### Shape of Our Earth

In this lesson, let us discuss the shape of our earth and its effect on air navigation. In simple terms, shape of our earth is a distorted sphere because it is not perfectly spherical in shape. Because of the constant rotation around its axis, there is more mass concentrated in its middle portion. In other words, earth is slightly flattened at the poles. Studies have shown earth to be pear shaped than that of an orange, due to larger concentration of mass in its southern hemisphere. This peculiar shape of earth is called an oblate spheroid. At times, shape of earth is also referred to as a geoid. Geoid simply means earth shaped in Greek language.

### Poles of Earth

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 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.

### Diameter 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 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

### Equatorial and Polar Diameter

We had understood, that the earth is slightly flattened at its poles. The impact of this flattening is that the horizontal diameter of earth is more than that of its vertical diameter. If we have used the correct terms, equatorial diameter of the earth is greater than that of its polar diameter. However, the difference is not much, the value is so small that the difference can be practically neglected for all all-navigational calculations. For theoretical purposes, the equatorial diameter is greater than the polar diameter by 23 statute miles or 43 kilometres. In terms of nautical miles which is commonly used in air navigation the value of difference is 27 nautical miles. If you are not clear about the term nautical mile, don’t worry, we will be discussing it in much more detail.

### Compression Ratio

We had seen that the earth is flattened at its poles. This flattening is called compression and is given by a ratio called the compression ratio. Compression ratio is the numerical difference between the equatorial and polar diameters divided by the equatorial diameter. The value of compression ratio is 1/300 or in percentage terms 0.3%, As you can see, the compression ratio is negligible and earth can be a perfect sphere for all practical purposes. Formula for 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 However, for equipment like global positioning system or flight management systems there was a need to have a model which was usable in all regions. Therefore, International civil aviation organisation (ICAO) adopted World Geodetic System 84 also called WGS 84 as a standard model for all aviation purposes. WGS 84 considers earth to be a perfect sphere and hence simple spherical trigonometry can be used for all navigational calculations

Tilt of the Spin Axis We had observed that the earth rotates around itself and revolves around the sun. Spin axis or polar axis is an imaginary line joining North and South Poles. Orbital plane on the other hand, is the three-dimensional plane on which earth revolves around the sun. We need to understand, that the spin axis is not perfectly perpendicular to the orbital plane. Spin axis of the earth is tilted to the orbital plane by 23 ½ degrees. You would learn in meteorology lessons that this tilt of the spin axis is the major reason for changing