An eminent astronomer writes (A twilight saga, part two)

**Caution- this blogpost contains no birds, and no estuary**

One e-mail stood out in my in-box (as, obviously, as it was the only one). An eminent astronomer, a Mr Trellis, MPhys, MSci, of The Gower, South Wales, had taken the time to correspond upon the recent Twilight Saga post.

Their' source of information was Norton's Star Atlas (the amateur astronomers bible), and provided some excellent specifics. I quote:

"Twilight from ancient times, has been reckoned as ending when the the Suns centre is 18 degrees below the horizon, 6th magnitude stars then being visible at the zenith; it has no definite duration, however, as meteorological conditions may modify it. The glow, in its later stages is a segment of a circle , brightest vertically over the sun. Directly opposite the indigo-blue segment of the unilluminated atmosphere rises from the east as the sun recedes from the horizon.

"Twilight lengthens with distance from the equator, and is shortest all over the Earth about the equinoxes. The total variation never exceeds half an hour below latitude 40 degrees, and in higher latitudes, some 10-20 minutes during autumn, Spring, and winter; but above Lat 40 degrees, in summer twilight lengthens, till it lasts all night above 50 degrees. Civil, Nautical, and Astronomical (British Nautical Almanac) end when the sun's centre is 6, 12 and 18 degrees below the horizon- the first about the limit when "ordinary outdoor operations become impractical without artificial light" "

I have written thanking Mr Trellis for this new information. He continued by providing some specific timings for twilight duration at several latitudes, including:

Latitude.                                                Winter solstice.       Equinoxes.       Summer solstice

50 (a tad below London)                       1hr 59min.           1hr 50min.          Lasts sunset to sunrise
55 (around about Edinburgh)                 2hr 17min.           2hr 4min.                                  "

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Having lured me in, Trellis then delivered his sucker punch. It appears knowledgeable astronomers insist twilight occurs only at dusk, never at dawn.

Had I upset the professionals? I'm no astronomer...

It called for some etymological research:
twilight (n.) "light from the sky when the sun is below the horizon at morning and evening," late 14c., a compound of twi- + light (n.). The exact connotation of twi- in this word is unclear, but it appears to refer to "half" light, rather than the fact that twilight occurs twice a day. Originally and most commonly in English with reference to evening twilight but occasionally used of morning twilight (a sense first attested mid-15c.).

So, used from time to time for dawn twilight for about 500 years.

It called for some ornithological research as well, by turning round and pulling the Poyser 'Birds by Night' off the shelves behind me (and yes, dear reader, perhaps I should have done so on the earlier post). The author is described by some as "the world's leading authority on nocturnal birds and one of the world's foremost vision researchers". Thankfully my initial post hadn't veered that far from his text. Admittedly, it took me to page 83, but there it was- "...passerines involved in morning twilight song..."

So, I am indebted to Mr Trellis for putting me through my paces, and causing me to stumble upon other excellent terms such as 'dusk chorus' and 'zeitgeber*' for the first time. I am aware Mr Trellis attends astronomical conferences at Taggs Coffee Shop at Hoo St. Werburgh from time to time, so I shall have to drop in to purchase him a cup by way of thanks.

* Zeitgeber- a rhythmically occurring natural
phenomenon which acts as a cue in the
regulation of the body's circadian rhythms.