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. The other days are not so obvious, because they are named after the Norse God equivalent for the Roman name of the planet. For example, Mars is the God of War and in Norse mythology, the God of War is Thor; thus Thursday, think "Thorsday" is for Mars.
The day is based upon one rotation of the Earth and the time of day is based upon the location of the Sun relative to the Meridian. Now the Meridian is an imaginary line that extends from due North up through the top of the sky and down to due South. It effectively splits the sky into an Eastern half and a Western half. When the Sun is in the Eastern sky, it is a certain number of hours "anti-meridian," or A.M. which means before the meridian, and when the Sun is in the Western sky, it is "post-meridian" or P.M.
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Follow your curiosity to the Fred G. Dale Planetarium at Wayne State College.
