One data point doesn't prove anything, but nonetheless this is interesting. I re-did the prediction for the 8 Feb '04 solar transit photographed by John Locker, based upon his updated elevation above sea-level of 86 feet (versus 50 feet). A - travel distance (miles) and direction B - date C - time D - elevation angle of the ISS E - azimuth angle of the ISS ( + is East from North; - is W from N) F - range (miles) G - latitude for observing the transit H - longitude I - how far (miles) can I be from the centerline? A------- B----- C----- D--- E----- F--- G------- H-------- I---- 3.9 SW 8 Feb 122740 21.6 -179.8 545 53.3561 -3.1708 5.8 0.0 SW 40.79 53.3874 -3.0919 5.8 1.0 NE 8 Feb 122741 21.5 -179.7 545 53.3957 -3.0711 5.8 The above used the Mission Control Center TLE that was posted on February 5: 1 25544U 98067A 04038.55066812 .00020000 00000-0 20000-3 0 9029 2 25544 51.6274 270.7867 0007082 138.3390 221.8310 15.66877780 17367 Next, I used the last Orbital Information Group TLE that John provided to me, which preceded the transit by 3.34 hours, or basically 2 orbits: 1 25544U 98067A 04039.37920139 .00012863 00000-0 12391-3 0 1508 2 25544 51.6289 266.5703 0007449 140.3946 216.4560 15.66900537297940 This produced the following transit track: A------- B----- C----- D--- E----- F--- G------- H-------- I---- 0.4 W 8 Feb 122742 21.6 -179.7 545 53.3867 -3.1024 5.8 0.2 NW 42.07 53.3895 -3.0954 5.8 4.6 NE 8 Feb 122743 21.5 -179.6 546 53.4262 -3.0025 5.8 Working out the distance more precisely, the OIG track indicated that John was about 1090 feet SE of the nearest point on the center line, while the MCC track indicated that he was about 60 feet (!) N of the center line. As nearly as I can measure (and that's probably no better than +/- 100 feet), John was actually about 200 feet / sin(21.5) = 550 feet NW of the closest point on the center line, so the MCC prediction was off by about 500 feet, while the OIG prediction was off by about 1600 feet. (It'd be nice if the MCC TLEs were always this good.) The angular error in the OIG prediction amounts to about 0.75 minutes of arc, though had the pass been nearly overhead, the error would have been about 1.5 minutes of arc. For a high elevation pass (e.g., at a range of 270 miles), the ISS's solar panels are about 40 seconds of arc across; the MCC error would have been about 30", versus 90" for the OIG prediction. Since Saturn currently has an angular diameter of about 20", a center-to-center miss by 30" might well show the ISS and Saturn "just touching," while a miss of 90" would leave a "gap" of about 60" between Saturn and the ISS. However, for such a high elevation pass, at 30 fps, the ISS moves about 128" per frame, so if you were trying to make a video of the ISS passing by Saturn, even if you scored a "direct hit," you might end up with the ISS centered 64" to either side of Saturn, in the closest frames. A center-to-center miss by 64" would leave a gap of about 34" of arc between Saturn and the ISS. Incidentally, if anybody's ever gotten a pic of the ISS any closer to the center of the sun or moon than John's, I'm not aware of it. ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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