The simmer dim is a very weird light. Affecting the Northern Isles of Scotland, it is a strange half light where evenings are endless and nights are brief or non-existent. Here, the light does not simply get darker, rather it becomes progressively less intense like a fading light bulb that needs to be changed. After nearly two weeks on Orkney, I’m getting used to it…almost!
To tourists, one of the fascinations of Orkney and the Other Northern Isles is their “nightless” summers. On the longest day in Shetland there are over 19 hours of daylight and complete darkness is unknown. This long twilight is known in the Northern Isles as the “simmer dim”. Winter nights are correspondingly long with less than six hours of daylight at midwinter. At this time of year the aurora borealis can occasionally be seen on the northern horizon during moderate auroral activity.
Additional information: https://en.wikipedia.org/wiki/Northern_Isles#Climate