In everyday life, when it comes to the shape of the Earth, we most often hear that it is spherical. However, if we talk in more detail and more precisely about the shape of the Earth, we can make some very, very interesting observations.
So, here are three interesting facts about the shape of the Earth:
Frankly, if you don’t pick on it too much, by and large, in a certain approximation, the statement about the spherical shape of the Earth is actually true, and quite sufficient for simple reasoning and problem solving, where the accuracy of calculations does not exceed 0.5%. However:
First, we certainly understand that the surface of the Earth is not perfectly smooth, and therefore cannot be exactly spherical. There are many mountains and depressions on it, violating the strict definition for such a figure as a sphere, where every point on the surface must be equidistant from the center.
Secondly, even if we neglect the irregularities on the surface of the Earth, nevertheless, it is also well known that the Earth is slightly flattened from pole to pole (we are talking about the geographical poles). It is connected with daily rotation of the Earth around its axis. Thus, the cross-section of the Earth, more precisely speaking, is not a circle, but rather an ellipse (or an oval). And the three-dimensional figure, which is a flattened ball, is called an ellipsoid of rotation. Thus, the Earth has the shape of an ellipsoid, which is nevertheless so faintly expressed that it is visually impossible to distinguish it from a sphere.
Sometimes it is also said that the Earth has the shape of a geoid. By definition, a geoid is a surface, everywhere normal to the force of gravity. And although, even the name of this figure came from the Greek word combination, literally meaning “something like the Earth” (i.e. here it is not the Earth that has the shape of a geoid, but a geoid that has the shape of the Earth), nevertheless, if we speak very strictly and meticulously, the Earth does not even have the shape of a geoid. The fact is that due to the influence of other large celestial bodies, primarily the Moon, which constantly act on the surface of the Earth causing the tides, the shape of the Earth in general constantly very slightly, but still varies depending on the mutual location in relation to the moon and other planets.
Sometimes it is possible to encounter the opinion that even in the 15th century people thought that the Earth was flat, and only the discovery of America and the first circumnavigation of the globe, which took place at the turn of the 15th and 16th centuries, proved the opposite. However, this was not the case. These voyages were undertaken to achieve quite different goals. And the fact that the Earth is not flat, but has the shape of a sphere, was not a secret in the 6th century BC.
Ancient Greek philosophers and thinkers, observing how ships go over the horizon, how lunar eclipses occur (when the round shadow of the Earth falls on the Moon), how celestial bodies move across the sky and how the picture of the starry sky changes if the observer moves along the Earth to the north or south, have long understood that the Earth is spherical.
Apparently, the ancient Greek scientist Parmenides was the first to express the idea of a spherical Earth. Pythagoras and his teacher Anaximander of Miletus (who also claims to be the discoverer of this question) were also in no doubt. All these philosophers lived, as already mentioned, in the distant 6th century B.C. And since then, no one seriously and did not try to disprove the fact of the spherical shape of the Earth, there was not even any discussion about it, just all civilized and educated people knew that the Earth is shaped like a sphere.
Moreover, in the third century B.C. Eratosthenes, knowing the distance from Alexandria to Siena and using the length of the shadow of the Library of Alexandria (whose height was also known to him) during the position of the Sun over Siena at zenith, was able to measure the length of the Earth’s meridian and calculate the radius of the Earth. Apparently, he then counted in modern units of measurement 6287 kilometers. Modern research shows that the radius of the Earth is 6371 kilometers on average. Thus, the accuracy of Eratosthenes’ calculations is amazing. Well, the first who has shown that the Earth actually has the form of an ellipsoid was Isaac Newton.
And finally, another interesting observation about the shape of the Earth. The fact is that there is no objective reason to expect that the shape of the Earth is exactly as we, its inhabitants, see and feel it. For example, particles of solar radiation, being quite material objects, and moving sometimes at a speed very close to the speed of light, can also be quite rightly considered as objective observers. So in the frame of reference connected with these particles, the Earth, according to the theory of relativity, will be compressed in the direction of motion of these particles.