That's a really great image! I'm interested to know if you have determined which flight this was, and where you got your distance value of 8km? Looking at the flights from Sydney to Brisbane that landed after 8:00pm local time, I found this list from the website Flightaware. Whether your plane is on this list would depend on whether it was actually one of the Sydney to Brisbane flights of course.
The arrival times listed above aren't accurate, but after clicking on the individual flights I could get more detailed info, including time stamped GPS data in approximately 30s increments for the duration of each flight. Very interesting.
Anyway, using the Earth-Moon distance at the time of your image, we can calibrate the image scale as you presented above at about 2.25"/px by making some measurements of known lunar features near the sub-Earth point. The plane can then be sized, although there is considerable inaccuracy in deciding where to measure and because of perspective distortion. I don't know whether this is an Airbus A320 or 737-800, despite looking at some images, but you probably know better than me. Anyway, I reasoned that the diameter of the engine cowling would be fairly free from distortion, and this value is about 2.44m for the 737-800, and very similar for the Airbus. Doing the trigonometry then places the aircraft at about 5200m line of sight and the altitude at about 5000m (since the Moon was high in the sky, these values are similar) but with some significant room for error. This would be an altitude of a bit over 16,000 feet.
Looking at the flight data, nothing matches perfectly unfortunately, probably due to some errors in my estimates, as well as perhaps inaccuracies in the GPS data on that free website (altitude is often fairly inaccurate). But the closest match was Virgin Australia #977, which was located at the indicated position below at the time your image was taken (within a few seconds), and at that time had a reported altitude of 12,225 feet (34% error from my estimate, but errors from measurement as well as GPS altitude would compound). I don't know if this makes any sense based on your location or not. There are a couple of other flights possibilities, although those aircraft were located about 80km further south at the time your image was taken, so that might make sense if you were imaging farther away. Those aircraft were about 25,000 feet altitude, though, so that would mean my estimates were off by about 50% in the other direction, but I suppose that's possible. That altitude would agree with your estimate of 8km distance line of sight however, but would require you to have been imaging from a considerable distance to the south in the map above. And the aircraft would have been over the land, not the water, which would be a good piece of information to clear things up.
And of course, maybe the aircraft isn't even on my list if it's a different flight route! And flight #977 could be crossed off the list if the aircraft in your image is not a 737-800, but I'm not confident on making that call. But I'd be interested to hear if you have more information. Once again, nice image!
Edit: As shown in the post below, I made an error reading the time stamp in the image. This changes the list of possible flights, but other parts of the analysis remain valid (albeit for the wrong time) so I'm leaving the post as written above. This will make sense after reading the posts below.
Edited by Tom Glenn, 13 September 2019 - 02:01 AM.