Parallels of latitude

Also called: Latitude, Parallels · Topic: Latitude, Longitude and Time · NCERT: Class 6, Ch 1 "Locating Places on the Earth"; Class 11, Ch 3 "Latitude, Longitude and Time"; Class 11, Ch 4 "Map Projections"

Meaning

Latitude is the angle of a place north or south of the equator, measured at the earth's centre along the place's meridian. A parallel of latitude is an east–west circle that joins all places with the same latitude. Parallels are the base of the global grid used to find any place on earth. They also mark climate limits like the Tropics and the Arctic Circle.

Explanation

What latitude measures

• Latitude is an angle, not a distance.
• It is measured in degrees, from the earth's centre, along the place's meridian (its north–south line).

• The equator is 0°. The North Pole is 90°N and the South Pole is 90°S.

• Each degree (°) has 60 minutes (′). Each minute has 60 seconds (″).

• One second of latitude covers about 30.7 m on the ground [2].
• Parallels never meet. They stay the same distance apart from the equator to the poles [2].
• Parallels get smaller as you move away from the equator. At 90° they shrink to a point (the pole), so the poles are points, not circles.
• Counting parallels drawn at every 1°:
• 89 parallels in each hemisphere (1° to 89°);
• 179 parallels in all, counting the equator;

• the poles are not counted, because they are points.

• Drawing a parallel (NCERT Class 11, Fig. 3.2): use a protractor to mark the angle at the earth's centre, measured from the equator. Then draw a line through that point that runs parallel to the equator.

How latitude is found

• The sun's altitude: altitude means how high the sun stands above the horizon at noon. The noon altitude and the date together give the latitude.
• The Pole Star (Polaris): in the northern hemisphere, the Pole Star's height above the horizon is roughly equal to the latitude.
• At the equator (0°), it sits on the horizon.
• At the North Pole (90°N), it is almost straight overhead.

• It cannot be seen from the southern hemisphere.

• Latitude has been used in astronomy and navigation since ancient times [2].

Equator, great circles and small circles

• The equator is the 0° parallel and the longest parallel.
• It divides the earth into the Northern Hemisphere and the Southern Hemisphere [2][3].
• Its circumference (the distance once around it) is about 40,075 km [3].

• Great circles that pass through the poles measure only about 40,000 km. The equator is longer because of the equatorial bulge (the earth is fatter at the equator) [3].

• Great circle: a circle whose plane passes through the earth's centre. It cuts the earth into two equal halves. It is the largest circle you can draw on a sphere [3].

• Small circle: a circle whose plane does not pass through the centre. It does not cut the earth into equal halves.
• The equator is the only parallel that is a great circle. It is the earth's only east–west great circle [3]. Every other parallel, such as the Tropic of Cancer or the Arctic Circle, is a small circle.
• Great-circle route:
• On a sphere, the shortest path between two points always lies along the arc of the great circle that passes through both [3].
• So ships and aircraft follow great-circle routes to save distance, fuel and time. Pilots still change these routes for weather and winds [3].
• On many flat map projections, such a route looks curved and bends towards the pole.

Length of a degree

Length of 1°	Equator	45°	Poles
1° of latitude	110.6 km	≈111 km	111.7 km
1° of longitude	111.3 km	≈79 km	0 km

• A degree of latitude is almost the same everywhere, about 111 km [2].

• More exact values are 110.567 km at the equator and 111.699 km at the poles [4]. NCERT gives 110.6 km and 111.7 km.

• Why a degree of latitude is longer at the poles:

• the earth is an oblate spheroid (a ball slightly squashed at the poles);
• so its surface curves less near the poles;

• on a flatter curve, you travel slightly farther on the ground to change your angle by 1°, so 1° is longer there [4].

• A degree of longitude shrinks towards the poles, because meridians meet at the poles.

• It is about 111.3 km × cos(latitude): 111.3 km at 0°, about 79 km at 45°, about 55.7 km at 60°, and 0 km at 90°.

Where it occurs

• Named parallels on the world map (approximate values) [2]:
• Arctic Circle: 66½°N
• Tropic of Cancer: 23½°N
• Equator: 0°
• Tropic of Capricorn: 23½°S

• Antarctic Circle: 66½°S

• India: the Tropic of Cancer crosses the country. It splits India into a tropical south and a sub-tropical or temperate north.

• USA, latitude watch: NOAA's National Ocean Service has tracked latitude since 1899 from observatories at Gaithersburg (Maryland) and Ukiah (California) [2].
• Great-circle routes:
• Vancouver (Canada) to Yokohama (Japan): this sea route across the North Pacific follows a great-circle arc (NCERT Class 12).
• Atlanta (USA) to Athens (Greece): flights are planned along a great circle [3].

• Many flights between Asia and North America pass over the Arctic for the same reason.

• Mapping in India: the latitude grid and accurate geodesy (the science of measuring the earth's shape and size) underlie Survey of India maps and satellite positioning.

Don't confuse with

• Meridians of longitude: meridians run north–south and all meet at the poles. Parallels run east–west and never meet.
• Great circle vs parallel: only the equator is both a parallel and a great circle. Each meridian joined with its opposite meridian (for example 0° + 180°) is a great circle, but no other parallel is.
• Degree of longitude: 1° of longitude is longest at the equator (111.3 km) and 0 km at the poles. 1° of latitude is the opposite: slightly longer at the poles (111.7 km) than at the equator (110.6 km) [4].
• Latitude as a distance: latitude is an angle in degrees measured at the earth's centre, not a distance in kilometres.

Prelims Hooks

• At 1° spacing there are 179 parallels: 89 in each hemisphere plus the equator. The poles are points, not parallels.
• The equator is the only parallel that is a great circle. The Tropics and the Arctic and Antarctic Circles are all small circles.
• 1° of latitude: 110.6 km at the equator, 111.7 km at the poles. It is longer at the poles because of polar flattening [4].
• The equator's circumference is about 40,075 km. That is more than the roughly 40,000 km of the meridian great circles because of the equatorial bulge [3].
• In the northern hemisphere, the Pole Star's altitude ≈ the latitude of the place. At the equator, Polaris sits on the horizon.
• One second of latitude ≈ 30.7 m [2]. The shortest route on the globe is a great-circle arc, for example Vancouver to Yokohama across the North Pacific.

Mains Points

• Great-circle navigation (GS-I/GS-III):
• Air and sea routes follow great-circle arcs to cut distance, fuel use and emissions.
• This explains flights over the Arctic between Asia and North America, and North Pacific shipping lanes that curve northwards.

• Weather, jet streams and airspace limits still change the actual route [3].

• Latitude as a control on climate (GS-I):

• The Tropics and the Arctic Circle mark the limits of the overhead sun and of the midnight sun.
• So latitude shapes insolation (the sunlight a place receives), and through it the temperature belt and vegetation.

• In India, the Tropic of Cancer divides a tropical south from a sub-tropical or temperate north.

• Earth's shape, measurement and mapping (GS-I):

• Because the earth is an oblate spheroid, the equator measures 40,075 km [3] and 1° of latitude is longer near the poles [4].
• Meridians converge, so east–west distances shrink with latitude. The same number of degrees covers less ground at high latitudes.
• This matters for map projections, gridded satellite data and reading area correctly on world maps.

Related concepts

• Equator
• Great circle
• Small circle
• Length of a degree of latitude and longitude

Read more

• Parallels of latitude: equator, great and small circles, length of a degree