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On day of transit of Venus, 8 June 2004, Tomáš Maruška pinpointed a narrow geographic corridor with help of the transit prediction feature of www.CalSKY.com. Observer from this corridor witnessed sometime very special, which was never seen before: a spacecraft that passes in front of Venus, which itself was in front of the sun. The spacecraft was the manend International Space Station ISS.
Tomáš used an off-the-shelf Philips TouCam Pro Webcam mounted on a Rubinar 5.6/500 with Baader Astro Solar filter. The framerate was 30 frames/sec. The transit occured at 10:09:18 UT. The exact observing positions was the village of Stupava, north of Bratislava in Slovakia, WGS84 coordinates latitude 48°15'29" N and longitude 17°01'39" E.
Tomas reported the WGS84 coordinates of his observing site with 2m estimated error:
WGS84: 17:01:38E / 48:15:28.6NCalSKY extracts an altitude of 164m above sea level from SRTM-3 DEM (in good accordance with the Slovac hikking map), and a geoid undulation of 44 m from EGM96. Hence the ellipsoidal height regarding WGS84 is 208m:
208m above WGS84 ellipsoid
NASA publishes orbit elements of ISS on: http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/orbit/ISS/SVPOST.html. These elements are used within CalSKY.com for predictions of future events, since they contain orbit maneuvers. We use following MCC element set:
1 25544U 98067A 04159.51929753 .00020000 00000-0 20000-3 0 9009 2 25544 51.6330 15.0298 0005537 207.1571 152.9300 15.68796178 36803
Using this data CalSKY predicts ISS to transit Venus at 8 June 2004 at 10h09m17.02s UT:
Crosses the disk of Venus. Separation: 0.00d Angular Velocity: 61.7'/s. Transit duration: 0.02s Angular diameter: 45.0" size: 73.0m x 44.5m x 27.5m Satellite at az: 158.5d SSE h: 63.1d dist: 411.7 km mag=11.7m Satellite apparently moves to direction 251.7° You are on the centerline of this transit! Path direction: 62.8d ENE ground speed: 7.406 km/s width: 0.1 km
Following graphic visualizes this transit; ISS moves from right to left with 0.1 seconds between drawn positions. The first position on the solar disk is at 10h09m17.0s UT.
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Obviously Tomas image reveals that ISS moved through its closest approach to Venus about 1/2 diameter of Venus south of the center of Venus. Since Venus was 58" across, this is about 30". At a distance of 412 km and a satellite elevation of 63.1° this sums up to 67m { = 412 km * tan(30") / sin(63.1°) }. Hence Tomas was 67m northwest (azimuth 333°) of the center line.
NASA publishes orbit elements of ISS on: http://oig1.gsfc.nasa.gov/. These elements are used within CalSKY.com for archiving and calculation of events before or up to one day after the observation of ISS by OIG. We use following OIG element set:
1 25544U 98067A 04160.42390752 .00014992 00000-0 13290-3 0 9491 2 25544 51.6329 10.4117 0005395 206.7073 225.7658 15.68815833316945
Using this data CalSKY predicts ISS to encounter Venus at 8 June 2004 at 10h09m17.94s UT:
Close to Venus. Separation: 0.03d Position Angle: 22.3d
Angular Velocity: 61.8'/s
Angular diameter: 45.0" size: 73.0m x 44.5m x 27.5m
Satellite at az: 158.5d SSE h: 63.2d dist: 411.4 km mag=12.1m
Satellite apparently moves to direction 286.9°
Centerline, Closest Point: Lon: 17d01m33s E Lat: +48d15m36s
dist: 0.24 km az: 332.9d NNW
Path direction: 62.8d ENE ground speed: 7.406 km/s width: 0.1 km
Following graphic visualizes this transit; ISS moves from right to left with 0.1 seconds between drawn positions. The first position on the solar disk is at 10h09m17.9s UT.
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Using this orbit data, the mass center of ISS would have missed the center of Venus by 1.9 diameters of Venus, 91". This makes about 204 m regarding the center line on the surface of the earth; Tomas would have been 204 m to far southeast (azimuth 153°) of the predicted center line.
To solve the mistery of the orbit elements (source MCC or OIG), we should try in future to pinpoint the exact time of a close encounter with or occultation of a planet to better than 1/10 second. I would prefer planets rather than stars as targets, since they allow a well defined scale within the image for the later analysis.
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