Twilight in astronomy

The current explanation is exceptionally poor and unusable in the 21st century -

"At Greenwich, England (51.5°N), the duration of civil twilight will vary from 33 minutes to 48 minutes, depending on the time of year. At the equator, conditions can go from day to night in as little as 20-25 minutes. This is true because at low latitudes the sun's apparent movement is perpendicular to the observer's horizon." Wikipedia

For world travelers who are familiar with the rapid transition from daylight to darkness as they travel from higher to lower latitudes the only thing they concern themselves with are the latitudinal speeds which cause the variation in transition periods as the Earth swings through the circle of illumination and out of it within each 24 hours. This means getting rid of any apparent motion of the Sun and focusing on the motions of the Earth. This is done though a simple graphic -

http://vaab.blog.kal.fr/2012/04/20/s...s-vs-earth.jpg

The greater the surface distance covered by a location from a maximum at the Equatorial circumference,the faster that location turns through the circle of illumination and the less drawn out twilight will be,likewise with dawn as that location turns back into solar radiation.

Polar twilight is exceptionally slow and arises from a separate cause,in this case the orbital surface rotation as the North/South poles turn through the circle of illumination creating the experience of with orbital twilight or orbital dawn.

"oriel36" wrote in message
...

The current explanation is exceptionally poor and unusable in the 21st
century -

"At Greenwich, England (51.5°N), the duration of civil twilight will vary
from 33 minutes to 48 minutes, depending on the time of year. At the
equator, conditions can go from day to night in as little as 20-25 minutes.
This is true because at low latitudes the sun's apparent movement is
perpendicular to the observer's horizon." Wikipedia

For world travelers who are familiar with the rapid transition from daylight
to darkness as they travel from higher to lower latitudes the only thing
they concern themselves with are the latitudinal speeds which cause the
variation in transition periods as the Earth swings through the circle of
illumination and out of it within each 24 hours. This means getting rid of
any apparent motion of the Sun and focusing on the motions of the Earth.
This is done though a simple graphic -

http://vaab.blog.kal.fr/2012/04/20/s...s-vs-earth.jpg

The greater the surface distance covered by a location from a maximum at the
Equatorial circumference,the faster that location turns through the circle
of illumination and the less drawn out twilight will be,likewise with dawn
as that location turns back into solar radiation.

Polar twilight is exceptionally slow and arises from a separate cause,in
this case the orbital surface rotation as the North/South poles turn through
the circle of illumination creating the experience of with orbital twilight
or orbital dawn.

================================================== ====
In Britain and probably many other places around the world we have something
called "lighting up time", the hour at which motorists are required to
illuminate their vehicles with red lights at the rear and white lights at
the front. Lighting up time depends on twilight.
Kelleher's ranting drivel is exceptionally poor and unusable in the 21st
century or any other.

Every student has a right to exercise their imagination in tandem with common where latitudinal speeds dictate how long it takes a location to change from daylight to darkness or what effectively amounts to the same thing - variations in twilight lengths across latitudes.

http://www.nasa.gov/images/content/2..._full_full.jpg

https://www.youtube.com/watch?v=YXCnxoixb-s

This is an issue which differentiates the intuitive mind and the mathematical mind for the latter will find an alternative explanation using not only the apparent motion of the Sun but a nonsensical appeal to the Earth's curvature as a means to explain what is actually the job of latitudinal speeds.

Despite the slow acceptance of the proper dynamics behind twilight lengths,not just across latitudes but also the partitioning between daily and orbital twilights, the better explanation will take hold and eventually be enjoyed by the wider community.

"oriel36" wrote in message
...

differentiates the intuitive mind and the mathematical mind

============================================
"Intuitive mind" - filled with supernatural dreams unrelated to reality.
"mathematical mind" - scientific, explanatory, reasonable, logical.

"oriel36" wrote in message
...
This is an issue which differentiates the intuitive mind and the mathematical mind for the
latter will find an alternative explanation using not only the apparent motion of the Sun but
a nonsensical appeal to the Earth's curvature as a means to explain what is actually the job
of latitudinal speeds.

It's only by considering the Earth's curvature, and using spherical trigonometry, that the true
variation in twilight times over different latitudes can be fully determined.

Your 'explanation' is overly simplistic and fails to take into account, for example, the
distance the Earth's surface has to turn at different latitudes during the twilight period.

On Thursday, April 17, 2014 7:43:39 PM UTC+1, Gavino wrote:
"oriel36" wrote in message

...

This is an issue which differentiates the intuitive mind and the mathematical mind for the

latter will find an alternative explanation using not only the apparent motion of the Sun but

a nonsensical appeal to the Earth's curvature as a means to explain what is actually the job

of latitudinal speeds.



It's only by considering the Earth's curvature, and using spherical trigonometry, that the true

variation in twilight times over different latitudes can be fully determined.


There is one dawn and one twilight within each 24 hour period as long as the Sun is in view daily so what will happen to you is that you will come to break out of that hard shell of indoctrination which assumes that the Earth is 3 minutes 56 seconds into the next rotation after 23 hours 56 minutes 04 seconds and up to 24 hours. By concentrating on the Earth's maximum Equatorial speed you will eventually come to understand that the Earth turns once in a 24 hour day and one of the effects is that as the surface speed increases towards the Equator,the twilight lengths become shorter.




Your 'explanation' is overly simplistic and fails to take into account, for example, the

distance the Earth's surface has to turn at different latitudes during the twilight period.

That's the style !,now you are beginning to think so keep up the good work and you will soon get into the stream of astronomy where there are so many of these causes and effects waiting to be discovered and promoted to a world that really needs a stable astronomical narrative. Be content that you are among the first people to encounter the true cause of latitudinal variations in twilight length in tandem with the separate polar twilight/dawn and the orbital generated surface rotation behind it.

Remember now,the Earth's motions are behind the effect and nothing to do with any apparent motion of the Sun ,if in doubt then just go back to the Sun/Earth comparison and the flawed current view won't take hold.

http://www.waltonandjohnson.com/cont...comparison.jpg

Cults hate common sense so you can easily judge where you are in the scheme of things through this basic insight.

Making a joke about sparkly vampires would probably do as much good as attempting any kind of serious reply to you.

I understand that you feel that referencing the Earth's rotation to the stars instead of to the Sun is somehow un-Copernican. But because the Earth's orbit is an ellipse and not a perfect circle around the Sun, and thus the Earth doesn't move along its orbit at a uniform rate (as Kepler tells us) we have no choice.

I know you don't think much of mathematics. I know you think it's somehow demeaning to think of the Earth as a heavy rotating body, subject to the same laws as a spinning top.

But your feelings are blinding you to the fact that today's astronomers know what they're doing, and their calculations and physical models lead to accurate results - allowing space probes to travel to distant worlds, allowing Neptune to be discovered through its subtle gravitational effects. So experience proves your approach is wrong and fruitless, which is why you aren't finding anyone here to take an interest in it.

John Savard

On Wednesday, April 16, 2014 11:13:57 PM UTC-7, oriel36 wrote:

The current explanation is exceptionally poor and unusable in the 21st century

The existing definition is 100% accurate and very easy to understand. Astronomical twilight is over, and it finally as dark as it is going to get, when the center of the Sun is 18 degrees below the horizon. Period. No need to thank me.

It doesn't matter how the Sun got there, the rotation of the Earth, the angle of the Sun to the horizon, the angle of Earth's poles to the Sun or to the ecliptic, no other variables, none of these make any difference at all (even if they all contribute one way or another), the only thing that matters is the 18 degrees below the horizon.

On Friday, April 18, 2014 4:10:55 AM UTC+1, palsing wrote:
On Wednesday, April 16, 2014 11:13:57 PM UTC-7, oriel36 wrote:



The current explanation is exceptionally poor and unusable in the 21st century



The existing definition is 100% accurate and very easy to understand. Astronomical twilight is over, and it finally as dark as it is going to get, when the center of the Sun is 18 degrees below the horizon. Period. No need to thank me.


The issue,as if you didn't know,is why twilight lengths vary with latitude so the answer is fairly straightforward and nothing like the unthinking idea of the apparent angle of the Sun's descent.

The actual answer requires the observer to think in terms of the Earth's motions,that there are two types of twilights to consider,the crucial size comparisons which breaks the idea of the Sun being above/below anything and all those things which carry the observer into new astronomical territory.

https://www.youtube.com/watch?v=YXCnxoixb-s





It doesn't matter how the Sun got there, the rotation of the Earth, the angle of the Sun to the horizon, the angle of Earth's poles to the Sun or to the ecliptic, no other variables, none of these make any difference at all (even if they all contribute one way or another), the only thing that matters is the 18 degrees below the horizon.


You should all go to Church today on what is the most sacred day in the Christian calendar just to get a feel for that human spirit that doesn't die .