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  #11  
Old October 21st 18, 03:11 AM posted to sci.astro.amateur
Quadibloc
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Posts: 6,965
Default OT? Amateur Astronomy

On Tuesday, October 16, 2018 at 1:13:56 AM UTC-6, Gerald Kelleher wrote:

I see this fella is around to remind you all to keep quiet and bully you into silence.


In fact, we are grateful to him for contributing an interesting post concerning
the topic that most of us came here to read about - genuine amateur astronomy.

John Savard
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  #12  
Old October 21st 18, 06:35 AM posted to sci.astro.amateur
Barry Schwarz[_2_]
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Default OT? Amateur Astronomy

On Sat, 20 Oct 2018 08:05:02 -0700 (PDT), Quadibloc
wrote:

On Thursday, October 18, 2018 at 11:35:06 PM UTC-6, Gerald Kelleher wrote:

A person capable of using their intelligence will recognise that the RA/Dec
framework is in competition with the Lat/Long system for the basic answer to the
question 'How long does it take the Earth to turn once ?'. The reasonable answer
is once in 24 hours with quite a long explanation covering timekeeping from its
emergence in antiquity to demonstrate the rules which tie cyclical timekeeping
to cyclical dynamics to a close approximation.


The naive answer is indeed 24 hours.

If the Earth didn't turn, wouldn't one part of the Earth always face the Sun,
the way Mercury was once thought to do, and the way one side of the Moon always
faces the Earth?


If the Earth didn't turn at all, then the time from one sunrise to the
next would be approximately 24*365.25 hours. One side of the Moon
always faces the Earth precisely because its rotation about its axis
"exactly" matches it revolution around the Earth.


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  #13  
Old October 21st 18, 07:11 AM posted to sci.astro.amateur
Quadibloc
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Posts: 6,965
Default OT? Amateur Astronomy

On Saturday, October 20, 2018 at 11:35:25 PM UTC-6, Barry Schwarz wrote:

If the Earth didn't turn at all, then the time from one sunrise to the
next would be approximately 24*365.25 hours. One side of the Moon
always faces the Earth precisely because its rotation about its axis
"exactly" matches it revolution around the Earth.


Yes, that's correct. However, I was ttying to explain things to someone who
doesn't agree with that, and so I was starting from his point of reference.

John Savard
  #14  
Old October 21st 18, 07:57 AM posted to sci.astro.amateur
Gerald Kelleher
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Posts: 1,443
Default OT? Amateur Astronomy

On Sunday, October 21, 2018 at 6:35:25 AM UTC+1, Barry Schwarz wrote:
On Sat, 20 Oct 2018 08:05:02 -0700 (PDT), Quadibloc
wrote:

On Thursday, October 18, 2018 at 11:35:06 PM UTC-6, Gerald Kelleher wrote:

A person capable of using their intelligence will recognise that the RA/Dec
framework is in competition with the Lat/Long system for the basic answer to the
question 'How long does it take the Earth to turn once ?'. The reasonable answer
is once in 24 hours with quite a long explanation covering timekeeping from its
emergence in antiquity to demonstrate the rules which tie cyclical timekeeping
to cyclical dynamics to a close approximation.


The naive answer is indeed 24 hours.

If the Earth didn't turn, wouldn't one part of the Earth always face the Sun,
the way Mercury was once thought to do, and the way one side of the Moon always
faces the Earth?


If the Earth didn't turn at all, then the time from one sunrise to the
next would be approximately 24*365.25 hours. One side of the Moon
always faces the Earth precisely because its rotation about its axis
"exactly" matches it revolution around the Earth.


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There is no hypothetical 'if'.

When daily rotation and all its effects are subtracted, the entire surface of the Earth still turns through a complete rotation to the Sun each year. It is the reason why there is a single Polar day/night cycle with sunrise/sunset on the Equinoxes and Polar noon/midnight on the Solstices.

The rotational characteristics of the Polar day/night cycle are strictly a function of the Earth's orbital motion and expressed in those terms -

http://afewbitsmore.com/img/2015_ecliptic.png

The orbital rotation rate across the maximum diameter is 24,901 miles/ 365.25 or roughly 68 miles per day (denoted by the broken line) while at the North/South polar latitudes and its rotational circumference of 10,975 miles, the rotational speed reduces to about 30 miles per day. For this reason the North and South Poles turn through a distance of 5,487.5 miles from one Equinox to the following Equinox so the Sun stays constantly in view for 6 months and out of sight for another 6 months .

To be fair, I see a few people try to play catch-up but that will change as better graphics and less strident objections exist. The Polar day/night cycle and its cause is a reality so no need to tiptoe around it for the sake of less productive explanations. I wouldn't be beholden to the old fool nor the other 'best boy in the class' contributors as you are going to have to work this out in your own mind with your own perceptions and not borrowed from others.




 




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