Defining Dusk

The Teen and I went for a bit of a wander this afternoon to shake off some school and work demons. It proved very effective really and we discovered that there's more to the fen than first meets the eye.
When we arrived, it was late afternoon and we were a bit perturbed that the gates are locked at dusk. When, exactly, is dusk?
We wandered through to the river and got some good shots - this one selected for its transience. Slight mishap of lens cap in the river and a wobbly pontoon but a much better humour ensued!