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| p.1 #13 · ISS - Jupiter. Transit capture failure. |
I'd never realized that the apparent diameters of Jupiter and the ISS were so similar.
So since you're about 4 diameters away from perfect alignment and you were 300 meters off does that mean you have to precisely position yourself to within less than 75 m to have a prayer of getting the shot you wanted? Isn't that like <1 second of lat/lon? Worse, getting a "clean" shot of the ISS passing through the center of Jupiter must mean that you 'd have to be within 20m or so of the transit line. Does the error in a standard GPS...Show more →
Excellent deductive reasoning there. Exactly right, that's what my calculations suggest. It does require quite a bit of precision. I still don't know why the line of transit was off it's calculated path.
As I mentioned earlier, I get my ISS orbital event data from the CalSky website:
You can select your viewing location by GPS coordinates or on the zoomable map/satellite view.
I did check the satellite view calibration by entering the coordinates (from GPS) into the CalSky interface. It placed the icon precisely at the overgrown turnoff to the farm field where I was.
When I first noticed this event was occurring near me, I began to research the relative apparent sizes and the crossing speed, to get an idea of the precision involved in trying to capture the actual transit.
Like you, I was surprised at the similar apparent size. CalSky supplied an angular diameter of 39.1 sec. of arc for the size of the ISS at the time of the transit. Jupiter's' angular size varies between 30 and 50 sec. of arc. Since Jupiter is near opposition to the sun right now, I figured the size was probably near it's upper limit. Even so, the ISS was going to be a tight fit in the disk of Jupiter.
As for the crossing speed and dwell time over Jupiter, CalSky somewhat cryptically reports the transit to last for .01 seconds. I'm not sure if this is supposed to represent the time the entire ISS is inside the disk of Jupiter or if this time interval is from leading edge contact to trailing edge exit. CalSky helpfully specified the angular velocity of the ISS at the moment of transit as 29.5 minutes of arc per second. Converted to arc seconds that is 1770 arc seconds per second. Since I had already determined that the ISS would 80% the diameter of Jupiter, then it would be ideally at the center of Jupiter with a 10% cushion on each side. So the window for timing the shot to contain the entire ISS within the disk of Jupiter is the time it takes the ISS to traverse 20% of it's apparent angular diameter. 20% of the angular diameter of the ISS is about 8 arc seconds and at 1770 arc seconds per second the ISS should traverse this distance in about 1/220th of a second. In essence, in order to guarantee the ideal capture, with the entire ISS contained within the disk of Jupiter, I would need to trigger a shot within the 4 millisecond window or capture at least 220 FPS. Obviously neither option is really viable.
Lowering my expectations a little, I computed the time it takes from the leading edge of the ISS to make contact until the trailing edge exits the disk of Jupiter. It turns out this window is about 1/20th of a second. I don't think I can trigger a shot within a 50 millisecond window but I can capture video at 30 FPS. So although 30 FPS only gives a 13% probability of getting the ideal shot, it assures at least 1 shot will show them with the edges touching.
I would still like to know where I went wrong. In my opinion, Jupiter would be the perfect backdrop for the ISS to be silhouetted on. It's 5 times further from the sun so it is quite a bit dimmer but it's still bright enough to show the shaded parts of the ISS in silhouette. The moon is either too bright to show much contrast between it and the illuminated parts of the ISS or so dark it doesn't show any silhouette from the shaded parts. When photographed against the sun it is obviously a silhouette only.
This may be an exercise in futility, but I will probably try again if the opportunity arises. Any advice or insight into where I went wrong would be greatly appreciated.
Thanks everyone for the encouraging comments.