## Eratosthenes

As we can read in the Wikipedia :

Eratosthenes of Cyrene (c. -276 to -195) was a Greek mathematician, geographer, poet, astronomer, and music theorist.

He was the first person to use the word "geography" in Greek and he invented the discipline of geography as we understand it.

He invented a system of latitude and longitude.

He was the first person to calculate the circumference of the earth by using a measuring system using stades, or the length of stadiums during that time period (with remarkable accuracy). He was the first to calculate the tilt of the Earth's axis (also with remarkable accuracy). He may also have accurately calculated the distance from the earth to the sun and invented the leap day.

He also created the first map of the world incorporating parallels and meridians within his cartographic depictions based on the available geographical knowledge of the era. In addition, Eratosthenes was the founder of scientific chronology; he endeavoured to fix the dates of the chief literary and political events from the conquest of Troy.

__Measuring the Earth circumference using Eratosthenes method__Now we are going to measure the size of the Earth following Eratosthenes method, although with a more comfortable way (without having to command anybody to travel a long distance !!).

If you want to do this experience please continue reading this article.

In the following pages you can follow (and even repeat !!) the process carried out by Eratosthenes.

To do this we can assume the following postulates :

**Equinox**: In this moment of the year the length of the day and of the night is the same; so, is a special date that we can use to carry out our experience

Wikipedia : |

- As the distance that separates Earth and Sun its very long, we can consider that incident
**solar radiation**on earth surface is parallel in any part of our planet - These parallel solar rays focusing at the same time in different latitudes and in the same longitude, allow us to calculate the value of the
**angle of the circle**that separates the two towns located, then, on the same meridian.

Wikipedia : |

- We can know what is the
**distance between the observation sites**. This data, together with the angular value previously obtained, will help us to know what is the radius of the Earth and, therefore, what is its length, quite accurately. **Another way to calculate the radius of the Earth**would be taking into account the ratios between the angle measured before and the arch circumference (distance between the observation sites) on the one hand, and the entire circumference (considering as perfectly spherical our planet ) on the other.- Practical examples
- Finally, we can carry out this experience without having to wait for the dates of the equinoxes