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