Timing the glow

You have to be quick these days.

As the days get shorter, the minutes of orange morning sky dwindle. I see the blaze in my window, run for the camera, pull off the lens cap, ... and the colour is already fading. I timed it this morning; from a pale orange to when there was just a faint tint of creaminess to the blue — 2 minutes.

Civil dawn, the period between the moment the sun reaches 6 degrees below the horizon and the actual sunrise, when it rises above the horizon, today, at this time of year and this latitude, 50° north, runs from 7:21 to 7:54 AM. On clear days the sky is mostly blue, but if conditions are right, an orange glow spreads out along the horizon, sometimes finding clouds to paint.

From my window, 7:21 AM, 10 days ago.

Our day here on the island varies in length from 16 hours, 23 minutes of daylight at the summer solstice, June 21, gradually decreasing to 8 hours (and 4 minutes) on Dec. 21. I was surprised to discover that the length of the twilight does not follow this pattern. This week, mid-October, here in Campbell River, civil twilight (the morning portion) lasts 33 minutes, but then it begins to lengthen, getting up to 39 minutes at the winter solstice. Then down again to 32 minutes in March, and up to 45 minutes in June.

It's all in the geometry; the angle of the sun from our position in the trip around it, combined with the angle at which the rays hit a round object (us), depending on our latitude, and then the angle of the axial tilt.

The migrating birds know more about this than we do.

I found this on Wikipedia. It shows the sunlight and twilight lengths near Frankfurt, Germany, but the latitude, 50° north, is the same as ours. The magenta line shows the civil twilight.

Carpet plot of sunshine at 50° north. (Wikipedia)

A Skywatch post.

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Hay que estar alerta en estos dias.

Los dias se están encogiendo, y por lo tanto, también los momentos en que el cielo se incendia. Veo el color por mi ventana, corro a recoger la cámara, le quito la tapa de la lente, y ... el color ya se va desapareciendo. Le medí el tiempo esta mañana; desde que vi las nubes anaranjadas hasta que nada más quedaba un tinte de crema en lo azul fueron 2 minutos.

El crepúsculo civil matutino, el intervalo entre el momento en que la luz del sol llega desde una altura de 6° debajo del horizonte hasta la salida del sol, hoy, en esta época del año, a esta latitud, o sea 50° Norte, corre desde las 7:21 horas hasta las 7:45 de la mañana. Si el dia es claro, el cielo es azul, pero si las condiciones son buenas, un brillo anaranjado se extiende desde el horizonte; a veces encuentra nubes y las pinta de amarillo o anaranjado.

Foto: Desde mi ventana a las 7:21 hace 10 dias.

Nuestro dia, aquí en la isla varía desde 16 horas con 23 minutos de sol en el solsticio de verano, el 21 de junio, y disminuyendo lentamente hasta comprender 8 horas con 4 minutitos el 21 de diciembre. Me sorprendió descubrir que los crepúsculos no siguen este modelo. Esta semana, a mediados de octubre, aquí en la isla, el crepúsculo civil matutino dura 33 minutos. Después aumenta, llegando a 39 minutos en el solsticio de invierno. Disminuye de nuevo hasta 32 minutos en principios de marzo. Y luego en junio, dura 45 minutos.

Todo es cosa de geometría. Depende del ángulo desde que nos llega la luz del son en nuestro viaje alrededor del astro, combinado con el ángulo de incidencia de la luz en el globo terráqueo según la latitud del observador, y otra vez combinado con el ángulo de la inclinación axial.

Las aves migratorias entienden todo esto mejor que nosotros.

Encontré esta gráfica en Wikipedia. Muestra las horas del dia y de crepúsculo a lo largo del año en Frankfurt, Alemania, que está a la misma latitud, 50° Norte, que nosotros. La linea de color magenta muestra el crepúsculo.

Gráfica: Plano del sol a 50° Norte. (Wikipedia)

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!