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3(i). Size of the Earth
Earth's widest point is the equator, which runs across the planet's center at zero degrees
latitude. This is because Earth is not a perfect sphere; it has a prominence across its center
called the equatorial bulge. According to NASA, Earth's radius at the equator is 3,963 miles
(6,378 kilometers), while the radius at the poles is 3,950 miles (6,356 km).
Therefore, the diameter of Earth through its center is 7,926 miles (12,756 km). Again, because
of the equatorial bulge, the planet's diameter is slightly smaller when measured from pole to
pole, where it is about 7,900 miles (12,712 km).
The equatorial circumference of Earth is 24,901 miles (40,075 km). However, from the North
Pole to the South Pole, the so-called meridional circumference is smaller, at 24,860 miles
(40,008 km). That reveals that our planet is flattened at the poles, meaning its shape is an
oblate spheroid.
Calculating the Size of Earth
The study of the size and shape of the earth is known as geodesy and has been practiced for
centuries. Contrary to common misconceptions, many historical mathematicians and scientists
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were aware that Earth was spherical. Technically Earth's shape is called the "geoid", an
ellipsoidal shape. Possibly starting with Eratosthenes, over 2,200 years ago, mathematicians
have attempted to calculate the size of a spherical or ellipsoidal Earth. The techniques of
Eratosthenes and his predecessors are considered elementary, or at least straightforward, for
today's mathematician, but are impressive for their scope in calculating the entire Earth
without being able to see or measure most of it and for their accuracy.
Eratosthenes' Calculation of Earth's Size: He observed that at noon on the summer
solstice of the northern hemisphere, at two different places, the sun’s rays fell upon the earth
at different angles. In the city of ancient Syene (modern Aswan, Egypt), at noon on the solstice,
the sun was directly overhead and cast no shadows on the objects below it (Aswan is very near
the Tropic of Cancer. This was proven by observing that the sun completely illuminated the
bottom of a very deep well in Syene at noon. The angle between the sun's rays and objects
perpendicular to the earth's surface was effectively zero.
In Alexandria, on that same day at noon, the sun cast slight shadows behind objects. By
measuring the dimensions of the shadows and the objects that cast them, Eratosthenes was
able to calculate that the sun’s rays arrived in Alexandria at an angle of 1/50 of a circle (about
0.13 radians or 7.2 degrees). He knew the distance between Alexandria and Syene to be 5000
“stades” from land surveys done between the two cities. A stade was an ancient Greek unit of
measurement (150-200 meters). He also assumed that Syene was due south of Alexandria and
on the same meridian of longitude. This would mean that the distance between the two cities
represented an arc of the earth's circumference (in reality, Aswan is about three degrees of
longitude east of Alexandria).
Comparison of the size of the earth with other planets
1. Earth is the fifth-largest planet in the solar system, behind Jupiter, Saturn, Uranus and
Neptune.
2. With a diameter of 86,881 miles (139,822 km), Jupiter — the largest planet in the solar
system — is 11 times wider than Earth. The gas giant has a volume of 343 trillion cubic
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miles (1,431 trillion cubic kilometers), meaning it would take around 1,320 Earths to fill
the volume of Jupiter. According to NASA, if Earth were the size of a grape, Jupiter
would be the size of a basketball.
3. The size difference between Earth and Jupiter is so pronounced that the Great Red Spot,
a storm that has been raging in the gas giant's upper atmosphere for 200 years, is wider
than our entire planet.
4. With a diameter of 30,599 miles (49,244 km), Neptune — the fourth largest planet in
the solar system —is around four times as wide as Earth. It would take 57 Earths to fill
the volume of Neptune. If Earth were the size of a nickel, Neptune would be the size of a
baseball, according to NASA.
5. Although Earth is dwarfed by the outer planets, it is still the largest of the inner, rocky
planets. Venus is the second largest rocky planet, followed by Mars and Mercury.
Horizontal and vertical dimensions of the earth
The Earth's dimensions can be described in terms of its radius, diameter, and circumference.
1. Radius: The average radius of the Earth is approximately 6,371 kilometers (3,959
miles).
2. Diameter: The diameter of the Earth is twice its radius. Therefore, it's roughly 12,742
kilometers (7,917 miles).
3. Circumference: The Earth's circumference, which is the distance around its equator, is
approximately 40,075 kilometers (24,901 miles).
As for horizontal and vertical dimensions:
Horizontal Dimension: This typically refers to measurements taken along the surface of the
Earth, such as distances between two points on the equator or any other parallel.
Vertical Dimension: This generally refers to measurements from the surface of the Earth
towards the center or from one altitude to another, like elevations or depths.
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Earth's circumference
Earth's circumference is the distance around Earth. Measured around the equator, it is
40,075.017 km (24,901.461 mi). Measured passing through the poles, the circumference is
40,007.863 km (24,859.734 mi).
Measurement of Earth's circumference has been important to navigation since ancient times.
The first known scientific measurement and calculation was done by Eratosthenes, by
comparing altitudes of the mid-day sun at two places a known north–south distance apart. He
achieved a great degree of precision in his computation. Treating the Earth as a sphere, its
circumference would be its single most important measurement. Earth deviates from spherical
by about 0.3%, as characterized by flattening.
In modern times, Earth's circumference has been used to define fundamental units of
measurement of length: the nautical mile in the seventeenth century and the meter in the
eighteenth. Earth's polar circumference is very near to 21,600 nautical miles because the
nautical mile was intended to express one minute of latitude (see meridian arc), which is 21,600
partitions of the polar circumference (that is 60 minutes × 360 degrees). The polar
circumference is also close to 40,000 kilometres because the meter was originally defined to be
one ten millionth (i.e., a kilometer is one ten thousandth) of the arc from pole to equator
(quarter meridian). The accuracy of measuring the circumference has improved since then, but
the physical length of each unit of measure had remained close to what it was determined to
be at the time, so the Earth's circumference is no longer a round number in metres or nautical
miles.
