Alteration (alteratio) is a philosophical term for a type of change/motion (motus or mutatio).
St. Thomas Aquinas defines alteratio in Physica V l. 4 [679.]:
motion/change in quality is called aleration.motus qui est in qualitate, vocatur alteratio
Thus, a green apple turning to a red apple would be an alteration in the quality of the apple.
What does St. Thomas Aquinas mean by 'alteration' of heavenly bodies in Summa Theologiae?
What St. Thomas is saying here is that the heavenly bodies are undergoing changes as to the fact that the entire universe is alive, growing and subject to change, just as the earth and it’s inhabitants are.
For example a sun is just a sun. But after a certain amount ...