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http://iss-transit.sourceforge.net/IssVenusTransit.html The transit path seems to be fairly well defined at this point- the weather's a murkier question, as usual :-( I nearly have GTOPO30 Digital Elevation Model working in my WorldView program (it's been about 80% done since January!), which will allow the program to look up the actual elevation above sea-level (to 30" resolution in latitude & longitude, or ~1 km at the equator). The transit path isn't strongly affected by elevation above sea-level, of course, when the angle of elevation is large. However, if you're shooting at a small target (e.g., a planet about the angular size of the ISS itself), it's desirable to minimize all the errors one can. In that regard- and for the sake of accuracy for those who'll be trying to make the observation- I think it'd be useful to have a discussion about all the factors that go into computing the ISS/Venus transit track. So, a bit later, I'll be posting some of my own results, to see if anyone else can confirm or refute them. The final results are already posted on the page given above (computed for an elevation above sea-level of 820 feet / 250 meters), if anyone wishes to test them against this or that program. More usefully, I hope, I also intend to post some of my intermediate results, such as the ECI position of the ISS that I compute at some conspicuous times; e.g., 5:32:55 UTC on June 8, "Contact II" when the disk of Venus is first entirely within the disk of the Sun (as seen by a demon at the center of the Earth). I did a lot of work back around Christmas time to figure out the effects of ray bending in the Earth's atmosphere (the results are on the ISS Transit source code page, for anyone that may be interested)... a lot of work for not much. For the ISS at least (and even more so, for satellites in higher orbits), I found that ray-bending can be completely ignored at elevation angles greater than about 37°, where it amounts to only about 6 meters (in other words, ray-bending at that angle of elevation has the effect of raising your elevation above sea-level by 6 meters, were the Earth's atmosphere suddenly to become non-refractive). At the horizon & sea-level, ray bending amounts to a couple kilometers, however (were it not for ray-bending, the light from the "rising" sun would pass a couple kilometers over your head, rather than being bent down to your eyes). So the critical factors affecting the final accuracy (at reasonably high angles of elevation) boil down to 1) the position of the satellite at a given instant, 2) the (apparent) position of the astronomical object (i.e., accounting for light delay), and 3) the compution of the line connecting the two, and its point of intersection on the real Earth's surface, which presumably will be modeled by the WGS84 ellipsoid (though my code still uses WGS72 constants!). ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html

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