月球

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月球(moon)

地球唯一的天然卫星。运行轨道从西向东，平均距离为384,400公里。大小不及地球的1/3（赤道直径大约为3,476公里），质量约为地球的1/8，密度是地球的2/3。其表面重力大约是地球的1/6，是造成地球潮汐的主要原因。月球反射太阳光，但其返照率仅为7.3%。月球围绕太阳公转一圈大概需要29.5天，和围绕地球运转一圈的时间一样，因此对着地球的一面总是不变。但随着月球的自转，这一面受太阳照射的角度不同，因此会出现一个月从新月到满月的不同状态。大多数天文学家认为，在太阳系早期的历史中，当一个火星大小的星体撞击了原地球，则被喷射到地球轨道的碎片云集形成了月球。1609年伽利略首次发现地球以来，人类一直使用望远镜研究月球表面，在阿波罗计画中月球上的岩石块还被带回地球。其主要影响力是来自微小陨石和陨石的撞击。微小陨石将岩石碎片碾成细微的粉尘，而陨石则於历史早期，大约在40亿年前在月球表面形成分布广阔的陨石坑。月海面积巨大，古代曾流动熔岩。1998年在月球两极发现了有水结冰的迹象。一般来说，月球可以指任何围绕恒星运转的天然卫星。

English version：

moon

Sole natural satellite of earth, which it orbits from west to east at a mean distance of about 239,900 mi (384,400 km). It is less than one-third the size of earth (diameter about 2,160 mi, or 3,476 km, at its equator), about one-eightieth as massive, and about two-thirds as dense. Its surface gravity is about one-sixth that of earth, and its gravitational pull is largely responsible for earth's tides. The moon shines by reflected sunlight, but its albedo is only 7.3%. It rotates relative to the sun in about 29.5 days, in exactly the time it takes to orbit earth, and therefore always presents the same face to earth. However, that face is lit by the sun at different angles as the moon revolves, causing it to display different phases over the month, from new to full. Most astronomers now believe the moon formed from a cloud of fragments ejected into earth orbit when a Mars-sized body struck the proto-earth early in the solar system's history. Its surface has been studied by telescope since Galileo first observed it in 1609, and lunar rocks were brought back to earth in the Apollo program. The dominant process affecting it has been impacts, both from micrometeorite bombardment, which grinds rock fragments into fine dust, and from meteorite strikes, which produced the craters profusely scattered over its surface mostly early in its history, over 4 billion years ago. The maria (see mare) are huge, ancient lava flows. In 1998, possible signs of water ice near the moon's poles were found. More generally, a moon is any natural satellite orbiting a planet.

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