Tuesday, February 17, 2015

Burning sky

We've had a couple of glorious days. Yesterday and today, I took Laurie down to the Garden Walk to bask in the sun. (He basked, I sought out too-narrow stripes of shade.) And then, on the way home this afternoon, I was treated to a head-swivelling, eye-blistering sunset. Pink everywhere overhead, to the south, east, and north; and to the west, flaming yellows and oranges, simmering down to reds in my rear-view mirror as I turned into our driveway.

The traffic was heavy, so I pulled into a bus stop by an open schoolyard near home to get a photo.

5:43 PM. 

This was 10 minutes past the "official" sunset time, as given by timeanddate.com. But isn't this a sunset?

Looking at one of the sunrise/sunset pages, I noticed that our "sunsets" fall after sunset, during the twilight period. To confuse the matter, there are three ways to measure this twilight: there's Civil twilight, Nautical twilight, and Astronomical twilight.

Definitions:
  • Sunset is
 "when the last part of the Sun is about to disappear below the horizon (in clear weather conditions)." (And over a flat horizon.) (timeanddate.com)
  • Civil twilight
... begins at sunset and ends when the geometric center of the sun reaches 6° below the horizon (civil dusk). Civil twilight can also be described as the limit at which twilight illumination is sufficient, under clear weather conditions, for terrestrial objects to be clearly distinguished; at the ... end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under clear atmospheric conditions. (Wikipedia, CT)
  • Nautical twilight begins
... at sunset and end(s) when the center of the sun is 12 degrees below the horizon. In general, nautical twilight ends when navigation via the horizon at sea is no longer possible. (Wikipedia, NT)
  • And Astronomical twilight goes on until 
... the center of the sun is 18 degrees below the horizon. From the end of astronomical twilight in the evening ... the sky (away from urban light pollution) is dark enough for all astronomical observations. (Wikipedia, AT)
There's a handy diagram on Wikipedia that sort of clears this up.

Wikipedia, by TWCarson


Complicated, and irrelevant to my purposes. I would like to propose a useful new category of twilight: Fiery Twilight. I'll define it as beginning when the first tinge of yellow or pink shows up in the eastern sky and ending when the last hint of glowing embers dies in the west. The length of time will vary, except that it will invariably be too short.

Another Skywatch post; two this week!

2 comments:

  1. I love that Burning Sky! I enjoyed reading the info! We have had a few burning sunsets here too!

    ReplyDelete
  2. I enjoyed your information and support your suggestion for Fiery Twilight, at least here on the west coast where we get fabulous sunset displays. - Margy

    ReplyDelete

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