Earth is a planet in the Solar System, which is located in the Milky Way galaxy. Earth is one of eight planets within the Solar System, but is the only one that has been confirmed to sustain life. It is also the largest of the four terrestrial planets, the others being Mars, Mercury, and Venus. In order to measure the size of the Earth, scientists consider several distinct measurements such as the planet's diameter, density, surface area, and size relative to other objects within the Solar System.

## History of Measuring the Size of Earth

Scientists have long been intrigued by the concept of the Earth's size, and several scholars have dedicated significant time trying to determine its exact size. In Ancient Greece, Pythagoras believed that Earth was spherical, and his theory gained significant traction among other Ancient Greek scholars, the most prominent being Aristotle and Archimedes. Aristotle's calculations indicated that the circumference of the Earth was roughly 400,000 stadia, which according to modern measurements, could be approximately between 39,250 and 46,250 miles long. In addition to Ancient Greek scholars, Egyptian scholars such as Eratosthenes also contributed to the ancient practice of measuring the Earth's size. The most significant part of Eratosthenes' calculations was that his measurements were extremely close, and were in fact only 0.4% higher than modern figures. Similarly, the ancient Indian mathematician Aryabhata's calculation of the diameter of Earth was only 1% less than modern measurements.

## Radius of Earth

When considering the size of Earth, radius is a key measurement. Since the Earth rotates on its axis, the poles have become flattened, and therefore the region near the equator has bulged. Due to its unique shape near the poles and the equator, the Earth is considered to have an imperfect sphere, which is commonly referred to as an oblate spheroid. Given this unique shape, the Earth may have different radii depending at which point it is measured. When measured from the equator, the radius of the Earth, commonly referred to as the equatorial radius, is approximately 3,963 miles. When measured from either of the poles, the radius of the Earth, commonly referred to as the polar radius, is approximately 3,950 miles long. According to standard suggested by the International Astronomical Union (IAU), the most frequently used radius of the Earth is the equatorial radius, since the polar radius does not encompass the entire breadth of the Earth's territory. The International Union of Geodesy and Geophysics (IUGG) has also provided several measurements of the Earth's radius.

## Surface Area of Earth

Historically, the surface area of the Earth has been one of the most hotly contested issues among scholars. This issue resulted from the fact that scholars could not agree on the Earth's exact shape, and therefore the appropriate formula needed to calculate its surface area. Another complicating factor in calculating the Earth's surface area is that its diameter varies from one part to another. As a result, in order to calculate the exact surface area, mathematicians have developed a specific formula which has detemined that the Earth's surface area is approximately 196,939,900 square miles (510.1 million km²) and of that 57,505,693.767 square miles (148.9 million km²) is the land area not covered by oceans.

## Density of Earth

The Earth's density is approximately 0.1992 pounds per cubic inch (pb/cu in). Since the Earth is made up of numerous materials, scientists obtain the average densities of every object on Earth. Based on this figure, Earth has the highest density of all the planets in the Solar System, which is due to a phenomenon referred to as gravitational compression. This phenomenon occurs when the gravity of a planet acts on objects within the planet, reducing the size of the object, and subsequently increasing its density by reducing its volume. If the gravitational compression not taken into account, Mercury would have the highest density of any planet in the Solar System.

## Volume of Earth

Since Earth is roughly spherical, calculating its volume is relatively simple. The equatorial radius is most commonly used to calculate the volume of Earth, so as not to leave out any territory. Using the mathematical formula for calculating the volume of a sphere, it has been estimated that the volume of Earth is roughly 259 billion cubic miles. Mathematicians have also developed a more accurate form to measure Earth's volume, which includes the use of the reference ellipsoid model. The volume calculated using the model is extremely accurate, and is similar to the value determined by NASA.

## Importance of Earth's Size

Scientists have shown that the size of Earth is one of the most important reasons it is able to support life. For example, the Earth's size dramatically influences the amount of gravity experienced on the planet. Scientists believe that if Earth was smaller, the amount of gravity experienced on the planet would be significantly lower, and if the gravity were lower, then the atmosphere would break apart and disappear into space. Without an atmosphere, life could not exist on Earth. Similarly, if Earth was larger, then its force of gravity would be significantly larger as well. A massive increase in the force of gravity would result in an increase in the quantity of various poisonous gases within the atmosphere, and these poisonous gases would decimate all life on the planet. Apart from the size, other factors that contribute to the Earth's optimal conditions for life are its distance from the Sun and the mixture of gases within its atmosphere.