----- Original Message ----- From: "Clint Bradford" clintbradford@mac.com To: "AMSAT BB" amsat-bb@amsat.org Sent: Wednesday, May 19, 2010 7:08 PM Subject: [amsat-bb] ISS and Shuttle Silhoutte Against Sun

I just don't tire of seeing these incredible photographs.

French astro-photographer Thierry Legault captured the ISS and Space Shuttle Atlantis 50 minutes before docking last week.

http://tinyurl.com/34hrt2m

Clint Bradford

Hi Clint,

As seen from the earth the time needed by the ISS to cross the disc of the sun passing through it's center can be roughly calculated.

The ISS Mean Motion is 15.73972181 rev/day and so one orbit of the ISS around the earth last 24 / 15.73972181 = 1.524804586 hours or 91.48827517 minutes.

The orbit of the ISS has an eccentricity of 0.0009642 with perigee hight = 340.7 km and apogee hight = 353.7 km so that the orbit of 360° can be considered circular and then the time for the ISS to run 1 degree of it's orbit in the sky is 91.48827517 / 360 = 0.2541340977 minutes.

As seen from the earth the sun's disc subtends an angle of about 0.5 degrees so that the time needed by the ISS to cross the full sun's disc as seen from the earth is 0.2541340977 x 0.5 = 0.1270670488 minutes or just 7.624 seconds.

This is why to search for the alignement earth--> ISS +Atlantis--> sun and shoot a picture of their shadow against the sun is extremely difficult particularly when the ISS is not crossing the sun passing through the center as the picture shows.

http://tinyurl.com/34hrt2m

and in fact for this transit near the edge of the sun's disc the maximum duration of visibility was only 0.54 seconds as seen from central Spain.

Succesfully the transit forecast was calculated with the software

www.calsky.com.

and solved the problem to french astro-photographer Thierry Legault that make a similar observation of the ISS and Atlantis against the sun on 29 september 2006 as you can see here:

http://www.dailymail.co.uk/news/article-407636/A-spot-sun.html

enjoy !

73" de

i8CVS Domenico