Re: length of day question

oriel36 wrote:

On Aug 8, 4:57 pm, palsing <pnals...@xxxxxxxxx> wrote:

On Aug 8, 8:15 am, oriel36 <kelleher.ger...@xxxxxxxxx> wrote:


It also helps you to understand why the transition from daylight to
darkness is more rapid at Boca Raton than at latitude of Cambridge as
that location rotates at a slower speed into the orbital shadow.The
new explanations for seasonal differences is more complicated yet just
as enjoyable but if you believe in the 'sidereal time' value for daily
rotation,it is not possible to enjoy the basic planetary facts

You have it backwards. If you cannot understand that it is primarily
the angle at which the suns hits the horizon at various latitudes that
determines the length of twilight as opposed to the varying surface
speeds at various latitudes, then you simply are not seeing the whole
picture and taking all available information into account.



Too embarrassing to even counter - the rotational speeds at various
latitudes with the maximum speed of 1669.8 km per hour at the Equator
explain why there is a rapid transition from daylight to darkness in
places like Kenya while the rotational speed of 837 km per hour at
60 degrees latitude,roughly the latitude of Helsinki,they experience
a longer transition to darkness or twilight as it is known.

http://www.ncgia.ucsb.edu/education/curricula/giscc/units/u014/tables/table02.html






There is no "new" explanation for seasonal differences, we have
understood these basics for a long time. The Earth does what it does,
we can't change anything, we can only observe and then develop a model
that explains it all.



This is the 21st century and with the power of modern imaging it is
possible to make planetary comparisons in order to arrive at a true
explanation for the seasons and the dynamics behind it,this includes
the replacement of 'tilt' as a dynamic and the assignment of its true
influence in determining equatorial or polar conditions .

http://hubblesite.org/newscenter/archive/releases/1999/11/video/b

The real innovation is not modelling but via planetary comparisons and
drawing conclusions based on traits,as Uranus is far enough away from
our planet and the central Sun,it is possible to use its unique polar
characteristics and its equatorial rings to facilitate a better
explanation for the Earth's climate and seasonal explanation by
replacing 'tilt' with the specifics of orbital motion.



In the end, it it YOU who does not really enjoy the basic planetary
facts.


Basic arithmetic - the Equatorial speed is 111.32 km per 4 minutes /1
degree, 1669.8 km per hour/15 degrees and 40,075 km every 24 hours/360
degrees,until you get that fact straight ,go away and join the other
'sidereal time' guys who can't accomplish basic multiplication -

http://www.ncgia.ucsb.edu/education/curricula/giscc/units/u014/tables/table02.html

Wrong again Gerald. The observer's latitude determines the angle that the setting sun makes with the western horizon and this determines the length of twilight. When the sun sets obliquely to the horizon it takes longer to travel any vertical distance below the horizon. This is because part of it's motion is parallel to the horizon. (Google- vector addition) Since the vertical component (vector) of the setting angle determines how fast it gets dark, twilight lasts longer at higher latitudes where the angle to the horizon is more shallow or conversely less vertical. At lower latitudes twilight is shorter because the setting angle is more vertical (less parallel) to the horizon. This is particularly noticeable at the equator where the motion of the setting sun has no parallel component with the horizon and consequently sets straight down. This vertical effect produces the shortest twilight period on the earth and it is also relatively constant throughout the year. At the equator all of the sun's motion contributes to it going below the horizon as fast as possible in comparison to any other location/latitude where it sets at an angle to the horizon. This is regardless of the seasons.

There is also another detail to this subject. In the northern summer the sun sets in an arc which curves northward away from the horizon. This results in the sun going below the horizon at a slower rate (less vertical motion) than it would if the path was a straight line but still at the same angle to the horizon as the arc. The curved path extends the length of twilight even longer than it would be if the path were straight. In the northern winter the sun still sets at an angle to the horizon but now the arc curves downward towards the horizon (more vertical motion) and twilight is therefore shorter than in summer. Of course all of the above is reversed for an observer in the southern hemisphere but the causes and effects are still comparable.

So the general rule regarding twilight for the northern hemisphere is that twilight lasts longer when the declination of the sun is north of the celestial equator (summer) than it does when the sun is south of the celestial equator (winter). All you have to do is look up the length of twilight for your location in the summer and winter, assuming you don't live on the equator. If the difference in rotational speed (tangential velocity) at the surface of the earth relative to latitude is responsible for the differences in the length of twilight, you have to explain why the length of twilight varies during the year for all locations other than the equator. Does the rotational rate of the earth vary during the year?

Looks like it's back to the drawing board Gerald.
.

Relevant Pages

  • Re: length of day question
    ... speeds at various latitudes, then you simply are not seeing the whole ... explanation for the seasons and the dynamics behind it,this includes ... the setting sun makes with the western horizon and this determines the ... the Earth rotates into the orbital shadow at different speeds so allow ...
    (sci.astro.amateur)
  • Re: A Couple of Nights ago
    ... twilight at different latitudes.Bad people I guess. ... is true that there are variations of twilight at different latitudes, ... the speed of the Earth at various latitudes, ... the angle at which the suns hits the horizon is closest to vertical ...
    (sci.astro.amateur)
  • Re: A Couple of Nights ago
    ... the Equator and a slower twilight at 60 degree latitude is due to the ... all points on the surface of the Earth ... and the rotational speeds at different latitudes. ...
    (sci.astro.amateur)
  • Re: length of day question
    ... at the equator and longer twilights towards the geographical poles is ... rotational speeds are the main cause of daylight/darkness transition ... these vacationers had come from Northern European latitudes. ... It's not surprising that they noticed this twilight effect in their ...
    (sci.astro.amateur)
  • Re: length of day question
    ... at the equator and longer twilights towards the geographical poles is ... rotational speeds are the main cause of daylight/darkness transition ... these vacationers had come from Northern European latitudes. ... It's not surprising that they noticed this twilight effect in their ...
    (sci.astro.amateur)