“a pale blue dot ....That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives.…There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world." Dr Carl Sagan
Carl Sagan's description of Earth as a pale blue dot refers to a unique picture of the our home planet taken by Voyager 1 in 1990.
Our rocky world has a region known as the biosphere, which includes the atmosphere and vast oceans covering more than two thirds of the planet.
The Earth's crust is up to 70km thick and sits on 2,890km of iron-rich, heated rock called the mantle. Below this, wrapped in a 960km deep liquid layer, is a 2,440km-thick hot core.
Earth's atmosphere is made up primarily of nitrogen 78% and oxygen 21%. It is densest at the surface, in the troposphere, but rapidly thins out so that it becomes difficult to breathe above a few thousand metres. Despite this, some traces of the atmosphere may be detected as high as 600km.
The Earth has kept its atmosphere for two main reasons. The size of Earth is such that its gravity is strong enough to hold on to the gas layers surrounding it. Because the Earth is not too close to the Sun the atmosphere's gas particles are not moving too quickly which makes it easier to hang on to them (the higher the temperature the harder the stronger the gravitational force would be required because temperature is a measure of speed and the higher the temperature means the higher the speed).
Its interesting that the Earth sits between a dead hot planet (Venus) and a dead frozen planet (Mars). Earth is the "goldilocks" planet as its not too big, not too small, not too hot and not too cold so its just right to support life.
Venus being that bit closer to the Sun probably had water in its early days but they would have evaporated to space, meaning there wasn't any rain to stop the build up of greenhouse gases which then prevented the head radiating from the planet - trapping the Sun's heat under a thick blanket of carbon dioxide and making the planet hostile and uninhabitable.
Mars, with a proportionately larger surface area to volume ratio, didn't have the gravity to hold on to its atmosphere and so its heat escaped, radiating into space. The result is a planet with a temperature nearly 3 times colder than Earth's south pole, frozen and unable to support life as we know it.
Also surrounding the Earth is a powerful magnetic field that protects us from the battering of radiation from space. When some violent outbursts from the Sun meet this shield, it can produce colourful displays called aurora, or northern and southern lights.
The Moon is made of rock that was once part of the Earth - this fact was established by examination of samples of rock brought back to Earth by the humans that visited the Moon (during the Apollo space programme). It is thought that a very large asteroid must have crashed into Earth during the early stages of the solar system's formation causing Earth to shed rocky debris into its orbit that eventually clumped together under gravity to create the Moon.
Large dark plains were called maria, or seas, by early astronomers though there are no oceans on this arid and airless world. However recent studies show that large amounts of ice are trapped in shadowed craters at the poles and more water is locked inside moonrock. The Moon’s mountainous surface is heavily pockmarked with craters from the impacts it has sustained over billions of years - lack of an atmosphere means these remain for eternity.
The Moon used to be closer to Earth and is very very slowly moving further away (about 4 centimetres per year).
The Moon is 400 times smaller than the Sun and is 400 times closer to Earth than the Sun, meaning that on Earth we can occasionally see a total solar eclipse. Earth is the only planet in the solar system where a total solar eclipse can occur. In millions of years when the Moon's orbit has edged further away a total solar eclipse won't be possible.