Discovery Science: Origins and Geology – Structure
As insignificant as the Earth may be within the totality of the universe, it does hold a special position in our solar system, giving us our daily, yearly, and seasonal patterns Its internal structure plays an important role in both the processes that affect its surface and its magnetic field, the invisible protective shield of the blue planet.
Shape of the Earth
From outer space, the Earth appears to be a uniform sphere. However, this appearance is misleading; the shape of the Earth is extraordinarily irregular. The reason for this is the unequal mass and density distribution of its interior.
Just like the other celestial planets, the Earth is never motionless. It orbits around the sun at a distance of about 93 million miles (149 million km) on a slightly elliptical trajectory. A complete revolution takes about 365 days (one year). Simultaneously the Earth is spinning counterclockwise on its own axis (the line running through its interior between the Poles). One complete rotation takes 23 hours and 56 seconds (a sidereal day), which gives us the alternating light and dark of day and night.
The annual seasons occur principally because the Earth’s axis is tilted by 23.5° from the vertical, relative to its orbital plane. In relation to space, its position remains virtually unchanged during Earth’s orbit around the sun, thus the angle of incoming solar radiation on any particular part of the Earth’s surface changes throughout the year. The speed that the Earth rotates depends on the respective latitude.
While certain points at the Poles will always remain in the same location during Earth’s rotation, a point at the Equator is moving at 1,525 feet per second (465 m/s). These high-speed revolutions cause centrifugal forces to develop, with the effect that the Earth is not strictly a sphere. Instead, it represents a rotational ellipsoid, slightly compressed at both Poles by about 13 miles (21 km) relative to the Equator. Mathematically, the Earth cannot be described as a simple geometric figure. Once the height and weight parameters are considered, a physical model of its shape is represented as a geoid.
The shape and rotation of the Earth influence its gravity, the force responsible for attracting all bodies toward its center, giving them “weight.” This depends on the mass of a particular object. The rule is: force of gravity (weight) equals the mass multiplied by the acceleration of Earth. Due to the compressed shape of Earth and its centrifugal force, gravity is lowest around the Equator at 32.086 ft/s2 (9.780 m/s2) while at the poles it measures 32.257 ft/s2 (9.832 m/s2). Inside the Earth, between the crust and the core, it reaches 34.4 ft/s2 (10.5 m/s2).
THE EARTH IN NUMBERS
The equatorial circumference is 24,901 miles (40,075 km); the equatorial radius is 3.963 miles (6,378 km)
Its mass is 6.585 x 10 :’ tons, the volume is 260 billion cubic miles (1,083.3 billion km3), and the mean density is 344 pounds per cubic foot (5,515 kg/m3).
Its surface measures about 317 million cubic miles (510 million km3); if this 224 million cubic miles (361 million km3) are covered by water.