I am not sure my question is on topic but I am going to fire it likewise.

I was wondering about the straight line captured in the picture posted at the bottom left side.

enter image description here

If not mistaken, I 've seen some similar pictures before, it must be the trail of the ISS station.

For instance posting the picture in astrometry.net doesn't quite work and I can't locate the ISS path at that date and time (21/08/2017 2:00am EST, SW sky as seen from Paros island, Greece).

Can I verify somehow this is actually the trail of ISS?


It is most likely to be an aeroplane. The image isn't very clear, but the line looks slightly dotted, indicating that it may be the flashing white light on an aeroplane as it's passing over during the exposure.

As far as I know, the International Space Station doesn't appear to regularly flash like this. The only time it would appear to flash is if the sunlight happens to reflect off one of the solar panels, but it's unlikely to be repeated consistent flashes.

How long was the exposure on this photo?

  • 30sec. f/8 18mm ISO25600 Nikon d3400. Would it help to share the .nef? – forgotten_novel_char Dec 5 '17 at 11:41
  • @kapelnick if you look at the original photo nearer to actual size, does the line appear to be dotted? If so, that would probably confirm that it's a plane – laurencemadill Dec 5 '17 at 11:54
  • Yes it's dotted. I also checked and the airport must be nearby towards the same direction. So I guess it is a plane either taking of or peparing for landing. – forgotten_novel_char Dec 5 '17 at 12:02
  • If it is dotted, it is not the ISS. Compare my shot carefully tracked and anticipated with the help of Heavens Above: flickr.com/photos/chili5558/9605278010 – chili555 Dec 5 '17 at 23:33

As the others answered, it's not the ISS, but I wanted to answer about the posibility of it being the ISS anyway.

At first I thought it can't be the ISS, because I like to observe the ISS at night and it's moving much faster than in your picture, but then I made some more precise calculations.

Your 18mm lens on a crop body has a 66°×47°; 76° (H×V; D) field of view. Your trail is 68 pixels long, and your image diagonal is 961 pixels, so your trail occupies 68/961 × 76° = 5.37°. At 30s exposure this means an average angular velocity of 0.179°/s.

Your trail is approximately 215 pixels over the horizon, and the image is 533 pixels tall, so 215/533 × 47° = 18.9° over the horizon. Let's assume the ISS is α=18.9° over the horizon relative to your position. Assuming the height of the ISS (h = 405km) is much smaller than the earth radius, some basic trigonometry put its distance relative to you to D = h/sin α = 1253km. Now its observed angular velocity depends on its motion relative to you, but let's go for the average scalar velocity, which is ωₐ = 1/(2π) * Integral[ω*|sin[x]|, 0, 2π], where ω is the best case (tangential motion) velocity. ω = v/D, where v = 7.76km/s, the orbital speed of the ISS, so ω = 6.19 × 10⁻³ rad/s. Evaluating the integral we get ωₐ = 3.94 × 10⁻³ rad/s. In 30s it would have moved `0.118 rad = 6.76°.

This number is comparable to your trail, which surprised me, because I know the ISS can move much faster. But of course, I observe it in the best circumstances, when it's directly above. If the ISS is directly above, D=405km and ω = 19.16 × 10⁻³ rad/s = 1.097°/s.


You could always go to http://www.heavens-above.com/ and set the proper location, date and time and use "Daily predictions for brighter satellites" - it works for past dates.

  • I used heaves-above and it seems all ISS passes where non visible that day due to daylight. It might as well have been an iridium flare. As per astronomy.net, it doens't work probably because of the noise in the image. – forgotten_novel_char Dec 14 '17 at 6:28

It could possibly have been Cosmos 1844, which was passing through Sagitarius, low in the SW, and tracking in the direction indicated by your photograph, at about that time (midnight UTC). The Ariane 5 rocket booster was also in the area, but not tracking in the direction indicated by your photo. Unfortunately, I am not skilled enough to identify the constellations in your photograph. Alas, the ISS was nowhere to be seen over Europe when you made your exposure. If you install Stellarium and set up your coordinates, you will be able to predict and watch satellite transits on your computer.

Even if the object in your photo was an aircraft, Stellarium is a very useful tool for an amateur astronomer, and great fun to use. Using Stellarium, I was able to predict an ISS transit over the UK, went outside, and to my great surprise, I saw the ISS chasing one of the last Space Shuttle flights across the sky. Of course, professional astronomers (and serious amateurs) use software like Stellarium to avoid satellite transits.

Stellarium screen-shot


  • 1
    I have used Stellarium before to map the night sky without ever using the satellites option. I will do a further research and let you know if anything comes up. – forgotten_novel_char Dec 6 '17 at 15:25
  • "Unfortunately, I am not skilled enough to identify the constellations in your photograph." ---> upload it on nova.astrometry.net – user258532 Dec 12 '17 at 21:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.