The peacock made of sunlight, hidden in a dome

Here is the Sheikh Lotfollah detail you can walk right under it and miss it. Everyone who steps into this mosque looks up, because the dome overhead really is one of the loveliest things in Iran, a slow swirl of cream and blue tilework rising to a medallion at the centre. On its own it would be worth the trip.

But at certain hours the dome does something extraordinary. Around its base sit sixteen lattice windows, and when the sun comes through them it lays a shaft of light across the tiles, and the whole pattern resolves into a peacock. Its body is the central medallion, and its shining tail is made entirely of that beam of sunlight fanning out beneath it. As the sun moves, the tail lengthens and shifts and slowly fades, so the bird is never the same twice and is gone for most of the day. The mosque was built as a private, inward place for the royal household, with no minaret and no courtyard, and this hidden peacock feels made for exactly that.

I painted it in watercolour and ink, the dome and its impossible tail of light, and the whole time I kept thinking about how the real masterpiece only shows itself if you happen to be standing there at the right hour, looking up. There's a poster with a letter home and a postcard if you'd like one.