Rob wrote:
"Plugging in the coordinates of your four sites, they're all a
bit west of the transit centerline, suggesting that either
a different elset was used, or perhaps there is a systematic
error in the lunar location." and
"While the errors only average about 0.12 degrees in longitude
(about 6 1/2 miles), this is more than enough for the track to
miss the moon."
I believe I've figured out what the problem is. It appears that
the Moon position on the pass charts on heavens-above are
geocentric. They do not take into the account horizontal parallax,
IF I've figured this out correctly.
Your 6.5 mile difference eastward is just about what would have
been required for the pass in question to lower the altitude of
the ISS by about two-thirds of a degree. And since the Moon was
roughly 45 degrees high for that pass, its parallax would be
just about two-thirds of a degree lower in the sky.
I also tried running some satellite passes for points along the
same longitude with latitudes of +60 and then -30. I found a pair
of passes that occurred at the same zone time to make sure that
they would be calculating the Moon's position at the same UT.
And sure enough, the Moon was mapped on the pass chart at what
seemed to be *exactly* the same position despite a 90 degree
difference in latitude. Obervers that far apart, comparing
simultaneous observations of the Moon would normally be expected
to see the Moon at positions against background stars that are
about a degree apart due to horizontal parallax.
IF this turns out to be a correct analysis, it would be relatively
easy for heavens-above.com to fix the problem, which should then
make that site a fun way to find lunar transits.
In the meantime, you can apply a good, approximate correction by
peforming a simple calculation:
d_away = r_sat / [60 * tan(h)]
where r_sat is the approximate satellite range, tan(h) is the
tangent of the satellite altitude, and the result d_away is the
distance that you should "step back" in order to get the satellite
lower in the sky to compensate for parallax. So, as a numerical
example, if the Moon is 25 degrees high in the southwest, and
heavens-above shows that ISS, or some other satellite, at a
range of 1200km will pass in front of the Moon as seen from
a specified location, you would correct the location by moving
43km towards the northeast ("away" from the direction of the Moon
in the sky). Note that for a high pass of the ISS the effect of
horizontal parallax is much smaller. If r_sat is 450km and h is
75 degrees, the correction is only 2 kilometers.
Please note the big "IF" above...
-Frank E. Reed
www.clockwk.com/fer
Chicago, IL
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