Latitude and Longitude: Essential Concepts in Physical Geography (UPSC)

Latitude and Longitude

The latitudes and longitudes are essentially imaginary lines drawn on the globe to locate any place accurately. The latitudes run from east to west, parallel to the equator, while the longitudes run from north to south and pass through the poles. The location of any place on the Earth can be determined by the intersection of latitude and longitude. Both latitudes and longitudes are measured in degrees.

Latitude

Latitude can be defined as an angular distance of a point on the surface of Earth from the center of the Earth. Latitudes are parallel to the equator, which lies halfway between the two poles. The latitudes become smaller towards the poles, following the shape of a sphere. The equator represents 0°, whereas the North and the South poles are at 90°N and 90°S respectively.

North of the equator, latitudes are assigned North degrees, and south of the equator, the latitudes are assigned South degrees. Between the equator and the poles, the lines or parallels of latitude are drawn at an interval of 1°. For precision of location mapping, each degree of latitude is divided into 60 minutes, and each minute into 60 seconds.

Important Latitudes

The most important latitudes are:

• Equator at 0°
• Tropic of Cancer (23 1/2° N)
• Arctic Circle (66 1/2° N)
• Tropic of Capricorn (23 1/2° S)
• Antarctic Circle (66 1/2° S)

Because of the spherical shape of Earth, the physical distance between the latitudes increases very gradually from the equator to the poles. For example, at the equator at 0°, the distance between two latitudes is 68.704 miles. At 45°N, the difference between two latitudes is 69.054 miles, and near the pole at 90°N, it is 69.407 miles. However, for ease of purpose, the average distance between two latitudes is taken as 69 miles.

Longitude

Longitude is the angular distance, measured in degrees, along the equator, west or east of the Prime Meridian. On the globe, longitudes are a series of semi-circles that run from north to south from one pole to the other. Each longitude cuts through the equator. Longitudes are also called Meridians.

Any longitude could have been taken as the prime meridian for the purpose of numbering. However, in 1884, by an International Agreement, it was decided that the zero meridian would pass through the Royal Astronomical Observatory at Greenwich, near London. The 0° Longitude is called the Prime Meridian, and all other meridians fall to the left or right of the prime meridian.

The longitudes are divided into 180° to the east and west of the Prime Meridian. The distance between two longitudes at the equator is calculated by dividing the circumference of the Earth by 360° (25000 miles ÷ 360 = 69.1 miles). However, the longitudes converge at the poles, so the linear distance between two longitudes narrows. For example:

• At the equator, the distance between two longitudes is 69.1 miles.
• At 45°N, it is 49 miles.
• At 75°N, it is only 18 miles.
• At the poles, it is 0 miles.

The degree of longitude thus decreases in length polewards.

Longitude and Time

The rotation of the Earth around the Sun means that at any point in time, different places on Earth will experience a different time of the day. Since the Earth makes one complete round of 360° in one day or 24 hours, it means that in one hour, it covers a distance of 15°, or 1° in four minutes.

As the Earth rotates from west to east, for every 15° eastwards, we gain an hour. That means the local time is advanced by an hour for every 15° eastward move. Conversely, if we go westwards, we lose an hour, meaning for every 15° westward move, the time is reduced by an hour.

Places to the east of Greenwich see sunrise earlier. A person traveling from Greenwich to the east will gain time on reaching the eastern destination compared to a person traveling to the west of Greenwich, who will lose time. The places west of Greenwich see sunrise later than sunrise in Greenwich. Generally, time is determined by the longitude of the place, compared with the local time of Greenwich, and then adjusted by adding or subtracting time depending on the longitude.