Moon: Our Nearest Celestial Neighbour
Moon is the most prominent object in the night sky. From time immemorial, moon is familiar to humans. Moon is the nearest celestial body to Earth and lies at a distance of about 384,000 km from here. And, Moon is the only natural satellite of Earth. It travels round Earth once in 27.3 days and takes the same time to spin around its own axis. Thus, one hemisphere of the moon ('the farside') is not visible to us.
In terms of diameter, moon is one fourth the size of Earth and its mass is 1/81 of Earth. Gravity on the surface of the moon is only one sixth of that on Earth.
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| Parameter | Earth | Moon | |
| Diameter | 12,742km | 3,474km | |
| Mass | 5.9x1024kg | 7.475x1022kg | |
| Volume | 1.08 3206x1021m3 | 2.199X1019m3 | |
| Density | 5.52g/cm3 | 3.34g/cm3 | |
| Surface Gravity | 1g | 0.166g | |
Like Earth, the moon too is a world with mountains, plateaus, plains, lowlands, and of course, craters. But, unlike Earth, the moon does not have an atmosphere. Thus, liquid water cannot exist on the moon. But, results from the recent unmanned spacecraft missions have raised the possibility of what appears to be the presence of water ice in the lunar polar regions. Formation and evolution of our moon are of importance in understanding the history of our solar system.