Solar System -
Description of Earth
Amazing to think that only a few hundred years ago, everyone thought the Earth was flat, and the center of the Universe! It orbits the Sun at a distance of between about 147,098,000 and 152,098,000 km, averaging out at around 149,598,000 km (93.02 million miles). This change in the distance to the Sun is not what produces the seasons, however. That is the result of Earth's axis of rotation being tilted 23.4° off perpendicular to the plane of its orbit. It actually takes 365.256363004 days to go once around the Sun, and 23 hours 56 minutes and 4.1 seconds to revolve once on its axis; one day. The Earth is very close to being spherical. The diameter at the equator is 12,756.2 km, and at the poles it is 12,713.6 km, averaging out at 12,742 km (about 7,920 miles).
The earliest material found in the Solar System dates back 4.5672 billion years, and it then took the planets about 200 or 300 million years to form. Earth formed some 4.54 billion years ago. Life first appeared on its surface within the first billion years. As the sun continues to burn hydrogen into helium, it gradually gets a little larger and more luminous. As a consequence, it is likely that we can survive on the planet for about a billion years more before it becomes too hot. Apart from the conducive climate and abundance of liquid water, we survive here because the ozone layer and Earth's magnetic field, both work to prevent harmful radiation reaching us from the Sun.
The interior of the Earth remains geologically active. It has a solid inner core composed of 80% iron, nickel and traces of other elements. The diameter of this core is about 1,500 miles. A liquid outer core, with a similar composition, surrounds the inner core to a depth of of about 1,400 miles. It is convection in the outer core, together with with the coriolis effect, that results in the Earth's magnetic field. The solid inner core is too hot to hold a permanent magnetic field. Recent research shows that the inner core rotates relative to the Earth's surface at a rate of once every 700 to 1200 years. The Mantle is the layer above the outer core and is about 1,800 miles thick. Although essentially solid, high temperatures in the mantle cause the silicate material to flow on very long time scales. It is convection currents in the mantle that causes movement of tectonic plates. Finally, the crust is between about 3 and 45 miles thick, with the area under the oceans being the thinnest, ands the continental crust being the thickest. While much of the crust comprises rock that is only around 100 million years old, the oldest rocks are about 4.4 billion years old. This means that there has been a solid crust on the Earth for at least that period.
On the early Earth, it was outgassing and volcanic activity that produced the original, primordial atmosphere. The water that produced the oceans was supplied from the interior, as water vapor that condensed, as well as, perhaps predominantly, from colliding with extra terrestrial objects like comets. While the inner planets, Mercury and Venus, have no natural satellites, Earth has the Moon. This formed about 4.53 billion years ago, probably as the result of a cataclysmic collision with another object. This is different process to the way most of the planets obtained their moons, as these, generally, either condensed from accretion disks around the planet or are captured asteroids. Here is an image showing the Earth & Moon to scale; both sizes and orbit.
Continents have formed, merged, broken apart and reformed many times in the life of the Earth. At present, the surface comprises a number of tectonic plates of varying size, that slowly move over the course of millions of years. Many are in contact and pushing or sliding against each other producing earthquakes and other phenomena. Over 70% of the Earth's surface is covered in water. Interestingly, 80% of the heat generated inside the Earth comes from the radioactive decay of stable isotopes of Thorium, Potassium and Uranium.