This week, let's continue discussing how we keep track of time. To summarize last week's discussion, the year is based upon the annual motion of the Earth around the sun, and the month is based on the moon's orbit around the Earth. The week is actually based upon the seven objects you can see in the sky that are moving differently that the stars. Specifically, the moon, the Sun, Mercury, Venus, Mars, Jupiter, and Saturn. A few days of the week are obvious: Monday for the Moon, Sunday for the Sun, and Saturday for Saturn.