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Re: [Phys-l] Moon's speed

"I also notice that tables of apparent moonrise depend steeply upon
refraction near the horizon *, a notoriously variable quantity."

Good point, I had forgotten. However, I suspect the table, whose URL ** I forgot to include, is derived from orbital formulae and ignores refraction, etc. I suspected the variation is due to eccentricities in the elliptical orbits, but anything else? e.g. precession; way too small an effect and periods too long?



* also since the moon subtends v. ~ 1/2 deg, and "moves" ~ 14 deg/hour, it takes v.~ > two minutes to rise. What's the definition of rise?
** http://aa.usno.navy.mil/data/docs/RS_OneYear.html


bc, will send to requesters a plot (JPEG) of this years first 65 rises.

Brian Whatcott wrote:

At 06:23 PM 9/1/2007, you wrote:

A friend wished to program his rotary table, so he could photo' the moon
over time w/ its position in the focal plane constant and supplied me w/
the Navy's moon rise table. He wondered about the variation. I do
also. I plotted the cumulative moon rise time and the differences. It
has a 3% variation (+/- from the mean). The variation has two periods
* one about 27 days (not surprising), and another of about ten days w/
about one third the amplitude.



* very obvious w/o FFT'ing the data.

Explanation please.


bc, too lazy to wiki.



Without searching the available resources or even examining the basis
of one of the excellent moon phase visualization programs, I first want
to suppose that it is not necessary that particular motions (like a
pendulum's instantaneous amplitude, for example) follow a sinusoidal path.

The consequence of a Fourier analysis of such a physical cycle is to
show odd harmonics for phase-symmetrical differences.
I also notice that tables of apparent moonrise depend steeply upon
refraction near the horizon, a notoriously variable quantity.



Brian (Mr. Indolence) Whatcott Altus OK
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