shape of the earth.

In the early times most of the people believed that the Earth has a flat shape. However, as time went on, new argument was put forward by Galileo Galilei and Ferdinand Magellan that the Earth is round in shape. It was around 500 B.C that Pythagoras of Samos first proposed a spherical earth, on aesthetic grounds rather than on physical evidence. Like many Greeks, he believed the sphere was the most perfect shape. There are several theories which elaborate on earth's shape which include:

1. Plato and Aristotle's Theory of Natural Place:
   This theory used a spherical Earth to explain why heavy objects fall downward (towards what Aristotle believed was the center of the Universe), while lighter elements like air and fire rise upward. In their geocentric model, the structure of the universe was thought to consist of a series of perfect spheres. Though initially not widely accepted, their theory gained support after the lunar eclipse observations and the journeys around the world, which provided evidence of the Earth's round shape.
2. Heliocentric Theory (Nicolaus Copernicus, 16th Century):
   This theory proposed that the Earth is spherical and that it is one of the planets revolving around the Sun. Copernicus considered the Sun to be the center of the solar system (and possibly the universe), and believed that planets, comets, and asteroids revolved around it. This model was a significant shift from the geocentric view, emphasizing the Sun's central role in the cosmos.
3. Centripetal Theory (Isaac Newton, 17th Century):
   Isaac Newton's mathematical evidence determined that the Earth is not a perfect sphere but is slightly flattened at the poles and bulging at the equator. This bulging along the equator is due to the Earth's rotation and the centrifugal forces acting on the material at the equator, overcoming gravity and causing the material to pull outward. Newton's work laid the foundation for understanding the Earth's actual shape.
4. Geocentric Theory (Claudius Ptolemy, 90-168 AD):
   Ptolemy's geocentric model argued that the Earth was spherical and at the center of the universe. He believed that the Earth was stationary, with all other celestial bodies, including the stars, the Sun, and the Moon, revolving around it. This theory, while influential for centuries, was eventually disproven by modern science, as it became clear that the Earth, along with other celestial bodies, is not stationary.

There are various evidences that prove on the spherical nature of the Earth, including the following:

1. Aerial photograph
   Numerous photographs have been taken by satellites at great distances from the Earth, and all of these show that the Earth is spherical

   

2. Circumnavigation of the earth
   The Earth has been circumnavigated many times by land, sea and air. When navigators sail on the sea or ocean, it has been proven that if they travel from one part following the same latitude they always come back to the same point (initial point). If the Earth were flat, this wouldn't be possible.

3. Lunar eclipse
   Lunar eclipse is the eclipse of the moon which is important evidence on the sphericity of the Earth which was discussed by Aristotle (384-322 BC). He cited two convincing arguments for the earths' sphericity. First is the fact that during a lunar eclipse, as the moon enters or emerges from the earth's shadow, the shape of the shadow seen on the moon is always rounded. Only a sphere can cast a shadow which is always circular.

   

4. Surveying with ranging poles on level ground
   It has been observed by surveyors that when they to insert three ranging poles of the same height on the ground at a distance of about one kilometer apart from one another in a straight line they found that, the middle one stands above other two poles . Thus, this proves the sphericity of the Earth. If the Earth were flat, the tops of all poles would lie on the line of sight .

   

5. Ship's visibility
   When a ship is approaching the coast, first the smoke will be seen followed by the chimney, and then the whole ship appears at last. Moreover, where two ships are coming along the coast the observer will be able to see the front ship, but if the Earth were flat, the observer would be able to see both ships at once.

   

   curved earth

   

   flat earth

6. Sunrise and Sunset
   The Earth rotates from West to East, which means that places in the East see the sun before places in the West. If the Earth were flat the sun would be seen at the same time in all places all over the earth's' surface. Also, sun rise and sun set would be at the same time in all places of the earth's surface.

   

7. Measuring weight using a spring balance
   When measuring weight using a spring balance the weight of something at the Equator changes a little bit at the poles since weight depends on gravitational pull. It is as if people are pulling the weight from the centre as the force tries to balance. It can be related as the people of equal weight pulling something in a circular motion at the equator (centrifugal force) while at the poles it seems as if they are pulling it towards the centre (gravitational force).

8. The Earth's curved horizon
   The Earth horizon when seen from a ship, a plane or a high altitude appears curved. The curved horizon widens as the observer's altitude increases until it becomes circular. If the earth were not spherical there would be no circular horizon.

Despite the above evidences which prove the spherical shape of the Earth, it is argued by Sir Isaac Newton that the Earth is not exactly sphere, it is oblate. It is a bit flat at the poles and bulge at the equator. This is because, near the equator the Earth rotates at very high speed compared to the poles. Due to the high speed the centrifugal force overcomes the gravitational force which leads to pull the matters away from the centre. This causes a bulging of the Earth at the Equator. On the other hand, the force of gravity is so great at the poles due to less speed leading to pulling of matters towards the centre of the earth. This situation causes a bulging at the equator and flattening of the polar region.

This shape is caused by the following factors

1. High Speed of Rotation along the Equator:
   The Earth rotates at a speed of approximately 460 meters per second at the equator (roughly 1000 miles per hour). This high speed generates a centrifugal force, which pushes outward from the center of the Earth. This force overcomes the gravitational pull at the equator, leading to a bulging effect. As a result, the equatorial diameter of the Earth is about 12,762 km, slightly larger than the polar diameter of 12,722 km.
2. Low Speed of Rotation at the Poles:
   At the poles, the rotational speed is effectively zero, meaning there's no centrifugal force to counteract gravity. Gravitational pull is strongest at the poles because the points there are closer to the Earth's center. This stronger gravity pulls material towards the center, causing the flattening of the polar regions. As a result, the Earth's shape is not perfectly spherical but slightly flattened at the poles and bulging at the equator.

The oblateness of the Earth is justified by five factors namely; inequality of circumference and diameters between equator and poles, measurement of latitude and recent satellite image data. Larger equatorial diameter of 12762 km results into larger equatorial circumference of 40085 km while smaller polar diameter results into small circumference of 39955 km. This proves that the planet Earth is not a perfect sphere. Variations in the measurements of latitude are caused by the difference in the curvature between the equatorial and polar areas. For example, near the equator 1o measure the length of 110 569 km while near the poles 1o measure the length of 111700 km. Recent satellite data of the Earth suggest that the southern hemisphere is relatively smaller than the northern hemisphere which also proves that the planet earth is not a perfect.

Oblateness of the Earth