Earth and the Moon
The third closest planet to the Sun is Earth and is the largest and densest of the inner planets. Earth orbits in a reasonably circular at 150 million km and is the first of the planets to have a moon. Earth is of course the only place that we know of that has life.
Earth takes 365.25 Earth days to orbit the Sun and rotates once every 23 hours, 56 minutes and 4 seconds. Because it rotates around the Sun the length of a day on Earth (sunrise to sunrise) takes 24 hours.
The Earth has an axial tilt of 23.4 degrees and a diameter of 12742km.
The Earth is thought to be 4.54 billion years old and has been accompanied by the Moon for most of that time. It is believed that the Moon was formed when a large Mars sized body impacted the Earth causing enough material to be ejected which eventually coalesced into the Moon. The Moon has had the effect of stabilising Earth’s axial tilt and is the source of the Earth’s ocean tides.
The Moon is 3,474km in diameter (27% that of Earth) and orbits at a distance of between about 362,000 to 405,000 km. It has also been affected by the gravitational pull of the Earth which has over time caused the Moon's rotation to be slowed until it matches the time it takes to orbit the Earth. This is why the same side of the Moon always faces the Earth.
Earth is protected from solar radiation by a strong magnetic field generated by movement of its core which is mainly comprised of molten iron.