请认真阅读下面短文,并根据所读内容在文章后的空格内填入最恰当的单词. 每空格1个单词，不得用文章中的单词。
A scientist once said that studying the stars from the earth is something like looking up at the sky from the bottom of a swimming pool. Even on the clearest night the earth’s dense atmosphere will dim stars in the sky as it screens out (遮挡) 30 percent of their light. Air turbulence (湍流) causes stars to winkle. Through a telescope, the stars seem to jump around dizzily.
Because the moon has no atmosphere, astronomers are looking forward to setting up an observatory there. On the moon, there will be no clouds to obscure the view. The sky will appear dark at all times, so astronomers can work twenty-four hours a day. Because there is not air turbulence, the stars will steady through a telescope.
Because the moon is relatively near the earth, moon-based astronomers will see the stars in familiar patterns known on earth. They will not need new star maps of the sky. Stars seen from the moon will appear to move very slowly. Because the earth rotates (自转) on its axis once every twenty-four hours, earth-based astronomers see the stars hurry across the sky each night. The moon rotates on its axis only once a month. To astronomers on the moon, the stars will appear to stand almost still. Moon-based astronomers will have much more time to focus their telescopes and take time ---exposure photographs.
On the earth, man-made radio noises jam the airwaves and interfere with the reception of radio signals from certain stars. A radio telescope located on the far side of the moon will be permanently protected from such interference.
It won’t be easy to put telescopes on the moon. However, astronomers are certain the job can be done. They are convinced that the moon is an ideal place to set up an observatory.
Title : The Reasons for establishing an observatory on the moon

Main Comparison	Context
Different	There is very   atmosphere on the moon.
On the earth, air turbulence causes stars to twinkle like   .
Different	On the moon, the sky will seem to be in    .
Because of clouds, there is a    view from the earth.
Different	Earth-based astronomers see stars move very
Stars are seen on the moon to move at a very low
In contrast	The moon is     to be an ideal place to set up an observatory.