With the ISS circling the globe every 90 minutes, chances are good it will fly through a patch of sky near you. But when ? There is a lot of math involved and helpful programmers and their computers solved the challenge. Various websites & phone-apps can keep track of this schedule. Looking at a few tools, I found results would differ significantly -- 30min discrepancy for an event lasting < 3 seconds isn't inspiring confidence in any of the tools.
The official site by NASA is spotthestation.nasa.gov/sightings/index.cfm
www.isstracker.com/ has a real-time view with map & Google-Earth background
www.calsky.com & heavens-above.com can provide you with tons of daily information, based on your location. And there are plenty more sources.
STELLARIUM & STARRY NIGHT (SE) can simulate a satellite's position in their sky maps and download updates to keep those simulations accurate. So far, it appears Starry Night's data isn't accurate or up-to-date enough for predictions more than a day or two in advance. It has the biggest mismatches of all I have tested when I asked for a simulation 5 ...10 days in advance. Once the date of the transit was only a few days into the future, Stellarium's simulation converged with CALSKY's predictions still had 2 minutes difference. With V0.12.4, Stellarium improved even further. On the day of the transit, Starry Night's simulation closely matched the results from the other tools

If you want to determine a TRANSIT of the ISS in front of the Moon or Sun, chances are slim it will happen right now, right where you are. To find IF, WHEN & WHERE this will occur within a reasonable driving distance, again CALSKY & HEAVENS- ABOVE can do that.: www.calsky.com/cs.cgi/Satellites/4 and other too even provide you with (daily) email updates and alerts. Starry Night & Stellarium don't provide any ground maps.

Some time ago, I introduced you to my SOFTWARE TOOLCHEST and now it is time to make a few additions to that to explain how you can predict (hopefully accurate) the position of the ISS and determine WHEN & WHERE you have to be to capture this event. CALSKY and similar WEB-resources are very helpful in providing DAILY (email) updates of all kinds of events & conjunctions. Usually these (email) predictions don't consider the local weather. The IMPORTANT fact of such tools is, THEY PROVIDE A STARTING POINT & REMINDER

A solar or lunar transit of the ISS doesn't occur frequently -- getting an alert 2 weeks in advance helps to prepare. Even if the initial details aren't 100% accurate, it still may be possible to see that event if you can drive to a suitable location. CAUTION : Some tools can make predictions even months in advance but the ISS frequently makes course / altitude adjustments, rendering month-long forecasts inaccurate or even useless.

All other tools I have found, their maps showed just ONE transit at a time and only showed the center path. Trying an old, now discontinued JAVA script, I found it would combine multiple transits and draw their paths using KML & Google Earth. To me that is a lot more useful, wheighing the driving to reach the different locations. CALSKY also shows only the CENTER PATH, seing the boundaries IMHO also is useful.
This JAVA script has a short window for it's predictions (just 10 days). Possibly to improve accuracy, since the ISS is frequently adjusting its orbit, thus voiding previous predictions..

Combining the data with Google EARTH also allows to better draw, rotate & tilt 3D-maps (you can't do that with Maps) --- the sample I show below is a screenshot, combining Google Earth & CALSKY timing predictions
STELLARIUM, CALSKY and STARRY NIGHT can simulate the view (and time) of the transit. To get an accurate predictions, you need accurate GPS coordinates. Look up your observation site on Google Maps or Earth or use your GPS or Phone to determine the location and enter that into the tools. Sadly none of the tools can import a list of locations stored in a KML/KMZ file.

EARTH & CALSKY don't simulate the view of the Sun & ISS and the path the ISS will take across the sun. As you can see in the map above, the transit is visible along a CENTER PATH where the ISS' transit lasts the longest and passes through the middle of the sun's (or moon's) disc. If you are not on that blue line, you still can see the transit but the closer you get to the red lines, the more off-center and shorter the transit will be. It helps to use STELLARIUM to simulate the view -- for that you must use VERY ACCURATE GPS coordinates of your (planned) observation site, e.g. your backyard. In the attached example you see my backyard is close but not on the blue center path. But close enough for me -- not worth the hassle of driving & setting up in a remote location !!

That brings us to the last detail -- HOW to capture this event. For a lunar ISS transit, use the longest lens you have. If you are familiar with shooting solar eclipses or the Transit of Venus, the preparations for a solar ISS transit are similiar. One important difference though, the ISS transit is a 50-meter dash compared to a 1mile run (eclipse totality) or a marathon (Venus transit). The ISS will stay in front of the sun for 3 seconds -- at the most. (totality usually is ~ 3 minutes, Venus transit lasted 5 hours).

The camera & lens setup will be SIMILAR, but not the same as for the "Transit of Venus". One key difference is the very short window of opportunity which doesn't allow much room for mirror lock-up delays and human errors.

Add to this the long uncertainty in the predictions (3s event, 30s uncertainty), I plan to NOT SHOOT still images and instead will attempt to capture high-def video instead.

Since the ISS isn't visible in the time leading up to the transit, pointing the camera at the exact location is tricky. With a wide FOV, you point it at the moon or sun and that's good enough. To get a higher resolution image of the ISS, I was exploring different options but so far haven't found a satisfactory answer to that tracking issue. On the left you see to examples of 2 camera's FOV attached to different telescopes.
In my shots during ToV and solar eclipse, the original size of the sun and moon were ~ 1800pix wide (900mm telescope). 1800 / 30 ==> ISS length ~80pix. OTOH, with a conventional 300mm lens & APS-C crop factor, the disc's diameter may be ~450 pixels, resulting in an ISS a mere 15x15 pixels. That probably is large enough to recognize the shape if the solar panels are spread across. 80x80 pixels hopefully should reveal a few more details, like attached Sojus transporters. Longer focal length will yield larger size of the ISS but with a smaller FOV, you also risk missing the transit if the actual position & predictions isn't very accurate.

I think the safest approach will be to capture video. No shutter vibrations and up to 90 frames during a 3sec transit. A D3 might capture ~27 frames if it weren't for the shutter vibrations, other SLR ~15 frames (or less, depending on buffer size). Of course even HD-video cannot match the 12...18MP resolution of a RAW file..

Other factors to consider when planning to observe a transit
  • for LUNAR TRANSITS, make sure the path of the ISS passes over the bright portion of the moon. Not all transits occur during full moon when the entire disc is illuminated.
  • Again for LUNAR TRANSITS, be wary of predictions that still have the Sun in the sky. ESPECIALLY when Sun & Moon are close together you risk burning your eyes by accident. In such situations, contrast may be insufficient to see the ISS.
  • For transits, you should disable the option "illuminated by the sun" because the ISS passes in front of either the sun or the brightly lit portion of the moon and that obstruction is what you see..
  • Be sure your lens magnifications are up to the task -- check how big the full moon is in your photos. The ISS will be 1/30 that diameter, possibly even less !! size comparison : /stargazer95050/28183923
  • Be aware of the distortions the air-mass will contribute to all objects close to the horizon : /stargazer95050/21434571
  • Test shots a day or more in advance under similar conditions help -- try looking at the rising moon and see how blurry all features are.
  • Use a lot of caution when attempting to capture SOLAR TRANSITS.

When using a SLR, pressing the shutter at the right moment will be a challenge. Not that I doubt your ability to hook up that cable release but seeing that tiny & fast dot against a bright light isn't easy. The footnote of these predictions often state error can be +- 30 seconds.
Starring through the viewfinder while pointing upward at the moon, a minute or two can feel like an eternity. I would advise against using LifeView because of the lower resolution (fewer display pixels & the compression might remove such tiny "dust specs" like the ISS. Also, with LifeView, you might add more delay. And you don't have much time.
Let me know how all this worked out for you.


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