Re: heavens-above Moon transit anomaly

From: Tom Wagner (sciteach@mchsi.com)
Date: Fri Apr 19 2002 - 05:07:00 EDT

  • Next message: Matson, Robert: "Transit of the sun by ISS from SoCal, Nevada, etc."

    Thank you Chris (and Frank),
    
    I appreciate it very much.
    
    Tom
    Iowa
    USA
    .....
    
    P.S. Up at 4:02 AM in a failed attempt to see a Starshine 3. Too much light
    pollution for sure. Gotta go back to bed now.
    
    ----- Original Message -----
    From: "Chris Peat" <chris.peat@heavens-above.com>
    To: <FrankEReed@aol.com>; <SeeSat-L@satobs.org>
    Sent: Friday, April 19, 2002 3:37 AM
    Subject: 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
    >
    > Chris Peat
    > Heavens-Above GmbH
    > E-Mail: chris.peat@heavens-above.com
    > Web site: www.heavens-above.com
    >
    >
    > > -----Original Message-----
    > > From: FrankEReed@aol.com [mailto:FrankEReed@aol.com]
    > > Sent: 19 April 2002 09:14
    > > To: SeeSat-L@satobs.org
    > > 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 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
    
    
    -----------------------------------------------------------------
    Unsubscribe from SeeSat-L by sending a message with 'unsubscribe'
    in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org
    http://www.satellite.eu.org/seesat/seesatindex.html
    



    This archive was generated by hypermail 2b29 : Fri Apr 19 2002 - 05:09:52 EDT