Moon - our nearest neighbor


Even without a telescope, it is easy to make out features on the Moon. Because it is close, it looms large, lit by reflected sunlight. Early Moon-observers, who started to use telescopes in the first decade of the 17th century, referred to the large, dark areas which are visible to the naked eye as 'maria', or seas. Galileo described the hundreds of small craters as 'spots'. Later, observations have shown that both 'seas' and 'spots' are the impact craters of meteorites. The great dark regions are smooth-floored, because volcanic activitiy has filled them with lava flows.

Earth, with its atmosphere, water, winds, and active core and mantle, has smoothed away nearly all traces of its early meteorite scarring. The Moon, on the other hand, has no atmosphere, because its weak gravity could not retain one. It is windless and waterless, so it displays the meteorite scars of millions of years sharply defined in its dead surface.