In the sentence "The moon goes round the earth," round is a preposition because it shows the relationship between "the moon" and "the earth" by indicating direction or location. Prepositions are words that connect a noun or pronoun to another word in the sentence, often showing movement or position. 

Here's why it's a preposition:

• Connection: "Round" connects the verb "goes" (referring to the moon) with its object, "the earth," indicating that the moon travels in a circular path around it. 
• Meaning: The word "round" here functions similarly to "around" and expresses the idea of movement in a circular path. 
• Object: A preposition typically takes an object (the noun or pronoun it relates to). In this case, "the earth" is the object of the preposition "round".