RE: heavens-above Moon transit anomaly

From: Chris Peat (
Date: Fri Apr 19 2002 - 04:37:17 EDT

  • Next message: Tom Wagner: "Re: heavens-above Moon transit anomaly"

    Hi all,
    Frank is correct. The Heavens-Above star charts do not account for the shift
    in the Moons position due to parallax. I have put this on my list of things
    to fix and will let you know when it's done. I must also say that the Moon
    size is also not to scale and no phase is shown, the idea was only to show
    where the Moon is. Now that so many people seem to be interested in Lunar
    transits of satellites, I will update the plots.
    Chris Peat
    Heavens-Above GmbH
    Web site:
    > -----Original Message-----
    > From: []
    > Sent: 19 April 2002 09:14
    > To:
    > Subject: heavens-above Moon transit anomaly
    > 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 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
    > Chicago, IL
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