EARTH

Home to more than 6 billion human beings and countless other species, our Home Planet, Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. Of the four terrestrial planets in the Solar System, Earth is the largest. It is believed that it is Earth’s unique location in the Solar System that has blessed it with life, color and abundance.  As far as we know, earth is the only place where life is known to exist. The planet formed 4.54 billion years ago, and the first living organisms appeared on its surface within a billion years. Earth's biosphere has considerably altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful solar radiation, permitting life on land. The physical features of the Earth, as well as its geological history and orbit, have allowed life to persist during this period. Our planet is believed to continue supporting life for at least another 500 million years.

Earth interacts with other objects in space, the Sun and the Moon in particular. At present, Earth orbits the Sun once for roughly every 366.26 times it rotates about its axis, which is equal to 365.26 solar days, or one sidereal year. The Earth's axis of rotation is tilted 23.4° away from the normal to its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days). Earth's only known natural satellite, the Moon, has been orbiting it ever since it formed about 4.53 billion years ago, and it provides ocean tides, stabilizes the axial tilt and gradually slows the planet's rotation.

Our planet is bound by a thin envelope of gas, its atmosphere. This atmosphere contains large amounts of molecular oxygen and therefore ozone, which protects us from the harmful solar UV radiation, and greenhouse gases like water vapor or carbon dioxide, maintaining mild climatic conditions, is unique in the solar system. It plays a critical role in preserving at the surface of the Earth an environment suitable for the preservation and development of life, and must be considered as a key component of the Earth system.

Change is perhaps the only constant in our planet’s history. Since the Earth’s beginning about 4.5 billion years ago, natural climate and the environment of our planet have been in constant flux. Solar variability, volcanic eruptions, meteor impacts, the emergence of life, the formation of an atmosphere with a robust supply of oxygen and greenhouse gases, changing ocean circulation patterns,wildfires—over the millennia these and other geological forces shaped an intricately intertwined global climate system that we are only just beginning to understand.

Questions to ponder:

• Why does the Earth have only a single Moon?
• What would happen if Earth had more than 1 moon?