# FACTS ABOUT THE SHAPE AND SIZE OF THE EARTH

international earth year

Earth, the third planet from the sun, is the fifth largest planet in the solar system. Earth is the largest of the terrestrial planets of the inner solar system, bigger than Mercury, Venus and Mars.

Earth, with an average distance of 92,955,820 miles (149,597,890 km) from the sun, is the third planet and one of the most unique planets in the solar system. It formed around 4.5 to 4.6 billion years ago and is the only planet known to sustain life. This is because factors like its atmospheric composition and physical properties such as the presence of water over 70.8% of the planet allow life to thrive.

Geodesy is the science that studies the shape and size of the Earth. Geodesy, a branch of science responsible for measuring the Earth’s size and shape with surveys and mathematical calculations is used.

In order to study the Earth and use geodesy today, researchers often refer to the ellipsoid, geoid, and datums. An ellipsoid in this field is a theoretical mathematical model that shows a smooth, simplistic representation of the Earth’s surface. It is used to measure distances on the surface without having to account for things like elevation changes and landforms. To account for the reality of the Earth’s surface, geodesists use the geoid which is a shape that is constructed using the global mean sea level and as a result takes elevation changes into account.

Throughout history, geodesy was a significant branch of science as early scientists and philosophers attempted to determine the Earth’s shape. Aristotle is the first person credited with trying to calculate Earth’s size and was, therefore, an early geodesist. The Greek philosopher Eratosthenes followed and was able to estimate the Earth’s circumference at 25,000 miles, only slightly higher than today’s accepted measurement.

Today, technology like satellites and global positioning systems (GPS) allow geodesists and other scientists to make extremely accurate measurements of the Earth’s surface. In fact, it is so accurate, geodesy can allow for worldwide navigation but it also allows researchers to measure small changes in the Earth’s surface down to the centimeter level to obtain the most accurate measurements of the Earth’s size and shape.

Earth’s Shape

The true shape of the Earth called an Oblate Spheroid. The term “Oblate” refers to it’s slightly oblong appearance. The term “Spheroid” means that it is almost a sphere, but not quite.

Earth’s orbit is not a perfect circle, but is rather an oval-shaped ellipse, like that of the orbits of all the other planets. Earth is a bit closer to the sun in early January and farther away in July, although this variation has a much smaller effect than the heating and cooling caused by the tilt of Earth’s axis. Earth happens to lie within the so-called “Goldilocks zone” around its star, where temperatures are just right to maintain liquid water on its surface.

Contrary to common misconceptions, many historical mathematicians and scientists were aware that Earth was spherical. Technically Earth’s shape is called the “geoid”, an ellipsoidal shape. Possibly starting with Eratosthenes, over 2,200 years ago, mathematicians have attempted to calculate the size of a spherical or ellipsoidal Earth. The techniques of Eratosthenes and his predecessors are considered elementary, or at least straightforward, for today’s mathematician, but are impressive for their scope in calculating the entire Earth without being able to see or measure most of it and for their accuracy.

Evidence Of The Earth Sphericity

There are many ways to prove that the earth is spherical in shape, these proves are;

1. Circumnavigation Of The Earth

Ferdinand megellian, a Spanish sailor with his crew sailed round the earth between 1519 and 1522, he and his crew were the first people to do so,

If the earth is flat, he and his crew would have fallen off the flat edge but instead they sail round the spherical edge because if the earth is truly flat, Ferdinand and his crew would have fell off the flat edge. Also modern air routes and area navigation are based on the assumption that the earth is spherical in shape.

2. Circular Horizon

When one view a distance horizon from the deck of a ship at a sea or from a cliff, the land and everywhere always look circular in shape with increase attitudes (height) which suggests that the earth is spherical in shape.

3. Sunrise And Sunset

As the earth rotates from west to east, place in the east experience the sun (earlier sunrise) than place in the west and also place in the west see the sun later (sunset) than place in the east.

If the earth is flat, then the whole earth should experience sunrise and sunset at the same time, but instead sun rise and set at different time for different place.

4. Ship Visibility

As a ship approaches the harbour, only the top of the mast is seen first before the hill, and also the same when it leaves the harbour, it disappearance over the curved surface is equally gradual. If the earth were flat, the entire body of the ship will be seen or observe all at ones because if the earth were flat the surface of the earth will also be flat.

5. Sphericity Of Planetary Bodies

All other planets including the sun, the moon and the stars they are all seen in circular shape when view from the telescope.

6. Driving Pole Of Equal Length On The Earth

when three poles of equal length are driven at the same depth into a level ground, the centre pole were found to be slightly projected above the other pole at either side because of the round curvature of the earth.

If the earth were flat, all three poles should have the same height.

7. Lunar Eclipse

During lunar eclipse, the earth cast a circular shadow on the moon which takes the outline of an area of a circle, and only a spherical can cast such circular shadow.

8. Aerial Photograph

When a picture of the earth is taken from a high altitude by rockets or drones, the picture clearly show the curved edge of the earth.

This proof is the most recent proof of the sphericity of the earth.

Earth’s Size

As the largest of the terrestrial planets, Earth has an estimated mass of 5.9736 × 1024 kg. Its volume is also the largest of these planets at 108.321 × 1010km3.

In addition, Earth is the densest of the terrestrial planets as it is made up of a crust, mantle, and core. The Earth’s crust is the thinnest of these layers while the mantle comprises 84% of Earth’s volume and extends 1,800 miles (2,900 km) below the surface. What makes Earth the densest of these planets, however, is its core. It is the only terrestrial planet with a liquid outer core that surrounds a solid, dense inner core. Earth’s average density is 5515 × 10 kg/m3. Mars, the smallest of the terrestrial planets by density, is only around 70% as dense as Earth.

Earth is classified as the largest of the terrestrial planets based on its circumference and diameter as well. At the equator, Earth’s circumference is 24,901.55 miles (40,075.16 km). It is slightly smaller between the North and South poles at 24,859.82 miles (40,008 km). Earth’s diameter at the poles is 7,899.80 miles (12,713.5 km) while it is 7,926.28 miles (12,756.1 km) at the equator. For comparison, the largest planet in Earth’s solar system, Jupiter, has a diameter of 88,846 miles (142,984 km).

Erastosthenes’ Calculation of Earth’s Size

He observed that at noon on the summer solstice of the northern hemisphere, at two different places, the sun’s rays fell upon the earth at different angles. In the city of ancient Syene (modern Aswan, Egypt), at noon on the solstice, the sun was directly overhead and cast no shadows on the objects below it (Aswan is very near the Tropic of Cancer. This was proven by observing that the sun completely illuminated the bottom of a very deep well in Syene at noon. The angle between the sun’s rays and objects perpendicular to the earth’s surface was effectively zero.

In Alexandria, on that same day at noon, the sun cast slight shadows behind objects. By measuring the dimensions of the shadows and the objects that cast them. Eratosthenes was able to calculate that the sun’s rays arrived in Alexandria at an angle of  of a circle (about 0.13 radians or 7.2 degrees). He knew the distance between Alexandria and Syene to be  “stades” from land surveys done between the two cities. A stade was an ancient Greek unit of measurement (150-200 meters). He also assumed that Syene was due south of Alexandria and on the same meridian of longitude. This would mean that the distance between the two cities represented an arc of the earth’s circumference (in reality, Aswan is about three degrees of longitude east of Alexandria).

Eratosthenes assumed that the sun was far enough away from the earth that its rays arrived at the Earth parallel to each other. In the diagram above,  is the angle that the sun’s rays arrived to Earth at Alexandria on noon of the solstice.  represents the angle subtended at the Earth’s center by the arc representing the distance between Alexandria and Syene. From the equivalence of alternate interior angles across a line transverse to two parallel lines,  of a circle. Since angle B is th of a circle and sweeps an arc at the earth’s surface of 5,000 stades, the Earth’s circumference would be  stades.

Because there is a scholarly debate as to what the definition of a “stade” is, there is no way of knowing exactly how close Eratosthenes was to today’s currently accepted value for the circumference of the earth. It is thought that a “stade” is defined as the length of an Ancient Greek stadium used for athletic competitions. Most of them are about  of a mile, but some are longer and some are shorter. There is evidence that the word stade meant different things in different contexts, at different times, and in different places. It is not certain which definition of stade Eratosthenes used. The actual circumference of the Earth is  km at the equator.

Regardless, he calculated the size of the earth to within an order of magnitude and possibly got very close. He also may have been lucky. Errors could have arisen from: a) the fact that Syene and Alexandria are not exactly on the same meridian of longitude; b) significant errors in measuring the distance overland between Alexandria and Syene; or c) in measuring the angles cast by the shadows at Alexandria.

It is a common modern misconception to assume that past civilizations believed in a flat earth of unknown dimensions. Eratosthenes’s calculation proliferated far and wide and was preserved by successive generations and geographically disparate civilizations. This estimate of the size of the earth and its method would remain canonical for the educated elite of the future civilizations of Europe, North Africa, the Middle East, and South Asia until the 17 century when more precise measurements could be made to constrain the size of the earth.

Earth’s formation and evolution

Scientists think Earth was formed at roughly the same time as the sun and other planets some 4.6 billion years ago, when the solar system coalesced from a giant, rotating cloud of gas and dust known as the solar nebula. As the nebula collapsed because of its gravity, it spun faster and flattened into a disk. Most of the material was pulled toward the center to form the sun.

Other particles within the disk collided and stuck together to form ever-larger bodies, including Earth. The solar wind from the sun was so powerful that it swept away most of the lighter elements, such as hydrogen and helium, from the innermost worlds, rendering Earth and its siblings into small, rocky planets.

Scientists think Earth started off as a waterless mass of rock. Radioactive materials in the rock and increasing pressure deep within the Earth generated enough heat to melt Earth’s interior, causing some chemicals to rise to the surface and form water, while others became the gases of the atmosphere. Recent evidence suggests that Earth’s crust and oceans may have formed within about 200 million years after the planet had taken shape.

The history of Earth is divided into four eons — starting with the earliest, these are the Hadean, Archean, Proterozoic and Phanerozoic. The first three eons, which together lasted nearly 4 billion years, are together known as the Precambrian. Evidence for life has been found in the Archaean about 3.8 billion years ago, but life did not become abundant until the Phanerozoic.

The Phanerozoic is divided into three eras — starting with the earliest, these are the Paleozoic, Mesozoic, and Cenozoic. The Paleozoic Era saw the development of many kinds of animals and plants in the seas and on land, the Mesozoic Era was the age of dinosaurs, and the Cenozoic Era we are in currently is the age of mammals.

Most of the fossils seen in Paleozoic rocks are invertebrate animals lacking backbones, such as corals, mollusks and trilobites. Fish are first found about 450 million years ago, while amphibians appear roughly 380 million years ago. By 300 million years ago, large forests and swamps covered the land, and the earliest fossils of reptiles appear during this period as well.

The Mesozoic saw the ascendence of dinosaurs, although mammals also appear in the fossil record about 200 million years ago. During this time, flowering plants became the dominant plant group and continue to be so today.

The Cenozoic began about 65 million years ago with the end of the age of dinosaurs, which many scientists think was caused by a cosmic impact. Mammals survived to become the dominant land animals of today.