TOPIC 7: THE EARTH AS A SPHERE
Difference between angles of latitude or longitude
Features and Location of Places
The Equator, Great Circle, Small Circles, Meridian, Latitudes and Longitudes
Describe the equator, great circle, small circles, meridian, latitudes and longitudes
Definition of latitude and longitude
The Earth is not a perfect sphere, as it is slightly flatter at the north and south poles than at the equator. But for most purposes we assume that it is a sphere.
The position of any point on earth is located by circles round the earth, as follows:
The earth rotates about its axis, which stretches from the north to the South Pole.
Circles round the Earth perpendicular to the axis are circles of Latitude and Circles round the Earth which go through the poles are circles of Longitude or meridians.
Consider the following diagram
Normally Latitude is defined relative to the equator, which is the circle of latitude round the middle of the Earth while Longitude is defined relative to the circle of longitude which passes through Greenwich in London (Greenwich meridian).
The latitude of a position tells us how far north or south of the equator it is while the longitude of a position shows us how far east or west of the Greenwich meridian it is.
Latitude; If we draw a line from the centre of the Earth to any position P , then the angle between this line and the plane of the equator is the latitude of P.
Longitude: This is the angle between the plane through the circle of any Longitude P and the plane of the Greenwich meridian .
Latitude can be either North or South of the equator while Longitude can be either East or West of Greenwich.
When locating the latitude and longitude of a place we write the latitude first then longitude.
Example 1
Dar es Salaam has latitude 7°S (i.e. 7° south of the equator) and longitude 39°E (i.e. 39° east of the Greenwich meridian). So Dar es Salaam is at (7°S, 39°E).
NB; Greenwich itself has latitude 51°N ( i.e. 51° north of the equator)and longitude 0° (by definition). Johannesburg has latitude 26°S (i.e. 26 south of the equator) and longitude 28°E(i.e. 28°east of the Greenwich meridian), therefore Johannesburg is at (26°S, 28°E).The north pole has latitude 90°S but its longitude is not defined. ( Every circle of longitude goes through the north pole).The south pole has latitude 90°s. Its longitude is not defined. So all points on the equator (such as Nanyuki in Kenya) have latitude 0°
Ranges; Latitude varies between 90°S (at the south pole) to 90°N (at the north pole).
Ranges; Latitude varies between 90°S (at the south pole) to 90°N (at the north pole). Longitudevaries between 180°E and 180°W. These are the longitudes on the opposite side of the Earth from Greenwich.
GREAT AND SMALL CIRCLES: There is an essential difference between latitude and longitude. Circles of longitude all have equal circumference. Circles of latitude get smaller as they approach the poles. The centre of a circle of longitude is at the centre of the earth. They are called great circles. For circles of latitude, only the equator itself is a great circle. Circles of latitude are called small circles.
Example 2
Find the latitudes and longitudes of A and B on the diagram below;
Solution;
The point A is 60° above equator, and 20° east of Greenwich.
So the point A is at (60°N, 20°E)
The point B is 10° below the equator, and on the Greenwich meridian.
So the point B is at (10°S, 0°).
Exercise 1
1. Write down the latitude and longitude of the places shown on figures below:
2. Copy the diagram show on the figure above and mark these points:
- (10°N, 30°E)
- (20°N, 20°W)
- (0°, 20°W)
3. Obtain a globe, and on it identify the following places.
- (40°S, 30°E)
- (50°S,20°W)
- (10°N,40°W)
- (40°N, 30°E)
- (80°N,10°E)
- (0°,0°)
Difference between angles of latitude or longitude
Suppose two places have the same longitude but different latitudes. Then they are north and south of each other.
In finding the difference between the latitudes take account of whether they are on the same side of the equator or not.
- If both points are south of the equator subtract the latitudes
- If both points are the north of the equator subtract the latitudes
- If one point is south of the equator and the other north then add the latitudes
Similarly, suppose two places have the same latitudes but different longitudes:
- If both points are east of Greenwich subtract the Longitudes
- If both points are west of Greenwich subtract the Longitudes
- If one point is east of Greenwich and the other west then add the longitudes
Suppose places A and B are on the same longitude, then the difference in latitude is the anglesubtended by AB at the centre of the earth.
Suppose places A and B are on the same latitude.
Then the difference in longitude is the angle subtended by AB on the earth’s axis.
Locating a Place on the Earth’s Surface
Locate a place on the Earth’s surface
Example 3
Three places on longitude 30°E are Alexandria (in Egypt) at (31°N, 30°E), Kigali (in Rwanda) at (2°S, 30°) and Pietermaritzburg (in South Africa) at (30°S, 30°E).
Find the difference in latitude between
- Kigali and Pietermaritzburg
- Kigali and Alexandria
Solution
(a)Both Towns are south of the equator. So subtract the latitudes. 30 – 2 = 28
Therefore the difference is 280
(b) Kigali is south of the equator, and Alexandria is north, so add the latitudes
31 + 2 = 33
The difference is 330
Example 4
A plane starts at Chileka airport (in Malawi) which is at (16°S, 35°E). It flies west for 50°. What is its new latitude and longitude?
Solution
Since it flies west, then subtract 35° from 50°. This gives 15°
The new longitude is now west of Greenwich, hence the plane is at (16°S, 15°W).
Exercise 2
1. In the diagram shown in the following figure find,
- The difference in longitude between A and B
- The difference in longitude between D and E
Distances along Great Circles
Distances along Small Circles
