The circle of the earth can be estimated on the longest parallel - the equator. However the last results of measurements of this parameter show that the standard idea of it not always is true.
The question of what planet circle size Earth is equal to, interested scientists long ago. So, the first measurements of this parameter were carried out in Ancient Greece.
Measurement of a circle
The scientists researching in the field of geology knew that our planet has the sphere form for a long time. For this reason the first measurements of size of a circle of the earth's surface concerned the longest parallel of Earth - the equator. This size, scientists believed, it is possible to consider correct for any other way of measurement. For example, was considered that if to measure a planet circle on the longest meridian, the received figure will be just the same.
Such opinion existed up to the 18th century. However scientists of the lead scientific institution of that time - the French academy - held the opinion that this hypothesis is incorrect, and a form which the planet has, it is not absolutely correct. Therefore, according to them, circle lengths on the longest meridian and on the longest parallel will differ.
As proof in 1735 and 1736 two scientific expeditions which proved the validity of this assumption were undertaken. Subsequently also the size of difference between these two lengths was established - it was 21.4 kilometers.
Circle length
Now planet circle length Earth is repeatedly measured not by means of extrapolation of length of any given piece of the earth's surface at its full size any more as it became earlier, and with use of modern high-precision technologies. Thanks to it it was succeeded to establish the exact length of a circle on the longest meridian and the longest parallel and also to specify the size of difference between these parameters. So, today in scientific community as the official size of a circle of the planet Earth on the equator, that is the longest parallel, it is accepted to give the figure making 40075.70 kilometers. At the same time the similar parameter measured on the longest meridian that is length of the circle passing through terrestrial poles is 40008.55 kilometers. Thus, the difference between lengths of circles makes 67.15 kilometers, and the equator is the longest circle of our planet. Besides, such distinction means that one degree of a geographical meridian is slightly shorter, than one degree of a geographical parallel.