Is it dark between 3 and 4 pm? That's when they decended to the minimum holding pattern of 6,000

Sunset was at 4:25 PM in Seattle on November 24, 1971. It was quite dark by 4 PM given the weather.

I'm reading sunset November 1971 was 4:20 to 4:55

The sunset was 4:24 PM for November 24, 2019. The sunset will be essentially the same time year in and year out. That said, I Googled for November 24, 1971 and it came in at 4:25 PM.

Thats when sunset begins..it becomes completely dark by 4:55.

Shutter, hold on here.

Sunset is a specific time which is 4:20PM in your illustration.  Quoting from Wikipedia:  "The time of sunset is defined in astronomy as the moment when the upper limb of the Sun disappears below the horizon.  Near the horizon, atmospheric refraction causes sunlight rays to be distorted to such an extent that geometrically the solar disk is already about one diameter below the horizon when sunset is observed."

Again quoting from Wikipedia:  "Evening civil twilight begins at sunset and ends when the geometric center of the sun reaches 6 degrees below the horizon."  Civil twilight starts at 4:20 PM and ends at 4:55 PM in your illustration.

And again quoting from Wikipedia:  "Evening nautical twilight begins when the geometric center of the sun is 6 degrees below the horizon in the evening and ends (nautical dusk) when the geometric center of the sun is 12 degrees below the horizon in the evening."  Evening nautical twilight begins at 4:55 PM and ends at 5:33 PM in your illustration.

The cloud cover does not figure into the above definitions.  But cloud cover means that things are going to get dark fast and maybe even before sunset depending on the circumstances.

Maybe Georger can provide additional information on the astronomy related to the above.

Shutter, if you really want to start something then define sea level.  It is not as simple as it appears. 

I was rebuilding my BBQ yesterday and noticed a few things pertinent to this conversation. I had to get my drop light to see adequately. It was 3:15 pm and clouds were coming in. It felt too dark to work by 4-ish. Also, I often turn my headlights on by 4 pm when I'm driving this time of year.

The darkest dark I have ever seen has been out here in Cooper Country at 5 pm with heavy think clouds. Driving in the rain under these conditions without lots of urban lighting is tough.

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Thats when sunset begins..it becomes completely dark by 4:55.

Shutter, hold on here.

Sunset is a specific time which is 4:20PM in your illustration.  Quoting from Wikipedia:  "The time of sunset is defined in astronomy as the moment when the upper limb of the Sun disappears below the horizon.  Near the horizon, atmospheric refraction causes sunlight rays to be distorted to such an extent that geometrically the solar disk is already about one diameter below the horizon when sunset is observed."

Again quoting from Wikipedia:  "Evening civil twilight begins at sunset and ends when the geometric center of the sun reaches 6 degrees below the horizon."  Civil twilight starts at 4:20 PM and ends at 4:55 PM in your illustration.

And again quoting from Wikipedia:  "Evening nautical twilight begins when the geometric center of the sun is 6 degrees below the horizon in the evening and ends (nautical dusk) when the geometric center of the sun is 12 degrees below the horizon in the evening."  Evening nautical twilight begins at 4:55 PM and ends at 5:33 PM in your illustration.

The cloud cover does not figure into the above definitions.  But cloud cover means that things are going to get dark fast and maybe even before sunset depending on the circumstances.

Maybe Georger can provide additional information on the astronomy related to the above.

Shutter, if you really want to start something then define sea level.  It is not as simple as it appears.

Sunset varies according to altitude above sea level - R99 is correct Usable light on the ground after sunset usually lasts 30-45 minutes after sunset. We usually start observing 30 mins after sunset in 'semi-dark' conditions. We define 'dark sky' as starting 2 hours after sunset. All of this is dependent on humidity, clouds, phase and position of Moon vs no Moon, so the first thing I want to know is the phase and position of the Moon that night relative to sunset and Cooper's jump time say at around 8:10-15. If Cooper bailed near an lighted urban area then there is ground lighting from reflection of light of cloud layers above. Was it foggy near the ground?

All of that is why Ive always been interested in getting ground reports near Vancouver for that night, or ground reports from people wherever Cooper jumped and drifted. I have yet to see any good ground reports from people in these different areas for that night. I contacted several observatories and they didn't have any recorded reports for that night. I contacted several small airports and they couldnt come up with any pilot or airport reports either, with one exception. The retired manager of the Troutdale airport gave a vague report based on a flight that came in about 7:00pm that night, from a small airport north of Troutdale near V23. I have that report in my files and will dig it out when I get time.   

If I am reading a very faded copy of the Portland sequence report for 8:00 PM PST on November 24, 1971, the ground visibility at Portland International Airport was 10 statute miles in light rain showers.  The observation reported in this sequence report would typically be made about 5 to 10 minutes before the hour.

4:20 pm

4:54pm

5:36...total darkness with visible overcast.