The important element left out of that scenario is a dark sky which is not glowing bright blue.simonshack » November 6th, 2017, 4:50 am wrote: Let's do a little thought experiment: Imagine yourself spotting a 747 flying overhead your garden one sunny afternoon - at about standard cruising altitude (ca. 10 km), moving from West to East (the plane, as it proceeds Eastwards, is therefore well lit by the sun from behind). As it moves away from you, the plane obviously becomes smaller and smaller. The cruise speed of a Boeing 747 is about 900 km/h. Now, imagine if (by magic) this 747 were capable of ascending to about 400 km of altitude. Well, let's say that (after having passed overhead of you in your garden) it starts slowly ascending and, 1440 km later, it finally reaches 400 km of altitude.
Now, since the plane would employ about 1.6 hours (1440km / 900 km/h = 1.6) to cover those 1440 km, you would have to patiently stand in your garden for 1.6 hours, watching your 747 fading away in the distance - while slowly ascending to 400 km of altitude. (So make sure you have adequate supplies of soda and popcorn at hand ... 1.6 hours [i.e. 1h and 36 minutes] is a pretty long wait!)
The question is : do you truly / honestly think that, even given the most ideal of visibility conditions imaginable (i.e. super-clear sky / sun shining directly on the ascending shape of the large 747 wings), the plane would still be visible to your naked eyes, 1.6 hours after it passed over your garden?
You wont see it in the daytime.
If the sky were clear and dark and the bottom of the plane was in bright sunlight, yes I think it would be visible. You wouldn't be able to tell it was a plane without some good magnification, but with the naked eye you would see a slowly moving point of light.