In this post, let us understand about the Shape of our Earth as our first topic in General Navigation. We had covered the scope of General Navigation in our earlier post.

### OBLATE SPHEROID OR GEOID

The earth had evolved from gas billions of years ago. A swirling ball of this gas then cooled and solidified to form the planet. As the gas solidified, the centrifugal force created by the spin of our earth, produced slightly more mass around its mid portion. Therefore, our planet became a Distorted Sphere, in the sense that the curvature of earth is slightly flat at the end points of the axis of rotation.

This unique shape is referred to as an Oblate Spheroid. The shape of our earth is also called at times as a Geoid, which means Earth Shaped in Greek language. Recent studies have shown that there is a slightly larger concentration of mass in the Southern Hemisphere, making it a bit of pear shaped.

### DEFINITION OF NORTH AND SOUTH POLE

We are aware that earth rotates around itself and revolves around the sun. Spin Axis is an imaginary line joining the North and South Poles. Therefore, Spin Axis is also called as the Polar Axis.

Poles of the earth are the two ends of Earth’s Spin Axis or the Polar Axis. Obviously, there are two poles, one of which is called the North Pole and the other is known as its South Pole.

If you look at the earth from over the North Pole, earth appears to rotate in an Anti-Clockwise direction. Similarly, earth would appear to rotate in a clockwise direction if you look at it from over the South Pole.

Normally, for directional reference, we take the direction of rising Sun or East as a datum reference. If you stand facing direction of rising sun, North Pole is to your left and South Pole is to your right.

### COMPRESSION OF EARTH

Let us understand a bit about the diameter of our earth. As we had discussed earlier, the Axis of Rotation of earth or the Spin Axis is an imaginary line joining North and South Poles. The plane perpendicular to the Spin Axis and dividing the earth in two equal halves is called its Equatorial Plane.

The equatorial plane divides the earth into two equal halves are called its Northern and Southern Hemisphere. Equatorial Diameter is the diameter of the earth along the Equatorial plane. Whereas, Polar Diameter is the diameter of the earth along the line joining the North and South Poles.

We had understood, that the earth is slightly flattened at its poles. Therefore, the horizontal diameter of earth is more than that of its vertical diameter. If we use the correct terms, Equatorial Diameter of the earth is greater than that of its Polar Diameter.

The earth, therefore, is not perfectly spherical in shape. It is slightly distorted, being more wide than it is high. This difference in diameter is called the Compression of Earth.

### COMPRESSION RATIO

The Compression of the Earth is described in terms of its height which is also called as the Polar Diameter, against its width which is called the Equatorial Diameter. The Polar Diameter is nearly 27 Statute Miles lesser than the Equatorial Diameter.

The Variation between the Equatorial and Polar Diameters is 27 Statute Miles. In other units, this difference in diameter is 23 Nautical Miles or 43 Kilometres. You would understand more about Nautical Miles in our later lessons. The difference between the Equatorial and Polar Diameter can also be expressed as a fraction or as percentage of the Equatorial Diameter. Value of earth compression is 1/300 or when expressed as a percentage it is equal to 0.3%.

### FORMULA FOR COMPRESSION RATIO

Earth’s Compression is given by a ratio called as the Compression Ratio. The formula for Compression Ratio is equal to

#### Compression Ratio = (Equatorial Diameter – Polar Diameter) / Equatorial Diameter

### WORLD GEODETIC SYSTEM

Cartographers are professionals who prepare Maps and Charts. They need to make mathematical models of earth to create accurate maps and charts for navigational purposes. Since, various agencies located at various parts of the world were involved in map preparation, they ended up creating different mathematical models, which were accurate for their region but not usable in other regions.

However, for aviation equipments like Global Positioning System (GPS) or Flight Management Systems (FMS) there is a need to have a model which was usable in all regions. In order to standardise Maps and Charts, International Civil Aviation Organisation (ICAO) adopted World Geodetic System, which is also called WGS 84 as a standard model for all aviation purposes. WGS 84 considers earth to be a perfect sphere and hence spherical trigonometry can be used for all the navigational calculations.

In the next section we shall understand about our Solar System and meaning of Obliquity of the Ecliptic.