Moon #1 - Why does the moon rise later each day?

The earth orbits the sun once per year, but also rotates on its axis once per day. The latter fact is of special interest in this discussion. The moon orbits the earth, as seen from the solar system, once per 29.53 days. This is also of interest in this discussion. Finally, the direction of rotation is also crucial. The earth's daily rotation is from west toward the east. The rotation of the moon around the earth is also from west toward the east.

If you have not thought about the above whatsoever, you may be surprised. After all, we see the moon rise in the east, make its way across the sky, and set in the west! To help explain this seeming discrepancy, it is useful to draw an analogy.

Analogy

Herbert is 3 years old and is driving with his mom from LA to a remote village in southern Arizona.

[Although not germane to this discussion, his mom is hoping to score some peyote in that village, first discovered when she was in college. This had proved useful in her relationships at that time, an experience she needs again.]

They are driving from west to east along an incredibly boring stretch; it's flat and all looks the same, hard to say they're even moving. There is a bicyclist ahead in their lane. Suddenly Herbert spots the biker who is coming closer and closer, then right next to the car, and finally Herbert sees the biker receding in the rear view mirror. From Herbert's viewpoint the biker looks like he's going backwards - first the biker is out front, then recedes out the car's back window.

Herbert says to his mom, "That biker is going backwards." Then looking at the compass built into his pacifier, Herbert continues "In fact he's going west." Herbert not only has a great vocabulary, but from his frame of reference (the car) that's what it does look like. His mom says, "Yes, dear".

Well as we (Herbert) sit on the earth (car), watching the moon (biker), it looks like the moon travels to the west before setting. And from his frame of reference, that is correct.

Now as Herbert, mom, and car travel past, there is a 2 year old girl (Malfeasa) standing on the side of the road, watching traffic pass by (a rare event). She sees the bicycle pass by heading east, then passed by the much faster car also headed east.

[Also not germane, Malfeasa's father typically sends her out to watch traffic.]

Malfeasa's frame of reference on the side of the road is not centered on the car, it's outside the car - the analogy is that "on the side of the road" is like a frame of reference tied to the sun where an observer can watch the earth (car) and moon (biker). Malfeasa sees them both (the car and the biker) heading east. And from her frame of reference, that is correct.

End of analogy

The prior analogy works OK for one traversal of the moon across our sky, but is only part of the story - but we're nearly there. The important result is that the earth rotates quickly (once per day) toward the east and the moon orbits the earth much more slowly (once per 29.53 days) also toward the east. So when the moon rises it has moved slightly eastward in its orbit around the earth compared to yesterday's moonrise. So it takes a little longer for an observer on earth to see the moonrise i.e. earth must 'catch up' to the moon.

Well then, how much later is the moonrise the next day? The moon's orbit is elliptical not circular. Further that orbit is tilted at 5 degrees relative to earth's orbit around the sun. And there are other slight complications. So the average time that the moon rises later is variable depending on where the moon is in its orbit and on our latitude as an observer. We will not descend into those details but note that the average moonrise is just under 50 minutes later than the day before. As an example of the variation, the actual time later can range from 30 to 63 minutes around that average at 50 degrees latitude.

Looking ahead:
We will, in a later blog entry, discuss why there are typically 2 tides per day i.e. 2 high tides and 2 low tides. Then we'll relate this to our moonrise lags as well.