On earth’s surface, locations are determined by the two reference lines known as latitude and longitude. In fact, these are the ‘geographical coordinates’ which are used by a pilot and the ship captain, to indicate the position on the map.

The surface of the earth is vast enough that it was difficult to locate any point, without the utilization of the mathematical method. For this purpose, fictional lines are drawn on the globe, known as latitude and longitude. Both the latitude and longitude are imaginary lines, which are used to locate points on the earth’s surface, measured in degrees.

Latitude

Often called parallels or circles of latitude, latitudes are imaginary circles parallel to the Equator. They are named after the angle created by a line connecting the latitude and the center of the Earth, and the line connecting the Equator and the center of the Earth. Greek letter phi (ɸ) is used to represent latitude.

In geography, latitude is defined as the angular distance of any point, north or south of the equator, i.e. it is a coordinate system, that is used as a reference point to locate places on earth.

Latitude ranges from 0 degrees to 90 degrees, wherein equator indicates 0° latitude and 90° is at the poles. The parallels lying in the northern hemisphere are considered as north latitudes and are assigned a suffix of N for north, while those lying in the southern hemisphere are called south hemisphere and are assigned a suffix of S for south.

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.

Latitudes are used to categorize heat zones, i.e. torrid zone, temperate zone and frigid zone. In contrast, longitudes are used to classify time zones.

The most important lines of latitude are the equator, the Tropic of Cancer is 23½ °N (marks the location where the sun reaches the zenith at this latitude and the summer solstice, which occurs on either June 20 or 21 of each year, marks the day on which the sun shines vertically over this parallel), the Tropic of Capricorn is 23½ °S (the winter solstice, which occurs on either December 21 or 22 of each year, marks the day on which the sun shines vertically over this line), the Arctic Circle is 66½ °N and the Antarctic Circle is 66½ °S (the Antarctic circle is 17,662 km long). The positions of these latitudes are determined by the Earth’s axial tilt.

Places in both the Arctic and Antarctic circles experience extreme weather, and experience the Midnight Sun. As the earth is slightly flattened at the poles, the linear distance of a degree of latitude at the pole is a little longer than that at the equator.

On a map where the orientation of the map is either due north or due south, latitude appears as horizontal lines.

Equator

The Equator is the line of 0° latitude, the starting point for measuring latitude. The latitude of the North Pole is 90° N, and that of the South Pole is 90° S.

The equator marks the locations on earth that are equidistant from the North and South Poles.  The equator crosses 78.7% water and 21.3% land and is about 24,901 miles (40,075 km) long.

The latitude of every point in between must be some degree north or south, from 0° to 90°. One degree of latitude covers about 69 miles (111 kilometers) i.e The distance between each degree of latitude is about 69 miles (111 kilometers).

The Sun is directly overhead the equator at noon on the two equinoxes (March and Sept. 20 or 21).

Most locations on the Equator experience consistently high temperatures throughout the year. They also experience at least 12 hours of daylight every day during the year. On the Equinoxes – September and March – the sun is directly overhead the Equator, resulting in almost exactly 12 hour days and 12 hour nights.

The Equator passes through 14 countries, including Uganda, Kenya, Somalia, Indonesia, Ecuador, Colombia, and Brazil.

Sometimes, latitudes north of the Equator are denoted by a positive sign. Latitudes south of the Equator are given negative values. This eliminates the need to add whether the specified latitude is north or south of the Equator.

Longitude

Longitude  can be described as regional coordinate which describes the east-west location of the point within the Earth’s surface. It is really an angular way of measuring, generally indicated in degrees as well as denoted by the Greek letter lambda Meridians (lines running from your North Pole towards the south Pole) hook up points with similar longitude. Through established practice, one of these simple, the Prime Meridian, which goes through the Royal Observatory, Greenwich, England, ended up being allotted the location of zero degrees longitude. The longitude associated with other locations is usually calculated because the angle east or even west from your Prime Meridian, covering anything from 0° at the Prime Meridian to +180° eastward and also -180° westward. Particularly, stands out as the angle coming from a plane that contains the Prime Meridian along with a plane containing the North Pole, South Pole and the location in question.

All the other longitudes are measured, and named after the angle they make with respect to the center of the Earth from the intersection of the Meridian and the Equator. Greek letter lambda (λ), is a symbol for longitude.

One of these lines must be designated as a prime meridian, or as the 0 ° mark, but which one? For many years, each country had its own prime meridian. France used a meridian passing through Paris, while England used a meridian passing through Greenwich, England. Either one worked just as well, as long as you were only communicating with people from the same country. But what happens as technologies improve, and travelers begin interacting with people from many different lands? If each country is using a different prime meridian to mark 0 ° longitude, then it would be impossible to give someone from another country a location, and have them be able to accurately find it.

Over time, more and more travelers began to recognize the meridian passing through Greenwich, England, as the Prime Meridian. In 1884, a group of scientists, navigators, and businessmen made Greenwich, England, the official worldwide Prime Meridian.

The further away from the Prime Meridian that one travels the higher their longitude becomes, until they reach 180 ° longitude. If an individual is in the Eastern Hemisphere, their longitude is measured in degrees east. If they are in the Western Hemisphere, their longitude is measured in degrees west.

Modern timekeeping systems use longitudes as references to keep time.

The Prime Meridian

The Prime Meridian is the imaginary line running between the North and South Poles that’s used to indicate 0º longitude. It passes through Greenwich in southeast London, and is therefore sometimes referred to as the Greenwich Meridian.

An international conference held in Washington D.C. in 1884 designated ‘the meridian passing through the centre of the transit instrument at the Observatory of Greenwich as the initial meridian for longitude.’ During the 1950s, the Greenwich Observatory moved to Herstmonceux Castle in East Sussex, where the skies were deemed to be clearer. In 1990, it moved again, to the Institute of Astronomy at the University of Cambridge, but the original site in Greenwich continues to serve as the location for 0º longitude.

As the prime meridian, the north–south line at Greenwich is used as the reference point for all other meridians of longitude, which are numbered east or west of it. The current system employs 24 standard meridians of longitude 15º apart, starting with the Prime Meridian.

The lines all run from the North to the South Pole, at right angles to the Equator. These meridians are the centres of 24 standard time zones. In theory, standard time becomes successively one hour earlier at each 15º longitude west of the Prime Meridian, and one hour later at each 15º longitude east. Time is the same throughout each zone. In practice, many of the zones have been subdivided or their shape altered for convenience. The mean solar time at Greenwich is now called Universal Time (formerly called Greenwich Mean Time or GMT).

When it is noon along the prime meridian, it is midnight along the International Date Line.

International Date Line

The International Date Line (IDL) is on the opposite side of the world to the Prime Meridian, located at 180 degrees. It is not straight but zigzags to avoid political and country borders and to not cut some countries in half. Together, the Prime Meridian and The International Date Line divide the earth into two halves: the Western and Eastern Hemispheres. The International Date Line can be anywhere on the globe. But it is most convenient to be 180° away from the defining meridian that goes through Greenwich, England.

Traveling eastward across the line, one subtracts one calendar day traveling westward, one adds a day. The date line is necessary to avoid a confusion that would otherwise result. For example, if an airplane were to travel westward with the sun, 24 hr would elapse as it circled the globe, but it would still be the same day for those in the airplane while it would be one day later for those on the ground below them. The same problem would arise if two travelers journeyed in opposite directions to a point on the opposite side of the earth, 180° of longitude distant. The eastward traveler would set his clock ahead 1 hr for each 15° of longitude (see standard time ), so that his clock would gain a total of 12 hr the westward traveler would set his clock back 1 hr for each 15°, resulting in a total loss of 12 hr. The two clocks would therefore differ by 24 hr, or one calendar day. The apparent paradox is resolved by requiring that the traveler crossing the date line change his date, thus bringing the travelers into agreement when they meet. The international date line does not follow the 180° meridian along its entire course but bends eastward around the eastern tip of Siberia, westward around the Aleutian Islands, and eastward again around various island groups in the South Pacific (mostly extremely around Kiribati) to avoid a date change within island nations and territories or between important trade partners.

Without the International Date Line travelers going westward would discover that when they returned home, one day more than they thought had passed, even though they had kept careful tally of the days. This occurrence first happened to Magellan’s crew after the first circumnavigation of the globe. Likewise, a person traveling eastward would find that one fewer days had elapsed than he had recorded, as happened to Phileas Fogg in “Around the World in Eighty Days” by Jules Verne.

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