Upside downside high up in the Chicago sky

This is a rare and great photo of Chicago taken from a plane as it descends to land. But why is it upside down?

Plane enthusiast and amateur photographer Mark Hersch captured the jaw-dropping scene of the city silhouetted in Lake Michigan from his seat on a Chicago-bound flight making its final approach.
The late-afternoon image shows the low sun casting a golden glow on the water, which is visible beneath cloud cover.
‘I was flying home to Chicago from a business trip recently,’ he said. ‘It was a cloudy day, late in the afternoon. We were flying eastbound, made a pass by O’Hare International Airport, then made a sweeping 180-degree left turn over Lake Michigan for our final westward approach into the airport.
‘I looked down and through a narrow break in the clouds, I saw the shadow of the Chicago skyline projecting onto the lake. Oddly enough, I am a very frequent flyer and almost always sit in an aisle set, but on this flight there were only window seats available.
‘I grabbed my iPhone and snapped off a single shot, hoping I captured the scene. When I got home I opened the photo on my computer. Not only was I amazed at the shot, but I noticed I also captured another plane on a parallel approach – which you can see if you look closely at the clouds above the skyline in the centre of the image.