12) decay. The American HeritageŽ Dictionary of the English Language: Fourth Edition. 2000.
...action; rot. b. Rotted matter. 2. Physics Radioactive decay. 3. Aerospace The decrease in orbital altitude of an artificial satellite as a result of conditions such...

13) turn. The American HeritageŽ Dictionary of the English Language: Fourth Edition. 2000.
...an object's own axis or center: Earth rotates on its axis once each day. Revolve involves orbital movement: Earth revolves around the sun. Gyrate suggests revolving...