Terminologies

Latitude and Longitude

The earth looks like a ball that is slightly flattened at the top and bottom, just like a big orange, and it is about 6400 kilometers wide from the middle to the edge. The earth is oblate spheroidal in shape with a radius often approximated to 6400km. This means the world is not a perfect circle but is a bit fat at the middle.

The earth spins around an imaginary line running from the top to the bottom, where the top end is called the North Pole and the bottom end is the South Pole. It rotates about the north-south axis, the north end of the axis being the north pole (N) and the south end of the axis being the south pole (S). Think of it like a spinning top or a gala wrapper spinning on a stick.

Equator

The Equator is the main line that cuts the earth into two equal halves, top and bottom, passing right through the middle. Equator: This is a circle formed by the intersection of the earth by a plane which is perpendicular to the N-S axis; which passes through the centre (C) of the earth. It is the biggest belt you can tie around the earth's belly.

Parallel of latitude

These are other circles drawn around the earth that stay level with the equator, getting smaller as they move toward the top or bottom poles. Parallel of latitude: Other planes parallel to the equator, intersecting the earth form circles which are called parallels of latitude. Just like how the rings on a sliced onion get smaller near the ends, these circles shrink as they leave the center.

The closer a parallel of latitude is to the pole, the smaller its diameter. This means as you go towards the North or South, the circles around the earth become tighter.

Great circle

A Great Circle is any circle drawn on the earth that is as big as the earth's full width, like the Equator. Great circle: A great circle is a circle formed by a plane which passes through the centre of the earth (equator). If you cut an orange exactly into two equal halves, the edge of that cut is a great circle.

Small circle

A Small Circle is any circle on the earth that does not pass through the very center and is smaller than the Equator. Small circle: This is a circle formed by a plane which doesn't pass through the centre of the earth (parallels of latitude). If you slice the top off an orange, the circle you see is a small circle because it did not touch the middle.

Meridian

A Meridian is a big circle that goes from the North Pole all the way to the South Pole. Meridian: This is the great circle which is formed by a plane passing through the axis of the earth. These lines stand upright on the earth, like the lines on a basketball.

Greenwich meridian

The Greenwich Meridian is the starting line for measuring all other upright lines around the world, and it passes through a place in England. Greenwich meridian: This is the reference meridian for all other meridians. It is the meridian which passes through Greenwich, a town in England. We use this special line to count where other lines are, just like using a 'Point Zero' on a ruler.