天王星(Uranus)

太阳系的九大行星之一，顺序排列是第七颗。赫瑟尔家族於1781年发现，以希腊象徵天空的神只乌拉诺斯命名。蓝绿色的气体巨星，质量约为地球的十五倍，体积超过地球的五十倍。密度小於地球；大气顶部的重力比地球表面小11%。赤道直径51,100公里。天王星有10条分隔清楚、细窄且暗淡的环，中间是宽阔的尘埃带，主要是由卵石大小的深色物质构成。卫星数目至少有十五个（英文名多以莎士比亚作品的角色命名），磁场强度大约和地球磁场相当。这个行星估计每17个小时左右自转一圈，绕着不寻常的水平轴转动，就像是躺着以侧边转动。绕行太阳一周要84年，平均距离28亿7000万公里。天文学家认为天文星内部是由冰和气体构成，可能具有小型的岩质核心。大气厚度达数千公里，外观的蓝色是因为大气的甲烷吸收了虹光所致。大气的上层主要是由氢和氦组成。

English version：

Uranus

Seventh planet from the sun. It was discovered in 1781 by Herschel family and named for the Greek god personifying heaven. A blue-green gas giant, it has almost 15 times the mass of earth and over 50 times its volume. It is less dense than earth; the gravity at the top of its atmosphere is 11% weaker. Its equatorial diameter is 31,800 mi (51,100 km). Uranus has 10 sharply defined, narrow, dark rings, with broad dust bands between them, consisting mainly of boulder-sized chunks of dark material. It also has at least 15 moons (most named after Shakespearean characters) and a magnetic field about as strong as earth's. The planet is estimated to rotate about every 17 hours, around an unusual horizontal axis, so it appears to spin on its side. It takes 84 years to orbit the sun, at a mean distance of 1.78 billion mi (2.87 billion km). Its interior is thought to consist of ice and gas, perhaps with a small rocky core. Its atmosphere appears to be thousands of miles deep, and its blue color comes from absorption of red light by methane in the atmosphere. The upper reaches of the atmosphere consist mainly of hydrogen and helium.