I would like to know in what positions of its orbit the moon is when
the time difference between its successive risings is shortest and
longest.
The "harvest moon effect" of the full moon rising at about the same
time on a few successive days in September or October is explained
with the moon beeing close to the vernal equinox point and thus
describing a shallow path to the horizon. The opposite, a long time
difference between successive risings, happens when the moon is close
to the autumnal equinox point. As I looked through a data set of
moonrise and -set times for an entire year, I found the time
differences between successive moonrises to be as short as that of the
"harvest moon" once in every cycle from one new moon to the next new
moon (with the moon always beeing in another phase of course).
My explanation would be that the moon is always close to the vernal equinox
point when the time differences between itīs successive risings is
shortest and it is always close to the autumnal equinox point when the
time differences between itīs successive risings is longest.
Please tell if my understanding of the matter is right or, if not,
where else the moon is if that position can be somewhat marked.
Thank you!

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Clarification of Question by joergs-ga on 01 Sep 2004 17:40 PDT

Just to get it perfectly clear for me : As the celestial equator is
just the expansion of the earthīs equator onto the celestial sphere,
it can also be said that the time difference between successive moon
rises is shortest when he is close to the vernal equinox point and
longest when he is close to the autumnal equinox point, right ?!

The moon orbits the earth in the same direction the earth spins on its
axis, so the moon tends to rise later each successive day.  In order
for the moon to reach the same position relative to earth, the earth
must rotate once plus a little more to catch up with the bit the moon
has moved in its orbit.  If the moon was in a circular orbit directly
above the equator, each moonrise would be about 50 minutes later than
the one the previous day.  However, this is not the case.  The moon's
orbit is tilted a bit with respect to the earth's axis, so sometimes
the point on the earth directly below the moon is in the northern
hemisphere, and sometimes in the southern.

In (northern hemisphere) winter, the point directly below the sun is
in the southern hemisphere, and days (in the northern hemisphere) are
short (that is, sunrises are late and sunsets are early).  Then in
summer, the sun is over the northern hemisphere, and the days are
long.  The same thing happens with the moon.  When it's over your
hemisphere, it stays in the sky longer than when it's over the other
one.  The main difference from the sun is that the moon goes through
its cycle (from north to south and back to north again) in just under
a month, whereas it takes the sun a year.  So the length of a
"moon-day" (the amount of time the moon is above the horizon) changes
from day to day much more dramatically than the length of the ordinary
day (when the sun is above the horizon).

Now, to get to the question about time differences between succesive
risings.  The day-to-day effect of the north/south motion of the moon
will be greatest when the moon is moving north/south most quickly,
which it does as it passes over the equator.  So, if you're in the
northern hemisphere, the time difference between successive risings
will be least when the moon is over the equator and moving northward,
and it will be greatest when the moon is over the equator and moving
southward.  This is because when the moon is moving northward, it
rises earlier than it otherwise would, and this partially cancels the
tendancy to rise 50 minutes later each day, while when the moon is
moving southward, it rises later than it would, and this delay is
added on to the 50 minute daily lag.

The technically correct answer to your question is that it depends on
your latitude.  But as long as you're well away from the tropics and
the poles, latitude doesn't matter too much.

To recap, both the biggest and smallest differences in successive
moonrise times generally occur as the moon passes over the equator.

RE:

"Just to get it perfectly clear for me : As the celestial equator is
just the expansion of the earthīs equator onto the celestial sphere,
it can also be said that the time difference between successive moon
rises is shortest when he is close to the vernal equinox point and
longest when he is close to the autumnal equinox point, right ?!"

No, that's not correct.  The plane of the moon's orbit precesses with
a period of 18.61 years.  So if at some point in time the moon's orbit
crossed the equator going northward near the vernal equinox point, in
which case the difference between succesive rises would be shortest,
then 9.3 years later the moon's orbit would cross the equator going
southward near the vernal equinox point, and the time difference
between successive rises would be greatest.  That said, there will be
a few-year period during each 18.6 years when what you say is
approximately true--that the difference is least when the moon is near
the vernal equinox point and greatest when near the autumnal equinox
point.