What is the difference between these two suffixes -ology and -onomy?

Answer 1

-ology is from the infinitive Logos which means 'to study'. It is understood that -ology is "the science or study of" a certain subject.

-onomy is from the Greek: Nomos, meaning law. Other sites say the suffix means "knowledge of"

The Concise Oxford defines '-logy' as denoting a subject of study or interest (much the same as above). It defines '-nomy' as denoting a specified area of knowledge or its laws (again, much like the above).

My opinion, which I have no good evidence for, is that '-nomy- denotes the observation of things as they are: Astronomy looks at the laws of the stars (the constellations and their movements). '-logy' is the practice of the subject: Astrology extrapolates "meaning" (if you believe that sort of thing) from the stars position. Gastroenterology or Radiology puts into practice what they know of a subject to give a result.

Answer 2

The suffix -ology refers to the study or science of a particular subject, such as psychology or Biology. The suffix -onomy typically refers to the rules, laws, or principles governing a particular subject, such as economy or astronomy.