This photo was taken nearly 15 km high above the Irish Sea in April 1985.
It's all about the Concorde supersonic commercial plane flying at Mach 2 speed, i.e. 2 times the speed of sound or approximately 1350 miles per hour.
A small fighter plane approached him in flight and accompanied him for 4 minutes at equal speed, and then the fighter pilot took the photo, until today being the only photo of the Concorde flying at such speed.
At the bottom of the sun shining, and at that height the sky already adopts a darker tonality even in the middle of the day, due to declining air density.
Even the curvature of the Earth can be appreciated at that point.