# Do parallels and meridians meet at right angles to the length

Nov 6, Longitude is related to latitude, the measurement of distance north The point in the grid where parallels and meridians intersect is called a coordinate. Coordinates can be used to locate any point on Earth. parallel, Noun. length; each is one-half the length of the equator. • All meridians converge at the poles and are true north-south lines. • All lines of latitude (parallels) are parallel. Each parallel marks off a fixed distance north or south of the equator. This distance is If you know the latitude of a place, you can thus estimate its distance from the equator. The poles have no longitude because all meridians meet there.

Like the prime meridian and the th meridian, all such pairs of meridians form circles that cut the Earth into hemispheres. These circles are known as great circles. Only one parallel, the equator, is a great circle. A cutaway drawing of the Earth demonstrates how latitude is determined.

It makes clear that latitude is a measure of the angle between the plane of the equator and lines projected from the center of the Earth. The cutaway drawing of the Earth also shows how longitude is determined. Longitude is seen to be a measure of the angle between the planes of two meridian circles, one of which is the prime meridian. Any location on Earth can be described as lying at a certain number of degrees and minutes of latitude either north or south of the equator and at a certain number of degrees and minutes of longitude either east or west of the prime meridian.

A degree of latitude can easily be changed into miles. Degrees of latitude vary a little in length—the variation between the shortest and the longest is less than a mile—because the Earth is not a perfect sphere but is flattened slightly toward the poles and bulges slightly around the equator.

The length of a degree of longitude, however, varies from about 69 miles at the equator to zero at the poles, where the meridians come together. Finding Latitude and Longitude The navigator of a ship or an airplane can determine latitude by using an instrument called a sextant. With it the navigator measures the altitude angle above the horizon of the Sun as the Sun transits, or crosses, the meridian longitude. The navigator then calculates the latitude by combining the observed altitude with information from an almanac—a book of data about the movement of the Sun and stars.

In the evening, latitude may similarly be found by observing stars see navigation. Longitude is more difficult to determine than latitude because the sextant and the almanac together do not yield enough information.

To calculate longitude, a navigator must also know the exact time at which the observations are made. The time is needed because the Sun and stars, as they appear to move across the sky, look the same at all places in a given latitude at some time during each day. The invention of clocks during the Renaissance was the first step toward the reliable calculation of longitude.

The clocks of that era, however, were too inaccurate for use in navigation. In the British Board of Longitude offered a large cash prize to anyone who could build a clock that would meet certain standards of accuracy throughout long ocean voyages. By John Harrisona British clockmaker, had submitted the first of several clocks, the last of which won the prize for him.

They were called chronometers. From that time on, sailors have been able to determine longitude accurately by comparing local time with Greenwich mean time GMT. Shipboard chronometers are set to show GMT. Special radio time signals allow navigators to check the accuracy of their chronometers. How the Prime Meridian Was Selected Before a prime meridian was agreed upon, map makers usually began numbering the lines of longitude on their maps at whichever meridian passed through the site of their national observatory.

Since Britain was a world leader in exploration and map making, navigators of other nations often used British maps. As a result, in the meridian of Greenwich was adopted throughout most of the world as the prime meridian.

In the s the royal Greenwich Observatory was moved about 60 miles southeast of Greenwich. The Greenwich meridian, however, remained the prime meridian. Travelers must change time by an entire day when they cross the th meridian. If this meridian crossed a large country, timekeeping and the establishment of calendar dates would be difficult.

But with the Greenwich meridian set at zero, the th meridian is near the middle of the Pacific Ocean.

## latitude and longitude

It crosses only a small land area in northeastern Asia and divides some island groups in the Pacific. To avoid differing dates in those areas, the nations of the world established a special line across which dates change.

It swerves from the th meridian whenever convenient. This line is called the international date line. Several lines of latitude have special significance. One of these is the equator. Two other special lines of latitude are the 30th parallels. The area between them, straddling the equator, is commonly referred to as the low latitudes.

The low latitudes are generally warm lands. The two 60th parallels are also special lines of latitude. The areas north and south of the 60th parallels, which center on the North and South poles, are commonly referred to as the high latitudes.

The high latitudes are generally cold lands. The areas between the 30th and 60th parallels in both hemispheres are commonly referred to as the middle latitudes. Generally, middle-latitude lands have four seasons — fallwinterspringand summer. The latitude of a place, accordingly, is a clue of its climate. This angle varies with distance from the equator latitude.

Regions in high latitudes, both north and south, get less insolation and are therefore usually colder than regions in low latitudes. The Tropic of Cancer and the Tropic of Capricorn mark the limits of the zone astride the equator in which the Sun appears directly overhead at some time during the year. The Arctic and Antarctic circles mark the limits of the areas around each pole in which the Sun at some time during the year does not rise or set for a period of 24 hours or more. The only special line of longitude is the prime meridian.

Direction is expressed in degrees, as shown by the compass rose in figure Because meridians converge toward the poles, course measurement should be taken at a meridian near the midpoint of the course rather than at the point of departure.

The course measured on the chart is known as the true course. This is the direction measured by reference to a meridian or true north. It is the direction of intended flight as measured in degrees clockwise from true north.

The true heading is the direction in which the nose of the airplane points during a flight when measured in degrees clockwise from true north. Usually, it is necessary to head the airplane in a direction slightly different from the true course to offset the effect of wind.

LONGITUDES - CBSE Class VI Social Science - Geography

Consequently, numerical value of the true heading may not correspond with that of the true course. This will be discussed more fully in subsequent sections in this chapter. For the purpose of this discussion, assume a no-wind condition exists under which heading and course would coincide.

To use the compass accurately, however, corrections must be made for magnetic variation and compass deviation. Variation Variation is the angle between true north and magnetic north. It is expressed as east variation or west variation depending upon whether magnetic north MN is to the east or west of true north TNrespectively. If the Earth were uniformly magnetized, the compass needle would point toward the magnetic pole, in which case the variation between true north as shown by the geographical meridians and magnetic north as shown by the magnetic meridians could be measured at any intersection of the meridians.

Actually, the Earth is not uniformly magnetized. In the United States the needle usually points in the general direction of the magnetic pole, but it may vary in certain geographical localities by many degrees.

Consequently, the exact amount of variation at thousands of selected locations in the United States has been carefully determined. The amount and the direction of variation, which change slightly from time to time, are shown on most aeronautical charts as broken magenta lines, called isogonic lines, which connects points of equal magnetic variation. The line connecting points at which there is no variation between true north and magnetic north is the agonic line.

An isogonic chart is shown in figure Minor bends and turns in the isogonic and agonic lines are caused by unusual geological conditions affecting magnetic forces in these areas. Magnetic meridians are in black, geographic meridians and parallels are in blue.

Variation is the angle between a magnetic and geographic meridian. On the west coast of the United States, the compass needle points to the east of true north; on the east coast, the compass needle points to the west of true north. Zero degree variation exists on the agonic line which runs roughly through Lake Michigan, the Appalachian Mountains, and off the coast of Florida, where magnetic north and true north coincide.

This conversion is made by adding or subtracting the variation which is indicated by the nearest isogonic line on the chart. The true heading, when corrected for variation, is known as magnetic heading. The black lines are isogonic lines which connect geographic points with identical magnetic variation.

### Latitude and Longitude

Remember, to convert true course or heading to magnetic course or heading, note the variation shown by the nearest isogonic line. If variation is west, add; if east, subtract. To determine compass heading, a correction for deviation must be made. Because of magnetic influences within the airplane such as electrical circuits, radio, lights, tools, engine, magnetized metal parts, etc.

This deflection is deviation. The deviation is different for each airplane, and it also may vary for different headings in the same airplane. For instance, if magnetism in the engine attracts the north end of the compass, there would be no effect when the plane is on a heading of magnetic north.

On easterly or westerly headings, however, the compass indications would be in error, as shown in figure Magnetic attraction can come from many other parts of the airplane; the assumption of attraction in the engine is merely used for the purpose of illustration.