EARLY this morning, at around 3 am, Irish time the sun attained the most northerly reach of its "annual apparent oscillation north and south of the equator. It was the summer solstice, the central point when the nights are pleasantly short, and are bracketed at each end by the lengthy twilights that we know as dawn and dusk.
"Twilight" describes the time when the sun is just hidden from view, but light from it is still reflected or scattered by the atmosphere. It begins at sunset which, strictly speaking, occurs the instant the upper edge or "limb" of the solar disc appears to coincide with the horizon. Allowing for the effects of refraction - the bending of the rays of light from the sun by the atmosphere - sunset occurs when the sun's centre is geometrically slightly less than one degree below the horizon.
The end of twilight, however, is less clear cut. "Civil twilight", for example, ends when the centre of the solar disc is 6 degrees below the horizon, but "nautical twilight" is for many purposes a more practical definition, and ends when the sun's centre is 12 degrees below; by this time, it is for all practical purposes, completely dark, the constellations can be distinguished overhead, and a distant horizon is no longer visible, except in moonlight.
At our latitude the length of twilight varies, throughout the year, depending on the angle that the path of the setting sun makes with the horizon. At a large angle of incidence such as occurs near the equinoxes - the sun shoots straight down behind the horizon with no nonsense, and nautical twilight lasts only for an hour so. But at this time of year the path of the setting sun is at a relatively shallow angle to the horizon, and takes considerably longer to reach a position where it is 12 degrees below; for this reason nautical twilight at the latitude of Cork, for example, lingers for a full 127 minutes.
Moreover, if you were to proceed northwards from Cork at present, you would find the twilight lengthening by about one minute for every mile travelled at Belfast, nautical twilight at the summer solstice lasts right throughout the night. And of course if you were to continue northwards and cross the Arctic Circle, you would find yourself at this time of year in the proverbial land of the midnight sun where there is no darkness, no twilight of any kind and where, when skies are cloudless, the sun is visible throughout the 24 hour day.