1. GREAT CIRCLES
On the earth’s surface, a great circle is a circle that has the centre of the earth as its centre. It is the circle you would get if you sliced the earth directly through its centre.
The radius of a great circle is the radius of the earth itself.

2. A great circle is what you get if you slice through the centre of a sphere.

3. The EQUATOR is the most famous great circle

4. SMALL CIRCLES
are circles on the earth’s surface that do not have the earth’s centre as their centre. The radius of a small circle is less than the radius of the earth.
A small circle is what you get if you slice through a sphere but miss its centre.

5. MERIDIANS are great circles that go through the earth’s north and south poles
Meridians are the great circles that measure
LONGITUDE
They run north to south.
The word ‘meridian” means “midday” because all points on the earth’s surface on the same meridian experience midday at exactly the same moment.
MERIDIANS are how we measure time across the earth’s surface.

6. Small circles parallel to the EQUATOR are known as Parallels of Latitude

7. A grid of longitude and latitude circles make up a co-ordinate system used to specify the position of locations on the earth’s surface:

8. Longitude measures the angular separation East or West from the prime meridian.
Demonstration of longitude
Geogebra demonstration

9. This is because the angular separation between 120W and 60E is equal to 120 + 60 = 180

10. Latitude demonstration
Geogebra demonstration

11. Latitude measures the angular separation of a point north or south from the equator.

12. LATITUDE and LONGITUDE Co-ordinates
Any point on the earth’s surface can be located from its co-ordinates of latitude and longitude.
For example, Melbourne is located at 37.8S and 144.9E.

13. The 145E meridian
The 40S parallel of latitude

14. Example 1

15. Example 2:

16. Example 3:

17. Example 4: