Time differences between successive moonrises

Dear forum,

after printing the moonrise and -set data for my position in central
europe from the US Naval Observatory and calculating the time
differences between successive moonrises from one new moon to the
following new moon for March/April, June/July/, September/October and
December/January I found the following regularity : The time
differences are shortest when the moon is at the most southern
position of it´s orbit and are longest when the moon is at the most
northern position. Can somebody explain me the reason behind that ?
Could it be that those positions fall into the points closest and
farest to/from earth of the moons excentric orbit ? Any input is very
welcome !

Mr.Adams

(Mr.Adams) wrote in message om...
after printing the moonrise and -set data for my position in central
europe from the US Naval Observatory and calculating the time
differences between successive moonrises from one new moon to the
following new moon for March/April, June/July/, September/October and
December/January I found the following regularity : The time
differences are shortest when the moon is at the most southern
position of it's orbit and are longest when the moon is at the most
northern position. Can somebody explain me the reason behind that ?
Could it be that those positions fall into the points closest and
farest to/from earth of the moons eccentric orbit ?

No, it has to do with changing declination of the moon (mostly).

First, let's calculate _average_ time differences between successive
moonrises:

After one full cycle of moon phases (29.53 days) moon will cross the
meridian in the same (or maybe approximately?) time, that is,
differences will accumulate to 24 hours. That means that _average_
time differences between successive moonrises is 24h/29.53 = 50.5
minutes.

But, since the moon's path is positioned with an angle to an equator,
moon's declination will change every day.

For the N-hemisphere observer, changing declination to the north means
moon will spent longer time above the horizon, so it will rise
sooner and set later than one can calculate by just adding 50.5
minutes to yesterday's rise and set times.

I wouldn't like to go more into details because moon's path is quite
complicated, if You take into account all the effects (inclination to
ecliptic,
nutation, precession, whatever...).

Hope this helps.

Ante Perkovic

"Mr.Adams" wrote:
after printing the moonrise and -set data for my position in central
europe from the US Naval Observatory and calculating the time
differences between successive moonrises from one new moon to the
following new moon for March/April, June/July/, September/October and
December/January I found the following regularity : The time
differences are shortest when the moon is at the most southern
position of its orbit and are longest when the moon is at the most
northern position. Can somebody explain me the reason behind that ?
Could it be that those positions fall into the points closest and
farest to/from earth of the moons excentric orbit ? Any input is very
welcome !

Objection: Asked and answered.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
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My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Mr.Adams wrote :

after printing the moonrise and -set data for my position in central
europe from the US Naval Observatory and calculating the time
differences between successive moonrises from one new moon to the
following new moon for March/April, June/July/, September/October and
December/January I found the following regularity : The time
differences are shortest when the moon is at the most southern
position of it´s orbit and are longest when the moon is at the most
northern position. Can somebody explain me the reason behind that ?
Could it be that those positions fall into the points closest and
farest to/from earth of the moons excentric orbit ? Any input is very
welcome !


Well, I think I was on the wrong track with the moon beeing in it´s
most southern position of the orbit when the time difference between
it´s risings is shortest and it beeing in it´s most northern position
of the orbit when the time difference is longest.
But there is a certian pattern in the table of the moons rise times
and the points where the time differences between successive risings
are shortest or longest. In combining the knowledge I got from
studying some articles about the harvest moon effect, I thought about
some other explanation for this pattern. The harvest moon effect of
the full moon rising at about the same time (short time difference) on
a couple of successive evenings in September or October is explained
by it beeing close to the vernal equinox point and describing a
shallow angle to the horizon. The opposite happens when the moon is
close to the autumnal equinox, it describes a steep angle to the
horizon and the time difference between it´s successive risings is
long. Now I ask for your combined approval for my conclusion based on
superficial knowledge : The moon is always close to the vernal equinox
point when the time differences between it´s successive risings is
shortest (about First quarter in winter, New moon in spring, Last
quarter in summer and Full moon in autumn) and it is always close to
the autumnal equinox point when the time differences between it´s
successive risings is longest (about Last quarter in winter, Full moon
in spring, First quarter in summer and New moon in autumn). I hope
somebody can tell me if I finally found the right explanation (sorry
for the long text) !

Mr.Adams