Introduction: Latitude And Longitude

Latitude and Longitude, coordinate system by means of which the position or location of any place on Earth’s surface can be determined and described.

Latitude:

Latitude is a measurement on a globe or map of location north or south of the Equator.

Technically, there are different kinds of latitude—geocentric, astronomical, and geographic (or geodetic) but there are only minor differences between them.

In most common references, geocentric latitude is implied. Given in degrees, minutes, and seconds, geocentric latitude is the arc subtended by an angle at Earth’s centre and measured in a north-south plane poleward from the Equator.

Thus, a point at 30°15′20″ N subtends an angle of 30°15′20″at the centre of the globe; similarly, the arc between the Equator and either geographic pole is 90° (one-fourth the circumference of the Earth, or ¹/₄ × 360°), and thus the greatest possible latitudes are 90° N and 90° S.

As aids to indicate different latitudinal positions on maps or globes, equidistant circles are plotted and drawn parallel to the Equator and each other; they are known as parallels, or parallels of latitude.

Different methods are used to determine geographic latitude, as by taking angle-sights on certain polar stars or by measuring with a sextant the angle of the noon Sun above the horizon.

The length of a degree of arc of latitude is approximately 111 km (69 miles), varying, because of the nonuniformity of Earth’s curvature, from 110.567 km (68.706 miles) at the Equator to 111.699 km (69.41 miles) at the poles. Geographic latitude is also given in degrees, minutes, and seconds.

Longitude:

Longitude is a measurement of location east or west of the prime meridian at Greenwich, the specially designated imaginary north-south line that passes through both geographic poles and Greenwich, London.

Measured also in degrees, minutes, and seconds, longitude is the amount of arc created by drawing first a line from the Earth’s centere to the intersection of the Equator and the prime meridian and then another line from the Earth’s centre to any point elsewhere on the Equator.

Longitude is measured 180° both east and west of the prime meridian.

As aids to locate longitudinal positions on a globe or map, meridians are plotted and drawn from pole to pole where they meet.

The distance per degree of longitude at the Equator is about 111.32 km (69.18 miles) and at the poles, 0.

The combination of meridians of longitude and parallels of latitude establishes a framework or grid by

means of which exact positions can be determined in reference to the prime meridian and the Equator: a point described as 40° N, 30° W, for example, is located 40° of arc north of the Equator and 30° of arc west of the Greenwich meridian.

Coordinate Reference System:

Coordinates are pairs (X, Y) in a two-dimensional space referenced to a horizontal datum. Whereas triplets (X, Y, Z) of points not only has position, but also has height referenced to a vertical datum.

In other words, the X- and Y-values represent horizontal position. Whereas, the Z-value represents the vertical position.

Geographic coordinate systems use an ellipsoid to approximate all locations on the surface of the earth. Whereas, the datum defines the surface.

A reference ellipsoid is the mathematical model of the shape of the Earth with the major axis along the equatorial radius. A geographic coordinate system uses longitude and latitude expressed in decimal degrees.

Cartographers write spherical coordinates (latitudes and longitudes) in degrees-minutes-seconds (DMS) and decimal degrees. For degrees-minutes-seconds, minutes range from 0 to 60.

Greenwich Mean Time (GMT):

Greenwich Mean Time or GMT is clock time at the Royal Observatory in Greenwich, London. It is the same all year round and is not affected by Summer Time (Daylight Saving Time) clock changes.

When the sun is at its highest point exactly above the Prime meridian, it is 1200 noon at Greenwich.

GMT is also a time zone, used by the United Kingdom of Great Britain and Northern Ireland UK when Daylight Saving Time is not in use, from October to March.

The Greenwich Meridian (Prime Meridian or Longitude Zero degrees) marks the starting point of every Time Zone of the time zone map.

Every 15° longitude represents one hour’s difference in time: (24 x 15 = 360, the degrees of a circle). You can work out the time at every location on earth if you know how many degrees it is east or west of Greenwich.

GMT was originally set up to aid naval navigation when travel around the globe started to open up with the discovery of the “New World” (America) in the fifteenth century.

Indian Standard Time (IST):

India is a very big country. In order to maintain the uniformity of time within the country the Standard Meridian of India is taken as 82 ½ ° E longitude.

It passes through the city of Allahabad. The local time along this Meridian serves as the Indian Standard Time (I.S.T) .

The local time of Mirzapur near Allahabad is taken as the standard time all over India . I.S.T is 5:30 hours ahead of G.M.T (Greenwich Mean Time) located in London.

Standard Meridian of India is needed to maintain time uniformity in India. 82 ½ ° has been selected as the Standard Meridian of India due to the following reasons :

• This is a central Meridian of India as it divides the country into two equal parts.
• This Central Meridian helps India to adopt time uniformity

International Date Line:

Drawn up in 1884

The 180° meridian was selected as the International Date Line because it mostly runs through the sparsely populated Central Pacific Ocean.

It was decided at the International Meridian Conference in 1884 in Washington, D.C. where 26 countries attended.

The International Date Line (IDL) is an imaginary line on Earth’s surface defining the boundary between one day and the next.

• The International Date Line is located halfway around the world from the prime meridian (0° longitude) or about 180° east (or west) of Greenwich, London, UK, the reference point of time zones. It is also known as the line of demarcation.

The Dateline Is Not Straight

• The dateline runs from the North Pole to the South Pole and marks the divide between the Western and Eastern Hemisphere.
  
  
• It is not straight but zigzags to avoid political and country borders and to not cut some countries in half.

What Happens When You Cross the Dateline?

• When you cross the International Date Line from west to east, you subtract a day, and if you cross the line from east to west, you add a day.
  
  
• Depending on which time zone the country follows, the time difference on either side of the line is not always 24 hours.

For example:

• if you travel the 1061 kilometers (659 miles) across the dateline from Baker Island to Tokelau you have to add 25 hours, or 1 day and 1 hour.

Three Dates at the Same Time

• Every day between 10:00 and 11:59 UTC, three different dates on the calendar are in use at the same time on Earth.

For example:

• our Time Zone Converter shows:

• At 10:30 UTC on May 2, it is 23:30 (11.30 pm) on May 1 in American Samoa (UTC−11)
  
  
• 06:30 (6:30 am) on May 2 in New York (UTC-4), and 00:30 (0:30 am) on May 3 in Kiritimati (UTC+14).

Changing Sides of the Dateline

• The dateline is not defined by international law. Countries are free to choose the date and time zone that they want to observe.

For example:

• when the Republic of Kiribati gained independence from being a British colony in 1979 some of the islands were on one side of the dateline, and the rest were on the other.
  
  
• They corrected the anomaly in the eastern half of Kiribati by skipping January 1, 1995 and ever since Kiribati has been the first country to enter the New Year.
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• In 2011, Samoa changed the time zone from UTC-11 to UTC+13 by shifting the dateline to the west and removing December 30, 2011 from the calendar.
  
  
• They did this to facilitate trade with Australia and New Zealand, and Tokelau followed Samoa for the same reasons.