The modern term Earth was derived from a number of different Middle English terms, which themselves were derived from the Old English word, eorðe. It is the only planet in our solar system that is not named after a Roman god/goddess. Earth is the fifth largest planet in our solar system and is the third farthest planet from the sun. It has a diameter of 12,756.3 km, a mass of 5.972e24 kg, and an orbit of 149,600.000 km from the sun. According to radiometric dating, the Earth is thought to be approximately 4.54 billion years old. It takes the Earth approximately 365.256 days to travel around the sun (this is one year) and it takes 23.9345 hours for the Earth to complete a full revolution (this is one day). One interesting fact is that the Earth’s rotation is gradually slowing, at about 17 milliseconds per one hundred years. At this rate, it would take about 140 million years to have an increase of 25 hours.
The shape of Earth is considered to be an oblate spheroid, meaning that it is flattened along the two poles and also has a bulge around the equator, which is caused by the Earth’s rotation. Its mass is mainly made up of iron, oxygen, silicon, magnesium, sulfur, nickel, calcium and aluminum (in order of decreasing percentage). The Earth’s core is made up of 88.8% iron, while the Earth’s crust is comprised of 47% oxygen and the rest in oxides. The Earth is divided into seven geologic layers: inner core, outer core, asthenosphere, mantle, upper mantle, crust, and lithosphere (which is made up of tectonic plates). The core and the crust are solid, while the outer core and mantle layers are semi-fluid or plastic.
The tectonic plates mentioned above are rigid segments of land that move past one another along three different boundaries: divergent (plates being pulled apart), convergent (plates coming together), and transform (plates sliding past one another). There are seven major plates: North American, Pacific, Eurasian, Antarctic African, South American, and Indo-Australian. The movement of these plates is what creates and destroys the Earth’s surface; hence, most of surface is relatively young and all traces of “old” surfaces are gone.
The Earth’s surface is covered by approximately 70.8% water, with most of it hidden below sea level (often referred to as the continental shelf). This underwater system is comprised of such features as volcanoes, submarine canyons, and mountains. Mountains, plateaus, deserts, plains, and other landforms cover the remaining 29.2% of the Earth’s surface. The Earth’s atmosphere is made of about 78% nitrogen and 21% oxygen, with minute amounts of carbon dioxide, water vapor, and other gaseous molecules. The ozone layer is an important part of the atmosphere because it blocks ultraviolet radiation, which is why life can exists on Earth. A big threat to the ozone layer is the greenhouse effect, which is where molecules capture within the atmosphere capture heat (or thermal energy), which is emitted from the ground; thereby, raising the average temperature on Earth.
The Earth also has a modest magnetic field, which is produced by the nickel-iron core of the planet and the Earth’s rapid rotation. The magnetic filed protects the Earth from solar winds, but also interacts with them to create auroras (which are glows in the Earth’s ionosphere, which can often be seen in the sky) and Van Allen radiation belts, which is a pair of rings of ionized gas (or plasma) that is trapped in orbit around the sun.