_ Source: _Volume 61, Issue 1, pp 33 - 42 _De caelo_ 2.6 describes irregular motion differently from the discussion at _Physics_ 5.4. The desire to make the one discussion congrue with the other has strained interpretation of the _De caelo_ passage. Aristotle provides a theory of irregular motion that is tripartite and the passage ought to be interpreted in such a way as to explain this tripartite motion. _Akmē_ is not a ‘top speed’ as it is generally translated, but (...) a point in an object’s motion when _epitasis_ must become _anesis_. Although the terms _epitasis_ and _anesis_ cover ‘acceleration’ and ‘deceleration’ they cannot be reduced to them. (shrink)