It has long been relegated to the past are the days when the Earth was considered the plane. Today, even children know that the planet is a ball.

But if the Earth is spherical, then we can determine its diameter.

The question of the diameter of the globe is not as simple asit may seem at first glance, because the term "earth" is very conditional itself. The real diameter of the ball will always be the same, no matter where or held segment connecting two points on the surface of the sphere and passes through the center.
With respect to the earth it is notpossible because of its spherical shape is not ideal (in nature does not happen perfect geometric shapes, they are abstract geometric concepts). For the exact designation of the Earth scientists even had to introduce a special term - "geoid".

## Official diameter of the Earth

The value determined by the diameter of the Earth in whichplace it will be measured. For the convenience of the officially recognized diameter are accepted two parameters: the diameter of the Earth at the equator and the distance between the North and South Poles. The first figure is 12 756.274 km, while the second - 12 714, the difference between them is just under 43 km.
These numbers do not make much of an impression,they are inferior to even the distance between Moscow and Krasnodar - two towns located in the same country. However, it was not easy to calculate them.

## Calculating the diameter of the Earth

The diameter of the planet is calculated according to the same geometric formula, like any other diameter.
To find the perimeter of the circle must beMultiply the diameter by the number of? and. Therefore, to find the diameter of the Earth is necessary to measure its circumference in the relevant section (at the equator or the poles in the plane) and divide it by the number of? And.
The first person to try to measurecircumference of the Earth, was the Greek scholar Eratosthenes Kirensk. He noted that in Siena (now - Aswan) at the summer solstice the sun is at its zenith, illuminating the bottom of a deep well. In Alexandria, as this day it is distant from the zenith to the 1/50 of a circle. From this, scientists concluded that the distance from Alexandria to Siena is 1/50 of the Earth's circumference. The distance between the two cities still 5000 Greek stages (about 787.5 km), therefore, is equal to the circumference of the Earth 250 thousand stages (about 39 375 km).
To name a few modern scholars are moresophisticated measuring instruments, but their theoretical basis consistent with the idea of ​​Eratosthenes. The two points are located a few hundred kilometers from each other, fix the position of the sun or of certain stars in the sky, and calculates the difference between the results of two measurements in degrees. Knowing the distance in kilometers, it is easy to calculate the length of one degree, and then multiply it by 360.
To verify the size of the Earth is used and laser ranging and the satellite surveillance system.
To date, it is believed that the circumference of the Earth at the equator is 40 075.017 km and along the meridian - 40 007.86. Eratosthenes is only slightly wrong.
The size and the circumference and diameter of the Earth is increasing due to the meteoritic matter are constantly falling on the Earth, but the process is very slow.