Why the Moon and not the Sun
The Sun moves roughly one degree a day and stays in a sign for about a month, so it describes the broad season of a life. The Moon crosses a sign in about two and a quarter days, which makes it the natural marker for anything that changes daily.
In Jyotish the Moon also governs the mind and the emotions — manas. A daily reading is a description of the emotional weather, so the Moon is the sensible reference point.