On Wednesday, March 16, 2016 at 7:31:01 AM UTC-6, wrote:
On Wednesday, March 16, 2016 at 10:33:35 AM UTC, oriel36 wrote:

"On the other hand the moon does not rotate on the axis of
its own body,as its spots prove " Kepler

An observer on the moon experiences sunrise and sunset once as the Moon orbits
the Earth, which as you have explained many times (for the Earth) is a result
of a surface rotation, showing that the Moon rotates once per orbit of the
Earth, and that Kepler was wrong.

Kepler, at least, had the intelligence to know exactly what he was saying.

As you and I both well know, the Moon has surface features that show that one
side of the Moon always faces the Earth.

Kepler - like Oriel - followed the naive convention that such a situation meant
that the Moon does not rotate. This is, in fact, quite a normal default
position for people to take. Just as most people will say in ordinary
conversation that the Earth rotates on its axis once every 24 hours so that we
can have days and nights.

Practicing scientists today, though, find this naive view inadequate.

The Equation of Time in the case of the Earth, and libration in longitude in the case of the Moon, show that neither the Earth nor the Moon has its rotation, as it were, "nailed down" to its orbital motion.

Tidal forces may keep the _period_ of the Moon's rotation equal to that of its orbit, but in the short run, the Moon rotates freely, and at a uniform rate, while its elliptical orbit makes its angular revolution non-uniform. Acknowledging, therefore, that the Moon does rotate lets us see that its rotation is uniform - without any back-and-forth wiggling that libration seems to imply.

That back and forth wiggling is just the difference between the Moon's uniform rotation and the nature of an elliptical and inclined orbital motion.

For Kepler to adhere to the naive view because it wasn't inadequate for his
purposes doesn't make him wrong.

If Oriel didn't take it upon himself to claim that today's practicing
astronomers _are_ wrong, in taking a more subtle and sophisticated view of the
matter as is appropriate for their purposes, I wouldn't be as critical of his
views either.

As I've noted, when I went to school, I had seen a table of the Solar System
that gave the length of the day on Earth as 23 hours and 56 minutes. That *was*
just wrong. And highly confusing.

But when I saw that the length of the day on Mercury was given as 88 days -
instead of forever - this was back when people thought Mercury always turned
one face towards the Sun, the way the Moon does towards Earth - I was able to
figure out what was going on.

The term "day", without qualification, means the synodic day, not the sidereal
day, and for a junior high school textbook to brutally confuse young students
in this manner is to be criticized. A reaction to that sort of thing is
warranted - unfortunately, Oriel is not the man to carry it out. He is rooted
in the naive perspective, which is not without value, but he lacks the
understanding to realize that the sophisticated perspective also has its place.

John Savard