diamond geezer

 Saturday, October 28, 2006

"It's getting dark earlier isn't it?"
"And it'll be even darker tomorrow when the clocks go back."
"Still, at least it'll be a bit lighter in the mornings."


Here's a scarily-detailed response to the above conversation...

A brief classification of darkness*
• Sunset
: the time at which the centre of the sun slips beneath the horizon.
Civil twilight: the period during which the centre of the Sun is less than 6 degrees below the horizon. Outdoor objects can still be clearly distinguished, the horizon is clearly defined and the brightest stars are visible. It's not completely dark.
Nautical twilight: the period during which the centre of the sun is between 6 and 12 degrees below the horizon. Outlines of outdoor objects may be distinguishable, but detailed outdoor operations are not possible and the horizon is indistinct. It's nearly completely dark.
Astronomical twilight: the period during which the centre of the Sun is between 12 and 18 degrees below the horizon. Illumination of the sky is practically imperceptible. It's completely dark, unless you're an astronomer.
Darkness: the period during which the centre of the Sun is more than 18 degrees below the horizon. It's pitch black.
(* under good atmospheric conditions in the absence of moonlight or other illumination)

How dark is it at 6pm?
Daylight: 17 March → 18 October
Sunset: 19 October (ie last week)
Civil twilight: 20 October → 28 October (would have been longer, but the clocks go back tomorrow)
Nautical twilight: (skipped when the clocks go back)
Astronomical twilight: 29 October → 24 November (suddenly very dark)
Pitch black: 25 November → 24 December
Astronomical twilight: 25 December → 28 January
Nautical twilight: 29 January → 20 February
Civil twilight: 21 February → 15 March
Sunset: 16 March (before the clocks go forward)

How dark is it at 5pm?
Daylight: 8 February → 28 October (and then the clocks go back)
Civil twilight: 29 October → 6 November
Nautical twilight: 7 November → 15 January (it never gets completely dark)
Civil twilight: 16 January → 6 February
Sunset: 7 February

How dark is it at 7am?
Daylight: 24 February → 20 October
Sunrise: 21 October (ie last week)
Civil twilight: 22 October → 28 October (and then the clocks go back)
Daylight: 29 October → 4 November (yes, it really is daylight again)
Sunrise: 5 November (but not for long)
Civil twilight: 6 November → 28 November
Nautical twilight: 29 November → 3 February (it never gets completely dark)
Civil twilight: 4 February → 22 February
Sunrise: 23 February

>> All figures are for London - even though it's never dark in sodium-glow London
>> Further in-depth information about twilight can be found here, here, here and here
>> You can find day-by-day twilight details for London here, and other world locations here


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