# Re: update to ISS Venus transit webpage

From: Thomas Fly (tfly@alumni.caltech.edu)
Date: Tue May 18 2004 - 11:16:47 EDT

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```On Thursday, I believe I should be able to make a (hopefully) accurate forecast
of the June 8 ISS / Venus transit track.  The basic idea occurred to me a while
back, but it required an orbit boost such as that scheduled for today.

The projected TLE for day-of-year 149.7 (May 28) published by the MCC yesterday
gives an orbital period corresponding to about 15.6839 orbits per day (there are
86400 seconds in a year, so that precision corresponds to about 1/2 second per
day):

1 25544U 98067A   04149.70875706  .00020000  00000-0  20000-3 0  9120
2 25544  51.6304  65.0080 0005811 166.6668 193.4647 15.68392604 35268

This TLE corresponds closely to the TLE for DOY 109.5 (April 18),

1 25544U 98067A   04109.51565041  .00020000  00000-0  20000-3 0  9033
2 25544  51.6252 269.8838 0010857  45.7908 314.4142 15.68393153 28958

With some offset in time (within half the orbital period, or about 46 minutes),
it should be possible to transform a position (such as crossing the equator,
northbound- at longitude 0, we'll pretend) predicted by the historic DOY 109.5
TLE to a position (also crossing the equator, northbound- at longitude 3.1416 E,
we'll pretend) predicted by the DOY 149.7 TLE, namely by adding 3.1416° of
longitude.

The transit of Venus begins at DOY 160.231192, or 10.5224351 days = 909138.39
seconds after DOY 149.708757.  So, by using the historic TLE for DOY 109.5 +
10.5 = 120.0; the nearest to which is DOY 119.5 (April 28),

1 25544U 98067A   04119.51743710  .00020000  00000-0  20000-3 0  9024
2 25544  51.6283 218.9203 0011040  78.8338 281.4063 15.68949863 30528

with the appropriate time offset & longitudinal correction, it should be
possible to use the well-known position of the ISS at that earlier time to
forecast the position during the Venus transit, and so to produce an acceptably
accurate ISS / Venus transit track for June 8!

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