Telescope Reviews: Satellite Passes in Front of the Moon

Kurt,

It's true there's a lot of stuff in orbit, but depending on how you define an "imaging opportunity" -- and whether you're willing to travel to witness it -- this may paint a somewhat optimistic picture. You may wish to check the apparent angular sizes of the transiting objects listed in the CalSky predictions, many of which are less than an arc-second in diameter. There is also the problem that some of the satellite elements, including perhaps those for Lacrosse 3, may be extremely out of date making it uncertain if the listed satellite will actually transit the Moon as seen from a given location.

To me, recording something the size of the shadow in one of the larger Plato craterlets (~1 arc-sec diameter) moving across the Moon in 1-2 sec would be quite challenging -- I should think you'd have to know exactly when and where to look.

For statistical purposes, the Sun is a good Moon-sized test target, and as I understand it, the CalSky page you reference has an "Astronomer" mode in which it prints out every satellite passing with 1.5° of the center of the Sun that has a potential to be as much as 1 arc-sec across in its largest dimension when passing directly overhead (of course many wind up much smaller because they are unfavorably oriented or seen over a much longer slant range). At this time of year, the Sun is up for about 12 hours a day, and from my location near latitude 34°N, CalSky says that for the next two weeks there will be, on average about 1 pass per day of something that could be larger than 2 arc-sec within 1.5° of the Sun, and a little under 10 passes of something possibly over 1 arc-sec.

To actually transit the Sun (or Moon), the satellite has to come within 0.25° of that object's center. Hence only 1/36th of the satellites that pass within 1.5° will even partially transit without moving the telescope to a more favorable location. This reduces the probabilities of witnessing a transit from a fixed location in a 12 hour observing session to about 0.3 for a 1 arc-sec object and 0.03 for a 2 arc-sec object. The predicted transit times for objects in this size range, incidentally, should one be lucky enough to be positioned where they cross the full diameter, range up to about 2 sec for the 2 arc-sec silhouettes and 4 sec for the 1 arc-sec ones. Most are briefer.

Switching to the CalSky "Hobby" mode, in 10 weeks of 12-hour samples I found 3 passes within 1.5° of the Sun in which the satellite was predicted to appear 5 arc-sec or more in diameter (the size of a 9 km crater). That gives a probability of 0.001 for observing a transit of this sort in a single 12-hour session. These larger-appearing transits (two of the three were the International Space Station) lasted at most 1.6 seconds.

I have very little practical feel for the difficulty of observing such things, but based on my own limited experiences trying to view Space Shuttle (up to ~40 arc-sec at 200 km range) and ISS (now up to ~50 arc-sec at 330 km range) passes many years ago, I should think trying to catch a 2 arc-sec object moving that rapidly as a dark silhouette on a bright background would be quite difficult, and catching a 1 arc-sec one extremely difficult. I would guess the satellite has to get up to 5-10 arc-sec diameter before you have a real "imaging opportunity". So I would think the chance of casually noticing a satellite pass over the Moon is very small. But our eyes are designed to see moving objects more readily than stationary ones, so I might be wrong about that.

The reason I didn't ask directly for Moon transits is that CalSky appears to list lunar transits on this page only when the Sun is near or below the horizon, so that the observing interval represented each day is shorter and less uniform; and, and least on the page referenced, it won't make predictions of this sort much beyond the current month, so I couldn't explore any but the current Full Moon day (you can make more comprehensive and extensive predictions for individual objects -- especially the ISS -- on other pages). However, the likelihood of a transit over some part of the lunar disk (illuminated or not) during any 12 hours while it is above the horizon should be very similar to the likelihood of a solar transit in the same time span.

From the numbers found above, it looks to me like, on average, from a fixed observing location one would have to watch the entire disk of the Full Moon continuously and intently for more than thirty 12-hour sessions before you could expect to witness the transit of a single satellite 2 arc-sec or larger in apparent size; and something like 1000 full nights to see one greater than 5 arc-sec. This makes me wonder if the 12 or so possible "orbital" objects that David saw drifting across the face of the last Full Moon in a single hour could really have all been space junk. If they were, David's eyes must have been able to see very tiny moving dots. Otherwise its several hundred times the expected rate.

Please note that the above numbers all refer to the "lazy" observer who is waiting for the satellite transit to come to their fixed observing location. Just as you have many opportunities to see a solar eclipse if you are willing to travel far enough, on any Full Moon night the ISS is casting its shadow somewhere on the dark side of the Earth about 50% of the time. If you are willing to go to that place at that time (and the weather is good), you can take a photo of it in transit. Most of the events that CalSky lists as passing within 1.5° of the Sun and Moon can be witnessed as transits by travelling less than 100 km -- sometimes much less. If all of these are included as "observing opportunities" then the probabilities of being able to see one are increased by 36x: each 12-hour Full Moon night there is likely to be an opportunity to witness a 2 arc-sec satellite somewhere within this kind of driving distance (although the predictions may not be accurate enough to say exactly where), and in ~30 12-hour Full Moon nights there is probably an opportunity to witness a 5 arc-sec (or larger) transit (most likely the ISS).

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Regarding "bright satellites," if CalSky has a button to restrict transit listings to those (I didn't see one), I'd avoid it, unless that's your special interest. "Bright" means the satellite is (at least partially) in sunlight, either because it's daytime or because the satellite is high enough up to catch some rays of sunlight even though the Sun is below the horizon from the observer's position on the ground. That's great for naked eye satellite observing because it means you can see a satellite too small to resolve as an artificial star in the darkened twilight sky (satellites that aren't "bright" can't be seen at night at all except in transit). However, for transit observations against the illuminated part of the lunar disk being "bright" may be a bummer: as with transits of the Galilean moons of Jupiter, it means the transiting object is going to blend in with the background. If it isn't very bright (like the ISS in Alan Friedman's daytime animation) the brightness may make it harder to see against the bright part of the Moon.

Just as they can easily be seen moving across any other dark part of the sky, bright satellites can be seen transiting in front of the dark part of the Moon. This web directory contains videos of two examples by Kevin Fetter. If I understand the notations correctly, CalSky identifies #15099 to be "Meteor 2-11" and "Cosmos 1624 Rocket", both about 5 m in maximum dimension and roughly 5th magnitude when seen in twilight at a range of 1000 km. In the videos they are readily visible as (unresolved) artificial stars making rather leisurely ~2 sec transits. But if seen as dark objects in silhouette against a bright Moon, they would be considerably less than 1 arc-sec in diameter.

-- Jim

Edit: try to make the distinction between the probabilities of seeing transits at fixed versus moving observing locations more clear. Link to videos of bright satellites crossing dark Moon.