 To tourists , one of the fascinations of the islands 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 . 
