1. ## Trying to Solve a Puzzle about satellite transits

A week or two back a friend wanted to catch the ISS transiting the Sun. He had made a simple solar projection scope for the eclipse, so since the center-line for an upcoming ISS transit was nearby, we planned to go watch and photograph it.

He has a camera that can capture a quick succession of photos, though it would only do this for a few seconds. I manned the watch while also pointing a smartphone taking video, and he took the photos. We did catch the ISS transiting, and he discovered that his camera was a bit out of focus and his solar projection scope was a bit out of focus, so the results were not at all great.

But as I was giving a count-down, we both saw what looked like something transiting the Sun several seconds ahead of the appointed time. And my friend also saw what looked like a transit several seconds before that as well.

So here's the mystery: His projection scope uses a 60mm objective, so the ISS made only a dim little spot on the projection scope's image (a fuzzy spot because it wasn't focused that well). But what in the world were those other transits? Today I see another post with a similar result, where in an attempt to get an ISS transit they also got another transit: ISS Transits

But the ISS is HUGE compared to other satellites. The ISS is the size of a football field and other decent sized satellites are maybe the size of an 18 wheeler, as someone once explained to me. So the ISS would be hundreds of times (or a thousand times?) the area of other satellites. If that's right, though, then I wouldn't expect the other satellite to be visible at all. For example, on the lunar transit video linked to in the above thread, the other satellite looks like it might be 1/4 the size of the ISS, or maybe even 1/8 the size, but not 1/500 the size. Is something wrong with my thinking that other satellites should be too small to see? Or could we be seeing something else? It seems unlikely that we were seeing a bug fly in front of the scope, and I wouldn't think a bug would be in focus since it's so low. A plane? We don't have very much air traffic here, so seeing two planes that happen to transit the Sun a few seconds apart seems incredible. And they would presumably be too large.

I was totally mystified, and the recent post on the ISS re-raised the puzzle for me. Any thoughts?

2. ## Re: Trying to Solve a Puzzle about satellite transits

My guess would be satellites. Yes, typical satellite is much smaller than ISS, but if it orbits closer to Earth it's angular size will be larger.

3. ## Re: Trying to Solve a Puzzle about satellite transits

Originally Posted by Bigzmey
My guess would be satellites. Yes, typical satellite is much smaller than ISS, but if it orbits closer to Earth it's angular size will be larger.
We thought about that but thought that the closer it would be to us the faster it would be transiting. Is this wrong? The transits we saw looked roughly like they were the same speed as the ISS. Similarly for the lunar transit the other thread linked to. And does it make sense that there would happen to be three transits all within 20 seconds??

4. ## Re: Trying to Solve a Puzzle about satellite transits

Satellites also vary greatly in their reflectivity for a given size. If you were viewing the ISS with several solar panels aligned within your angle of view, it appears much brighter than when you are seeing it at an oblique angle. When the ISS transits from horizon to horizon, it sometimes "disappears" before the horizon if the sun light is no longer reflected back to your viewing position.

5. ## Re: Trying to Solve a Puzzle about satellite transits

It could be a supply ship on its way to or from ISS - it would be smaller but still a fair size. I once saw a space shuttle on its way to the ISS and trailing it by a few seconds.

6. ## Re: Trying to Solve a Puzzle about satellite transits

Originally Posted by Mark Moyer
We thought about that but thought that the closer it would be to us the faster it would be transiting. Is this wrong? The transits we saw looked roughly like they were the same speed as the ISS. Similarly for the lunar transit the other thread linked to. And does it make sense that there would happen to be three transits all within 20 seconds??
Sometimes I see more than one satellite in the EP FOV when I observe with my scopes, but supply ship idea would make sense to.

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