A metaphor is a figure of speech where one thing is said to be another, implying some similarity. (This contrasts with a simile where the comparisons is made explicit.)
An extended metaphor goes beyond a single reference to a comparison, but uses several related references. An extended metaphor may function as an implicit analogy, but where the mapping between target ideas and the analogous comparison is not made explicit but left for the reader/listener to appreciate.
In this example, astronomy is compared to an animal with several modes of locomotion – reflecting how the science proceeds at different rates and sometimes with false steps [sic].
