Twilight cause

oriel36

With a friend heading to Kenya, the issue of a quick onset of darkness there arose compared to the long twilight of Ireland.

The current explanation for twilight and twilight length falls below a flat Earth notion -

https://en.wikipedia.org/wiki/Twilight


It attempts to explain the feature from an individual perspective based on a circumference demarcating the horizon with the observer at the centre which can, under certain circumstances, exist as a number of miles. Even twilight length is 'explained' as the angle of the Sun's descent with variations across latitudes.


Anyone who visits Equatorial regions from Northern latitudes immediately notices the difference in the transition from daylight to darkness with a rapid transition closer to the Equator compared to the slower transition in Northern European latitudes. I enjoy explaining that it is due to rotational speeds as they pass through the planet's divisor separating daylight and darkness.

At the Equator the rotational speed is a maximum 1037.5 mph with speeds diminishing towards either poles reflecting a round and rotating Earth -

10 degrees: 1,021.8 mph (1,644.4 km/h)
20 degrees: 974.9 mph (1,569.1 km/h)
30 degrees: 898.5 mph (1,446.1 km/h)
40 degrees: 794.8 mph (1,279.1 km/h)
50 degrees: 666.9 mph (1,073.3 km/h)
60 degrees: 518.7 mph (834.9 km/h)
70 degrees: 354.8 mph (571.1 km/h)
80 degrees: 180.1 mph (289.95 km/h)


The slower the speed the longer the twilight with a variation based on the orientation of latitudes to the planet's divisor (circle of illumination) across the year.

The exception to this is the Polar day/night cycle and its separate rotational cause with its own rotational speeds. The twilight at the North and South Poles lasts for 6 weeks or so post-Equinoxes but this requires a completely separate treatment

endacl

Jaysus. He’s off again...

oriel36

What would the forum do without the plaintive howl of the Irish urban redneck !.

The Northern Polar twilight which lasts for an extended 6 weeks after the September Equinox is a result of the rotation of the North Pole through the planet's divisor once each orbit -

https://en.wikipedia.org/wiki/Deep_Space_Climate_Observatory#/media/File:EpicEarth-Globespin-tilt-23.4.gif


In terms of this surface rotation parallel to the orbital plane, the North Pole scribes a small circle to the Sun coincident with the circumference of the Arctic circle so easy enough to calculate the average speed of that rotation compared to the maximum circumference on the Equator coincident with the orbital plane.

ps200306

oriel36 wrote: »

Anyone who visits Equatorial regions from Northern latitudes immediately notices the difference in the transition from daylight to darkness with a rapid transition closer to the Equator compared to the slower transition in Northern European latitudes. I enjoy explaining that it is due to rotational speeds as they pass through the planet's divisor separating daylight and darkness...

The slower the speed the longer the twilight...

God help anyone who has had the misfortune to be subjected to your tutelage. The onset and end of twilight are defined by the sun's angle below the horizon. The Earth turns at a constant angular speed. So one thing we know the length of twilight has nothing to do with is the linear speed of the surface. If you doubled the size of the Earth and kept the same angular speed, twilights would remain the same even though the equator is now moving at 2,000 mph instead of 1,000 mph.

Although twilight does get longer as you move away from the equator it's not because the surface is moving more slowly but because the solar parallactic angle -- the angle that the rising or setting sun makes to the horizon -- is changing. Check out the calculations for the duration of twilight:

http://www.gandraxa.com/length_of_day.xml

You won't find the linear speed of the surface entering into the calculation. No doubt you will smell a seventeenth century plot at the mention of mathematics.

endacl

I maintain the differing onset of twilight at different latitudes is a direct result of the prevelance of pine trees at those latitudes. Prove me wrong, OP.

Should keep him occupied for a while. Créatur.

oriel36

ps200306 wrote: »

The Earth turns at a constant angular speed. So one thing we know the length of twilight has nothing to do with is the linear speed of the surface.

Considering you can put the Sun in an RA/Dec sine wave to explain the seasons -

https://community.dur.ac.uk/john.lucey/users/solar_year.gif

Or put the Sun in a hideous figure 8 motion to explain variations in the length of the natural noon cycle -

https://community.dur.ac.uk/john.lucey/users/solar_year.gif

It is inevitable that you would now choose what is effectively a flat Earth circumference where the Sun moves across from horizon to horizon which can amount to 20 miles in some cases -

https://www.weather.gov/images/fsd/astro/twilight.png


The urban redneck's explanation has more validity but then again lack of consideration is that stamp of those who are not astronomers.

Hector Savage

ps200306 wrote: »

God help anyone who has had the misfortune to be subjected to your tutelage. The onset and end of twilight are defined by the sun's angle below the horizon. The Earth turns at a constant angular speed. So one thing we know the length of twilight has nothing to do with is the linear speed of the surface. If you doubled the size of the Earth and kept the same angular speed, twilights would remain the same even though the equator is now moving at 2,000 mph instead of 1,000 mph.

Although twilight does get longer as you move away from the equator it's not because the surface is moving more slowly but because the solar parallactic angle -- the angle that the rising or setting sun makes to the horizon -- is changing. Check out the calculations for the duration of twilight:

http://www.gandraxa.com/length_of_day.xml

You won't find the linear speed of the surface entering into the calculation. No doubt you will smell a seventeenth century plot at the mention of mathematics.

Wow very interesting indeed!

I have to admit I found the OPs explanation of linear speed making sense when I first read his opening post ... :o

oriel36

At the Equator, 15 degrees of geographical separation is 1037.5 miles and also 1 hour time difference as per the 24 hour and Lat/Long system.

https://www.timeanddate.com/sun/ecuador/quito

Quito, Ecuador is close to the maximum circumference of the Earth so it covers a distance of roughly 12,450 miles from the time it exits the planet's divisor (sunrise) and re-enters the divisor 12 hours later (sunset). The length of daylight/darkness is roughly symmetrical in Quito throughout the year.

Dublin, Ireland covers a distance of roughly 600 mph in its rotation with a total circumference 14,400 miles in its daily rotation at a latitude of 53 degrees where 15 degrees of separation is 600 miles. Around the Equinox next month, Dublin will exit the planet's divisor (sunrise) and rotate 7,200 miles until it re-enters the circle of illumination at sunset.

https://www.timeanddate.com/sun/irel...th=3&year=2020


As rotation in anchored to the central Sun and noon, Dublin exits and re-enters the planet's divisor today across a distance of roughly 5,900 miles as the period of daylight is 9 hours 38 minutes presently so a simple matter of multiplication by 600 mph.


A similar latitude like Vilnius, Lithuania shows that wonderful passage of a location through the planet's circle of illumination with the distant and stationary Sun disappearing from view as the planet turns each 24 hour day -

https://www.youtube.com/watch?v=gYEoLn_bqjc

It is possible to fall in love with that spectacle once people are aware of the relationship between rotation beneath our feet and the line dividing daylight and darkness as we pass through it in the evening and back out of it at dawn. It also explains why twilight varies across latitudes as the faster latitudinal speed towards the maximum circumference (equator), the quicker the transition from daylight to darkness. People should spend time with that time lapse, not just for the sake of the technical view but also that it belongs to all life on Earth.

ps200306

Here is Planet X:



We are looking down from the north pole at Dusky City on the equator of Planet X. The planet rotates at 15 degrees per hour. Twilight is defined to end when the Sun is 6 degrees below the horizon. Sunset occurred when Dusky City was at position A. Twilight ended 24 minutes later* when Dusky City was at position B.

What's the linear speed of Dusky City? You'd need to know the scale of the picture. Actually, it's life size. Planet X has a diameter of 4cm, Duskunians are a hundred times smaller than an atom, and Dusky City moves at five millionths of a kilometre per hour. Its twilight lasts about as long as at Nairobi on planet Earth ... BECAUSE ONLY ANGULAR -- NOT LINEAR -- SPEED MATTERS.


* Actually, twilight's a couple of minutes shorter because sunset is defined by the disappearance of the upper limb of the Sun while twilight is measured by its geometric centre. And the apparent Sun is elevated by refraction at sunset, but not at the end of twilight.

ps200306

oriel36 wrote: »

Quito, Ecuador is close to the maximum circumference of the Earth so it covers a distance of roughly 12,450 miles from the time it exits the planet's divisor (sunrise) and re-enters the divisor 12 hours later (sunset). The length of daylight/darkness is roughly symmetrical in Quito throughout the year.

Dublin, Ireland covers a distance of roughly 600 mph in its rotation with a total circumference 14,400 miles in its daily rotation at a latitude of 53 degrees where 15 degrees of separation is 600 miles. Around the Equinox next month, Dublin will exit the planet's divisor (sunrise) and rotate 7,200 miles until it re-enters the circle of illumination at sunset.

As rotation in anchored to the central Sun and noon, Dublin exits and re-enters the planet's divisor today across a distance of roughly 5,900 miles as the period of daylight is 9 hours 38 minutes presently so a simple matter of multiplication by 600 mph...

Here is Planet X again. Orange Island is on the equator, Cerise City is more northerly. We see both locations at the equinox, and then at the northern solstice:



Count the number of lines of longitude visible in the daylight hemisphere for each location at the equinox, and again at the solstice. At Orange Island it doesn't change. But Cerise City has a greater angle to turn to pass into darkness at the solstice than Orange Island. Since everywhere on Planet X rotates at a constant angular speed the day is longer in Cerise City at the solstice. It's really that simple. And it has nothing to do with linear speed as both cities are moving slower than a sleepy amoeba. It is purely a function of latitude. (I already provided the math where you can check this).

oriel36 wrote: »

Dublin, Ireland covers a distance of roughly 600 mph in its rotation with a total circumference 14,400 miles in its daily rotation at a latitude of 53 degrees where 15 degrees of separation is 600 miles.

600 mph is a speed, not a distance. And if you multiply that 600 mph by your fifteen degrees per 600 miles, you get 15 degrees per hour. That is the constant angular speed of the Earth -- the same at Dublin as it is as Quito. It's also the constant angular speed of Planet X which is why, although it is 300 million times smaller, it has the same day lengths and twilights as Earth for a given latitude.

endacl

Any word on the pines?

ps200306

He's working on it.

oriel36

ps200306 wrote: »

He's working on it.

Shame that those two talented men and their song is misused by those who lack inspiration and talent because of a self-inflicted adherence to an empirical ideology that lacks astronomical integrity and substance.

Twilight brings on the softening of human nature unknown to urban rednecks which is why it is loved the world over as dusk, the gloaming, the end of the day and just simply twilight when the lights are low -

https://www.youtube.com/watch?v=gYEoLn_bqjc

https://www.youtube.com/watch?v=uUSIRmzJc3w

( Lovely playing Jim McCann's singing with the time lapse footage simultaneously as our planet turns through the divisor each day)


We are fortunate to live at a latitude where there is a soft transition from daylight to darkness due to our rotational speed.

ps200306

oriel36 wrote: »

We are fortunate to live at a latitude where there is a soft transition from daylight to darkness due to our rotational speed.

It's got nothing to do with our rotational speed. It's to do with the fact that the Sun intersects the horizon at a shallow angle at sunset. At the solstice it is moving much faster in azimuth than in altitude so it sinks very slowly. Both the (linear) rotational speed and solar parallactic angle are functions of latitude. But the rotational speed is also scale dependent and entirely unnecessary.

oriel36

If other contributors want to discuss the sinking Sun, the Sun in a sine wave or the Sun doing a figure 8 then so well and good, this material on the other hand is for those who actually like the links between a rotating and orbiting Earth with experiences on the surface including why the transition from daylight to darkness (twilight) varies across latitudes.


At the North Pole presently, the first signs of Polar dawn are beginning to appear leading to Polar Sunrise on the March Equinox while at the South Pole, the same Equinox will begin events after Polar sunset and the long, slow twilight into Polar night -

https://www.timeanddate.com/sun/antarctica/south-pole?month=4


The North Pole will turn through the Earth's divisor over 5 weeks time but it will be already bright there whereas the South Polar latitude will sink or descend into darkness over 6 weeks.

The Sun tracks in opposite directions at either Poles but this too is a feature of the Earth's daily and orbital motions rather as the Sun is stationary and central to the spectacle -

https://www.youtube.com/watch?v=okw6Mu3mxdM

https://www.youtube.com/watch?v=xOCCSegL8ic&t=26s