Re: ?
It is synonomous because the sun rises in the morning and it sets in the evening. The evening and morning were there first, and then God made the sun fit that schedule. Even if they are not synonomous, they still happen at the same time, so if it pleases you: the Bible defines a day as one morning and one evening.
No, the moon is not always present. I am well aware that the moon can be seen in the daytime, but it is not always there. The only time that it is present throughout the entire daytime is during a new moon, and with the exception of the new moon, the moon is always present at night. Even if seeing it does not make it night, the moon is still a symbol of nighttime, and it is the primary light source at night.
Anyway, what I was meaning with this governing business is the day and night and lightness and darkness were already there, and the sun and moon were set into that cycle after the fact. So at least after the fourth day, you could say that one day was sunrise to sunset, and on that day, the previous days, and the following days the word "day" was used in the exact same context, which is reason enough to believe the time of one day had not changed.