This maybe the wrong ng, but why is it that after the winter solstice, for
sometime, the evenings get lighter, while the morning continue to get
darker?

Joe

On Tue, 9 Dec 2003 21:55:34 -0000, Joe wrote in


This maybe the wrong ng, but why is it that after the winter solstice, for
sometime, the evenings get lighter, while the morning continue to get
darker?

This is a very tricky question to answer in a few lines. Suffice it to say
it is partly down to the earth's elliptical orbit around the sun, and the
varying speed of rotation at different times of year. One has to
understand the connection between clock time and true sloar time. This URL
will help understand it - I hope:-)

http://www.analemma.com/Pages/framesPage.html

--
Mike Coleraine posted to uk.sci.weather 09/12/2003 22:06:23 UTC
My aurora images here http://www.mtullett.plus.com/29a-oct and
http://www.mtullett.plus.com/20-nov/

On Tue, 9 Dec 2003 22:06:23 +0000, Mike Tullett wrote in

snip
One has to
understand the connection between clock time and true sloar time
snip

LOL In case that needs deciphering "sloar" should read "solar". BTW the
darkest evening is in about 5 days time, yet the darkest morning is at the
end of December, another 20 days. The shortest day length remains at the
winter solstice of course.

--
Mike Coleraine posted to uk.sci.weather 09/12/2003 22:10:52 UTC
My aurora images here http://www.mtullett.plus.com/29a-oct and
http://www.mtullett.plus.com/20-nov/

On Tue, 9 Dec 2003 21:55:34 -0000, "Joe"
wrote:

This maybe the wrong ng, but why is it that after the winter solstice, for
sometime, the evenings get lighter, while the morning continue to get
darker?



One of Keppler's Laws says that the area swept out by a body in its
orbit rotating about another is a constant for a given time. In an
elliptical orbit the distance of the orbiting body varies with respect
to the orbited body. For the area swept in a given time to be
constant, the speed of the orbiting body will be greater when its
closer to the orbited body and slower when it's further away.

Because the orbit of the Earth is elliptical, it's orbital speed
varies, but it's rotational speed about its axis is constant. The
effect of this is to skew sunrise, noon and sunset times
with respect to Standard Time. The amount of this skew is called the
Equation of Time and tables are published giving its value throughout
the year. Hence, to labour the point, the Sun is only due south at
noon at the Greenwich Meridian on about four dates in the year. At
other times it can be as much as twenty minutes earlier or later.
However, sunrise and sunset are roughly symmetrical about the time
that the Sun crosses the meridian so that if the Equation of Time on a
particular day is ten minutes then the Sun will cross the meridian ten
minutes after noon and sunrise and sunset will also occur ten minutes
later than if the Equation of Time didn't exist, but not symmetrically
about noon.

After the Winter Solstice logic indicates that sunrise should occur
earlier. However, around the Winter Solstice the Equation of Time is
increasing at a greater rate than sunrise is decreasing. Hence,
sunrise actually increases after the Solstice until early January when
the two rates equalise and after that sunrise moves in the 'right'
direction.

The Equation of Time is zero on or about 15th April, 14th June, 1st
September and 25th December.Only on those days will the Sun cross the
Greenwich Meridian at noon.

Phew!

--
Alan White
Twenty-eight miles NW of Glasgow.
Overlooking Loch Goil and Loch Long in Argyll, Scotland.
http://tinyurl.com/55v3

On Tue, 09 Dec 2003 23:17:16 +0000, Alan White wrote in


On Tue, 9 Dec 2003 21:55:34 -0000, "Joe"
wrote:

This maybe the wrong ng, but why is it that after the winter solstice, for
sometime, the evenings get lighter, while the morning continue to get
darker?


snip the good explanation
The Equation of Time is zero on or about 15th April, 14th June, 1st
September and 25th December.Only on those days will the Sun cross the
Greenwich Meridian at noon.

Phew!

Well done Alan! As you know full well, trying to use few words in that
explantion is very difficult. There is the added factor of the tilt angle
effect, but both that and the orbital speed effect work togther this month
and into the new year, as seen on the graph on this page:

http://www.analemma.com/Pages/Summat...Summation.html

--
Mike Coleraine posted to uk.sci.weather 09/12/2003 23:41:15 UTC
My aurora images here http://www.mtullett.plus.com/29a-oct and
http://www.mtullett.plus.com/20-nov/

On Tue, 9 Dec 2003 23:41:15 +0000, Mike Tullett
wrote:

Well done Alan! As you know full well, trying to use few words in that
explantion is very difficult.

Thanks, Mike.

This question came up at about this time last year so I'd had some
practice.

--
Alan White
Twenty-eight miles NW of Glasgow.
Overlooking Loch Goil and Loch Long in Argyll, Scotland.
http://tinyurl.com/55v3

"Alan White" wrote

This question came up at about this time last year so I'd had some
practice.

I had a letter of explanation published in New Scientist many years ago.

My 84 year old mother-in-law is currently staying with us. She managed to
grasp just a little of the explanation - well at least, she now knows that
the shortest day and the earliest sunset do not coincide and has taken on
board a vague idea why.

I now plan to explain to my eleven year old daughter's hamster and see if
Elvis - yes, that is his name - can understand:-)

Jack

On Wed, 10 Dec 2003 15:16:38 -0000, "Jack Harrison"
wrote:

I now plan to explain to my eleven year old daughter's hamster...

That could be really difficult!

--
Alan White
Twenty-eight miles NW of Glasgow.
Overlooking Loch Goil and Loch Long in Argyll, Scotland.
http://tinyurl.com/55v3

"Alan White" wrote in messagein reply to


I now plan to explain to my eleven year old daughter's hamster...

That could be really difficult!

Not really. I did initially get unstuck with the partial differentials, but
after I had checked my College notes from 1959, it soon became quite clear
again.

But of course, some of us have problems with the most basic of concepts-
Can't help them. Sorry.

Jack

In message , Jack
Harrison writes

"Alan White" wrote in messagein reply to


I now plan to explain to my eleven year old daughter's hamster...

That could be really difficult!

Not really. I did initially get unstuck with the partial differentials, but
after I had checked my College notes from 1959, it soon became quite clear
again.

How did you check if the hamster understood?


But of course, some of us have problems with the most basic of concepts-
Can't help them. Sorry.

Jack



--
Five Cats
Email to: cats_spam at uk2 dot net