Could you please tell me the meaning of the bracketed part?
The huge mothership floated noiselessly through dark space, seemingly motionless, traveling at twenty-two thousand miles an hour [in exact synchronization with the orbit of the earth.]
Thanks a lot.
This is known as "geosychronous orbit" - Wikipedia does a nice, but technical description. http://en.wikipedia.org/wiki/Geosynchronous
In simple language, this would meand than the 'spaceship' would appear to be over exactly the same location although it is travelling incredibly quickly.
The furthar away it is, the faster it has to go to appear to be directly overhead.
Communication satellites are normally "in exact synchronization with the orbit of the earth" to remain over the same area.
To be syncronised is to be in keeping the same time or orbit or motion.
eg. syncronised watches, syncronised swimmers.