There is a science that defines the twilight of evening in three stages: when the sun has just dipped below the horizon, up to six degrees below, and when the stars are just visible, it is 'civil twilight'. This is the blue hour beloved of nineteenth-century French writers and painters.
When the sun is six to twelve degrees below the horizon, it is 'nautical twilight', deepening cobalt on a clear night, at the end of which time the horizon is no longer defined by visible brightness. On a summer night a remarkable amount of colour will linger surprisingly late in such a twight sky.
From twelve to eighteen degrees below the horizon it is 'astronomical light' apparently dark to the casual observer, to whom the constellations are as bright as they would be in full darkness.
Civil twilight lasts all night at midsummer at 60 degrees - that glimmer of redness in the northern sky seen from the north of Scotland is the faint reflection of the 'simmer dim' over the northern isles. Nautical twilight lasts thorughout the summer night at 54 degrees north, astronomical twilight at 48 degrees north. Civil twilight over Shetland, nautical twilight over Newcastle and the Scottish Borders, when the sky grows dark on midsummer night over London, it is actually astronomical light.
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