The phase of the moon we see depends on from what direction the sunlight’s hitting it, and angle we see that from earth. The moon orbits the earth roughly once per month. In fact, that’s where the word “month” comes from; “month” and “moon” are cognates, words that have similar etymological histories, and in most languages, including English, the two words are very similar. The length of time we call the month derived from the length of time it takes the moon to go through all its phases 29.5 days.