Earth & Moon
The Earth
Earth is the only planet in the Solar System known to harbor life. Our planet's rapid spin and molten nickel-iron core give rise to an extensive magnetic field, which, along with the atmosphere, shields us from nearly all of the harmful radiation coming from the Sun and other stars. Earth's atmosphere protects us from meteors, most of which burn up before they can strike the surface.
The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km).
The crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents. The inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid. The various layers are separated by discontinuities which are evident in seismic data; the best known of these is the Mohorovicic discontinuity between the crust and upper mantle.
The core is probably composed mostly of iron. Temperatures at the center of the core may be as high as 7500 K, hotter than the surface of the Sun. The lower mantle is probably mostly silicon, magnesium and oxygen with some iron, calcium and aluminum. The upper mantle is mostly olivene and pyroxene (iron/magnesium silicates), calcium and aluminum.
The Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. The presence of free oxygen is quite remarkable from a chemical point of view. Oxygen is a very reactive gas and under "normal" circumstances would quickly combine with other elements. The oxygen in Earth's atmosphere is produced and maintained by biological processes. Without life there would be no free oxygen.
The Moon
The diameter (the distance through the centre) of the Moon is 3,476 km (2,160 miles). That is just under a quarter of the length of the Earth's diameter.
The Earth is nearly 50 times bigger than the Moon, and the Moon is 384,402 km (238,867 miles) from the Earth. If the Earth was reduced to the size of a football, the Moon would be the size of an apple. The distance between them would then be 7 m (23 ft) - about the length of the average sitting room.
Because there is no air on the Moon there is no wind and no weather. The footprints made by astronauts will last forever unless someone disturbs them. There is no sound on the Moon either as there is no air to carry it.
The surface of the Moon has been bombed by meteorites and asteroids for thousands of millions of years. It is pitted with craters of different sizes. Some are tiny, but the largest, the Orientale Basin, must have been made by a huge meteorite. It is 965 km (600 miles) across. If London in the United Kingdom was on one side of the crater, Berlin in Germany would be on the other.
The Moon has huge mountain ranges. The highest mountain peaks are 6,000 m (19,686 ft) above the ground, two thirds the height of Mount Everest. There is no weather to wear down the Moon's mountains, so they will always remain that high.
During the day the Sun warms the Moon - up to 100 degrees celcius. That is twice as hot as the hottest place on Earth and hot enough to boil water. At night the temperature plummets far below zero, down to -150 degrees celcius. That is twice as cold as an Antarctic winter.