[amsat-bb] Graph of eclipse/sunlight

andy thomas andythomasmail at yahoo.co.uk
Sat Jan 23 14:21:57 PST 2010

I've been looking at some cosmonauts' historic accounts of missions on the Salyut series of orbital spacecraft. Characteristically they have drawn a graph of the eclipse and sunlight times of the long duration mission. The vertical axis is time after EQX of the first orbit of the day (the entry into and departure from eclipse is marked on this axis in 5 minute intervals) and the horizontal axis is the mission day. 

The result is a graph in which lines extend horizontally conencting the eclipse times (both in and out), and they have characteristically filled in the darkness time with dark pen. On one print the light time has another contour with what appears to be sun angle.

The graph has then been added to by showing the plan and actual activity on that mission day.

I would find it interesting to compute a similar graph for a long duration mission of the ISS. But I have some difficulty. From a TLE I can do the following:

*calculate first eqx in utc

but can't find the time of crossing into eclipse without knowing the position of the sun and the position of the spacecraft in relation to the sun (Subsatellite point?).

Does anyone have any idea about the calculation and data involved? It must have been a hand calculation carried out by cosmonauts, but once a simple program is written I could use excel to draw the graph. Maybe a standard predictor program can produce a data output that could be rewritten into excel? I'd need the eclipse times on the orbit following the first eqx of the day.

I would appreciate any input, also any thoughts of why they would want to draw it (have only thought of observation and power).

many thanks

73 de andy G0SFJ


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