This is a timeline of humanity's understanding of the shape and size of the planet Earth from antiquity to modern scientific measurements. The Earth has the general shape of a sphere, but it is oblate due to the revolution of the planet. The Earth is an irregular oblate spheroid because neither the interior nor the surface of the Earth are uniform, so a reference oblate spheroid such as the World Geodetic System is used to horizontally map the Earth. The current reference spheroid is WGS 84. The reference spheroid is then used to create a equigeopotentialgeoid to vertically map the Earth. A geoid represents the general shape of the Earth if the oceans and atmosphere were at rest. The geoid elevation replaces the previous notion of sea level since we know the oceans are never at rest.
Shape
From the apparent disappearance of mountain summits, islands, and boats below the horizon as their distance from the viewer increased, many ancient peoples understood that the Earth had some sort of positive curvature. Observing the ball-like appearance of the Moon, many ancient peoples thought that the Earth must have a similar shape. Around 500 BCE, Greek mathematician Pythagoras of Samos taught that a sphere is the "perfect form" and that the Earth is in the form of a sphere because "that which the gods create must be perfect." Although there were advocates for a flat Earth, dome Earth, cylindrical Earth, etc., most ancient and medieval philosophers argued that the Earth must have a spherical shape.
The pronouncement by Pythagoras (c.570-495 BCE) that the Earth was a sphere prompted his followers to speculate about the size of the Earth sphere. Aristotle (384–322 BCE) writes in De caelo,[7] writes that "those mathematicians who try to calculate the size of the earth's circumference arrive at the figure 400,000 stadia." Archimedes (c.287-212 BCE) felt that the Earth must be smaller at about 300,000 stadia in circumference. These were merely informed guesses. Since the length of a stadion varied from place to place and time to time, it is difficult to say how much these guesses overstated the size of the Earth.
Eratosthenes (c.276-194 BCE) was the first to use empirical observation to calculate the circumference of the Earth. Although Eratosthenes made errors, his errors tended to cancel out to produce a remarkably prescient result. If Eratosthenes used a stadion of between 150.9 and 166.8 meters (495 and 547 feet), his 252,000-stadion circumference was within 5% of the modern accepted Earth volumetric circumference.
Subsequent estimates employed various methods to calculate the Earth's circumference with varying degrees of success. Some historians believe that the ever optimistic Christopher Columbus (1451–1506) may have used the obsolete 180,000-stadion circumference of Ptolemy (c.100-170) to justify his proposed voyage to India. Columbus was very fortunate that the Antilles were in his way to India.
It was not until the development of the theodolite in 1576 and the refracting telescope in 1608 that surveying and astronomical instruments attained sufficient accuracy to make precise measurements of the Earth's size. The acceptance of Newton'soblate spheroid in the 18th century opened the new era of Geodesy. Geodesy has been revolutionized by the development of the first practical atomic clock in 1955, by the launch of the first artificial satellite in 1957, and by the development of the first laser in 1960.
Timeline
Some historical estimates of the size of the Earth
^Ancient units of length such as the cubit, stadion, yojana, Roman mile, Arabic mile, Italian mile, or toise varied considerably by author, location, era, and use. The conversion to modern units used here are only approximations. Other assumptions will yield substantially different results. (Some modern authors will use a conversion that will best illustrate their point.)
^Spherical deviations are calculated for a sphere of the same volume as the World Geodetic System 1984 (WGS 84) oblate spheriod model (1,083,207,320,000 km3).
^In De caelo,[7]Aristotle writes that "those mathematicians who try to calculate the size of the earth’s circumference arrive at the figure 400,000 stadia." Prominent among those mathematicians was his tutor Plato.
^ abcdeThe stadion was a unit of length used in ancient Greece that could range from about 150 to 210 meters (492 to 689 feet). This calculation assumes a stadion of 160 meters (524.9 feet).
^ abcdThe yojana was a unit of length used in ancient India and Southeast Asia that could range from about 3,500 to 15,000 meters (11,483 to 49,213 feet). This calculation assumes a yojana of 8,000 meters (26,247 feet).
^The lǐ is a Chinese unit of distance that varied from about 300 to 576 meters (984 to 1,890 feet). This calculation assumes a Tang dynasty distance of 443.25 meters (1,454.23 feet).
^The Arabic mile was a historical Arabic unit of length that could range from about 1,800 to 2,000 meters (5,906 to 6,562 feet). This calculation assumes a Arabic mile of 1,973.2 meters (6,473.8 feet).
^The cubit used by al-Biruni may have ranged from about 40 to 52 centimeters (15.7 to 20.5 inches). This calculation assumes a cubit of 45 centimeters (17.717 inches).
^The Italian mile is an old Italian unit of distance equal to about 1,624 meters (5,328 feet).
^ abcdeThe toise is an old French unit of length equal to about 1.949 meters (6.394 feet).
^In 1865, Isaac Newton calculated the oblateness of the Earth from the meridional circumference measurement of Jean-Félix Picard to be 3/692 ≅ 0.4335%. Newton later revised his calulation of oblateness of the Earth to 1/230 0.4347%. Had Newton known that the density of the Earth increased with depth, he would have calculated a smaller oblateness.[4]
