our Earth
Earth is the third planet from the Sun. It is the first planet from the Sun to have a moon, a common feature of all of the other planets beyond our orbit. The Earths atmosphere reaches to approximately 80km above the surface, but this is only about one hundredth of the planets radius, and so occupies a relatively thin band around the planet. It is composed mainly of nitrogen (78%) and oxygen (21%), with the remainder built up from small traces of other gases. These relative proportions remain fairly constant up to around 80km, but the heavy gases become rare at greater heights and only hydrogen and helium are found in the outermost regions. At the height of the stratosphere, there is a concentrated layer of ozone, at about 25km. This ozone layer is vital to life on the planet, as it stops nearly all of the Suns harmful ultraviolet radiation from reaching the surface.
Atmosphere & Surface The Earth's atmosphere also contains a large amount of water vapour, although its concentration varies in different places, and is subject to seasonal variations. Most of it is visible as clouds, and most of these extend only to the top of the troposphere - up to 13km. It is in this region that the dominant weather systems are found. Above this lies a layer of warm air which holds cooler air below containing most of the water vapour, and the clouds that we see.
The surface of the planet is subject to much geological activity, and is shaped by continental drifting (plate tectonics). This makes the planet's surface relatively young, and taken in conjunction with other factors, such as the existence of an atmosphere and weathering, explains why few visible impact craters are to be found. The surface of the moon, which is heavily cratered, gives a more accurate a more accurate guide as to the quantity of collisions with debris making its way through the solar system. Here, the surface features date back to the formation of the solar system, and have remained undisturbed due to the moon's geological inactivity.
the Moon
The Moon is the Earth's sole companion, orbiting at an average distance of 384,400km. It follows us on our path around the Sun, making one orbit of us (as seen against the background of stars) every 27 days, 7 hours, 43 minutes. This period is referred to as the sidereal month. The Moon's equatorial diameter measures 3,476km making it less than one third the diameter of the Earth. The tides in the Earth-Moon system have slowed the Moon's rotation and locked it to Earth, this is tidal coupling and occurs throughout the Solar System.
Fleeting Eclipses As the orbital plane of the Moon lies within 5 deg of the apparent orbit of the Sun as seen from Earth, occasionally their positions in the sky coincide, giving rise to a solar eclipse.
At other times the Moon's orbit takes it into the shadow cast by the Earth into space, causing a lunar eclipse. Unlike solar eclipses, Lunar eclipses are readily observed from large areas of the Earth's surface, due to the greater extent of the Earth's shadow than that of the Moon's.
Changing Shape As we see the Moon only in reflected light, it shows distinct phases, depending on the angle made between itself, the Sun and the Earth, as the diagram shows.
At New Moon (1), all three bodies are aligned, with the Moon between the Earth and the Sun. During this phase, despite the absence of direct illumination by the Sun, the lunar surface is just visible in light reflected from the Earth. Full Moon (5) occurs when the Earth lies directly between the Sun and the Moon, with first (3) and last (7) quarters occurring at the 90 deg and 270 deg positions. The time taken from New Moon to New Moon, called the synodic month, is 29 days, 12 hours, 44 minutes.
