radio signals and locate their positions with much accuracy. TIME The measurement of time is based largely upon the rate at which the earth rotates on its axis and revolves around the sun. A year, for example, is measured by the time it takes the earth to make one complete revolution around the sun. This poses a serious problem to astronomers and space scientists since such a measurement cannot be accurately made. The period of the earth’s revolution about the sun will vary, depending upon the starting point and the point at which the year ends. How then can we tell how long a year is? The Year. In general, what we mean by a year is a tropical year; that is, the interval of time between two vernal equinoxes. (The vernal equinox occurs on or about March 21 and marks the beginning of spring in the Northern Hemisphere.) The tropical year has a length of 365 days, 5 hours, 48 minutes, and 46 seconds. Since our calendar year is exactly 365 days, we are left with an extra ¼ day (about 6 hours) every tropical year. To make up for this excess or lost time, Julius Caesar devised a calendar as the Julian Calendar. Caesar ordered that one extra day (February 29) be added to the calendar every fourth year (which was to be known as the “leap year”). However, a new problem was created when one day was added every fourth year. The year became 11 minutes, 14 seconds too long. In 1582, a new more accurate was introduced by Pope Gregory XIII. This calendar, known as the Gregorian Calendar, kept Caesar’s principle of every fourth year being a leap year so long as the numeral of the year was divisible by 4. Thus, 1584 was a leap year, but 1585 was not. The next leap year occurred in 1588. The major difference between the Julian Calendar and the Gregorian Calendar concerned the century years. While all century years would be leap years (divisible by 4) according to the Julian Calendar, only those century years divisible by 400 would be leap years according to the Gregorian Calendar. Thus, the year 1600 was a leap year, but the years 1700, 1800, 1900 were not. The next century year which will be a leap year will be the year 2000. An error of less than one day in 300 years still exists in the Gregorian Calendar which is the one we still use today. A Day. The length of a day is based on the rotation on its axis. It is determined by the passing of the sun of another star across a meridian twice in succession. If the heavenly body crossing the meridian is the sun, we call the day a solar day; if it is another star then we refer to the day as a sidereal day. Apparent Solar Day. The time it takes the sun to cross the observers meridian twice in succession is called an apparent solar day. Apparent solar noon occurs when the sun is directly over the observers meridian. However, since the earth is moving in it’s orbit around the sun at the same time it is rotating, the earth must rotate about 1˚ more on it’s axis to bring the sun over the meridian again. Thus, the sun would arrive about 4 minutes (1̊ = 4 minutes) late the next day. During the course of the year, the observed solar day would vary in length. To compensate for the variation in time, an average, or mean length of soalr time has been calculated. The result is a 24- hour day which is called a mean solar day. Mean solar time is the time at which we set our clocks. across the earth with the prime meridian as their starting point. INTERNATIONAL DATELINE Sidereal Day. It is important to realize that although the earth rotates on it’s axis we do not feel the sensation of movement. Instead, it seems as if all the stars in the celestial sphere revolve around us. Therefore, a sidereal day is defined as the period of time for a particular star to cross a meridian twice in succession as the celestial makes one full turn. If we compare the length of a sidereal day to that of a mean solar day we find the sidereal day to be 4 minutes short. Thus, the length of a sidereal day is 23-hours, 56-minutes. Standard Time. Before 1883, each city or town set its clocks to correspond to apparent solar time. Thus, 12 o’clock noon would occur in each city when the sun was in its highest point in the sky. As people began traveling across the United States, they found they had to keep resetting their watches every time they crossed another meridian. Railroad timetables caused confusion due to this lack of standardized time-keeping. Finally, in 1883, four standard time zones were set up across the country. Each time zone was 15o wide with the time within each zone uniformly that of the meridian located near its center. Therefore, standard is the mean solar time of the meridian of that particular zone. The four standard time zones with their center points are (1) Eastern Standard time (750 W.), Central Standard time (900 W.), Mountain Standard time (1050 W.) and Pacific Standard time (1200 W.). Twenty-four standard time zones have been set up Opposite the prime meridian on the other side of the earth lies the 1800 meridian. Running close to this meridian is the International Dateline. It is here that the new day begins. If you were to travel westward across the International Dateline, you would continue to turn your clock back one hour. In addition, you would have to move your calendar ahead one full day when you crossed the International Dateline. If you crossed the dateline while traveling eastward, you would have to turn your calendar back one full day. Thus, if you left New York on Wednesday at 3 pm, and could travel completely around the world going westward in six hours, you would arrive back in New York the same day. If not for the dateline, you might arrive in New York the day before you left.