Earth

Earth is the third planet from the Sun and it is the fifth largest planet in the solar system. About 70% of the Earth's surface is covered by water. The Earth's atmosphere acts as a temperature modulator, shielding the planet from much of the Sun's harmful radiation. The Earth's tilt on its axis causes seasons.Earth - Planets & Astronautic Development in China
==Rotation==
Earth's rotation is the rotation of Planet Earth around its own axis. Earth rotates eastward, in prograde motion. As viewed from the north pole star Polaris, Earth turns anticlockwise.
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where Earth's axis of rotation meets its surface. This point is distinct from Earth's North Magnetic Pole. The South Pole is the other point where Earth's axis of rotation intersects its surface, in Antarctica.
Earth rotates once in about 24 hours with respect to the Sun, but once every 23 hours, 56 minutes, and 4 seconds with respect to other, distant, stars (see below). Earth's rotation is slowing slightly with time; thus, a day was shorter in the past. This is due to the tidal effects the Moon has on Earth's rotation. Atomic clocks show that a modern day is longer by about 1.7 milliseconds than a century ago, slowly increasing the rate at which UTC is adjusted by leap seconds. Analysis of historical astronomical records shows a slowing trend of about 2.3 milliseconds per century since the 8th century BCE.
==Changing Surfaces==
One thing that causes slow changes to the Earth's surface and two things that cause fast, abrupt changes on Earth's surface.
===Erosion Change the Surface===
Erosion is the major cause of changes that are done to the Earth's surface. Erosion changes the surface by creating rivers, streams and valleys. The act of erosion is also known for the wearing down of mountains.
==Atmosphere==
The present atmosphere of the Earth is probably not its original atmosphere. Our current atmosphere is what chemists would call an oxidizing atmosphere, while the original atmosphere was what chemists would call a reducing atmosphere. In particular, it probably did not contain oxygen.
==Water==
Water is found in many places on Earth including on the surface of the planet, within rocks below the surface, and in the atmosphere. Water travels between different areas of the Earth through the water cycle. About 70% of the Earth surface is covered with water, and most of that is the ocean. Only a small portion of the Earth's water is freshwater, which is found in rivers, lakes, and groundwater. Freshwater is needed for drinking, farming, and washing. In addition to liquid water, water is also present on Earth in the form of ice. Without water, life as we know it would not exist.
==Climate and Global Change==
Warm near the equator and cold at the poles, our planet is able to support a variety of living things because of its diverse regional climates. The average of all these regions makes up Earth's global climate. Climate has cooled and warmed throughout Earth history for various reasons. Rapid warming like we see today is unusual in the history of our planet. The scientific consensus is that climate is warming as a result of the addition of heat-trapping greenhouse gases which are increasing dramatically in the atmosphere as a result of human activities.
==Weather==
Weather is the state of the atmosphere at a given time and place. Most weather takes place in the troposphere, the lowest layer of the atmosphere.
Weather is described in a variety of ways by meteorologists, scientists who study and predict weather. Air temperature and pressure, the amount and type of precipitation, the strength and direction of wind, and the types of clouds are all described in a weather report.
Weather changes each day because the air in our atmosphere is always moving, redistributing energy from the Sun. In most places in the world, the type of weather events expected vary through the year as seasons change. While weather can change rapidly, climate changes slowly, over decades or more, in response to changes in the factors that determine our climate.
==Seasons==
Over much of the world, the temperature and weather vary with the seasons. Temperatures are lowest in the winter, start to rise in the spring, are highest in summer, start to drop in the autumn, and reach their lowest point again in the next winter.
These seasonal changes are caused by a tilt in the Earth's axis. The Earth does not spin upright, or on a perpendicular axis, as it travels round the Sun. Its axis is tilted 23 1/2° from the perpendicular.
The axis always points in the same direction in space as the Earth circles the Sun. This means that during the year, the Earth's axis is tilted alternately towards, then away from the Sun. So a place on the Earth's surface is slanted more towards the Sun at some times than at others.
In the Northern Hemisphere, a place slants the closest towards the Sun on about June 21 each year. On this date the Sun climbs highest in the sky at noon, its rays are strongest, and the hours of daylight are longest. The temperature is high; it is the summer solstice.
On the other hand, a place slants the furthest away from the Sun on about December 21. On this date the Sun climbs to its lowest noontime position, its rays are weakest, and the daylight hours are shortest. The temperature is low; it is the winter solstice.
==Structure==
The internal structure of the Earth is layered in spherical shells: an outer silicate solid crust, a highly viscous asthenosphere and mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. Scientific understanding of the internal structure of the Earth is based on observations of topography and bathymetry, observations of rock in outcrop, samples brought to the surface from greater depths by volcanoes or volcanic activity, analysis of the seismic waves that pass through the Earth, measurements of the gravitational and magnetic fields of the Earth, and experiments with crystalline solids at pressures and temperatures characteristic of the Earth's deep interior.
==Plate Tectonics==
Movements deep within the Earth, which carry heat from the hot interior to the cooler surface, cause the plates to move very slowly on the surface, about 2 inches per year. There are several different hypotheses to explain exactly how these motions allow plates to move.
Interesting things happen at the edges of plates. Subduction zones form when plates crash into each other, spreading ridges form when plates pull away from each other, and large faults form when plates slide past each other.
==Life==
Life on Earth occurs in a bewildering array of forms—plants, animals, fungi, protists, and bacteria, which together compose Earth's biosphere. The biosphere is connected to the Earth System through biogeochemical cycles.
===Origins of life on Earth===
The Earth was formed about 4,500 million years ago. For the first 500 million years a rain of rocky, metallic, and icy bodies pounded the surface. Gas leaking from its hot interior provided an atmosphere and oceans formed as the surface cooled down. Complicated molecules (groups of atoms) soon began to form in seas and pools and the first primitive life forms (single cells) appeared 3,500 million years ago. Advanced living creatures evolved first in the oceans and then moved onto the land.Solar System: Earth (page 2) - Blast Off Into Space (SpaceQuest)
===Early Life on the Earth===
The first life on the Earth was primitive plants that took carbon dioxide from the air and released oxygen during photosynthesis. Animals evolved when there was enough oxygen in the atmosphere to sustain them. Knowledge about evolving life forms comes in the form of fossils in the rocks. However, life forms survive only if environmental, conditions on the Earth are suitable for them. The dinosaurs, for example, though perfectly adapted to their age became extinct about 65 million years ago.
==Moon==
The Moon (Luna) is the only natural satellite of the Earth, and the fifth largest satellite in the Solar System. About one-sixth of the Earth's gravity.Earth: Moons and Rings - Windows to the Universe
While there are only two basic types of regions on the Moon's surface, there are many interesting surface features such as craters, mountain ranges, rilles, and lava plains. The structure of the Moon's interior is more difficult to study. The Moon's top layer is a rocky solid, perhaps 800 km thick. Beneath this layer is a partially molten zone. Although it is not known for certain, many lunar geologists believe the Moon may have a small iron core, even though the Moon has no magnetic field. By studying the Moon's surface and interior, geologists can learn about the Moon's geological history and its formation.The Moon
==References==
 
Category:Terrestrial planets
Category:Habitable zone planets