simmer dim
n.— «At the moment the sun sets around 10.30 in the evening, and rises again about 3.30am. The five hours in between are a time of prolonged dusk, known in Shetland as the simmer dim. If the sky is clear then it remains light enough to read outside for the entire night.» —“Fair Isle: The simmer dim” by Malachy Tallack New Statesman (London, United Kingdom) June 25, 2007. (source: Double-Tongued Dictionary)
Further reading
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