Solstices and Equinoxes and Day-Length.

The Solstices are when the Sun, considered as a body moving round a
spinning but otherwise fixed Earth, is at its furthest north and south;
and the Equinoxes are when it is over the Equator (the formal
definitions may be a little different).

Those terms were introduced in or near Europe, where the latitude is,
say, +45+-15 deg. It is well-known that the Summer Solstice marks the
longest daylight of the year and the winter the shortest, and that the
Equinoxes are when daylight and darkness are of equal duration, give or
take twilight, refraction, ... ; and that the same occurs in
corresponding southern latitudes --- but that named events occur six
months displaced, and with any north & south in the descriptions
exchanged.

Clearly, for any day of the year, the length of the day must vary
smoothly along the length of the Greenwich Meridian; and at any point on
the meridian it must vary smoothly throughout the year.

But what happens in the Tropics, within ~23deg of the Equator?

At the equator, to first order day and night are always each 12 hours,
which just makes the word Equinox linguistically unjustified there; but
the word Solstice is still etymologically appropriate there as
everywhere.

But how does the day-length behave through the year at latitudes of,
say, 15deg? Near Senegal, Honduras, Vietnam, Mauritius, Darwin, Lima,
for example?

While the daylight duration will not vary much in such places, it will
vary; and ISTM that it will have two different minima at the Solstices,
with therefore maxima in between.

Does anyone know of a good description or graphical representation?

--
© John Stockton, Surrey, UK. Turnpike v4.00 MIME. ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
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"Dr John Stockton" wrote in message
...
The Solstices are when the Sun, considered as a body moving round a
spinning but otherwise fixed Earth, is at its furthest north and south;
and the Equinoxes are when it is over the Equator (the formal
definitions may be a little different).

Those terms were introduced in or near Europe, where the latitude is,
say, +45+-15 deg. It is well-known that the Summer Solstice marks the
longest daylight of the year and the winter the shortest, and that the
Equinoxes are when daylight and darkness are of equal duration, give or
take twilight, refraction, ... ; and that the same occurs in
corresponding southern latitudes --- but that named events occur six
months displaced, and with any north & south in the descriptions
exchanged.

Clearly, for any day of the year, the length of the day must vary
smoothly along the length of the Greenwich Meridian; and at any point on
the meridian it must vary smoothly throughout the year.

But what happens in the Tropics, within ~23deg of the Equator?

At the equator, to first order day and night are always each 12 hours,
which just makes the word Equinox linguistically unjustified there; but
the word Solstice is still etymologically appropriate there as
everywhere.

But how does the day-length behave through the year at latitudes of,
say, 15deg? Near Senegal, Honduras, Vietnam, Mauritius, Darwin, Lima,
for example?

While the daylight duration will not vary much in such places, it will
vary; and ISTM that it will have two different minima at the Solstices,
with therefore maxima in between.

Does anyone know of a good description or graphical representation?

--
© John Stockton, Surrey, UK. Turnpike v4.00
MIME. ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms &
links;
some Astro stuff via astro.htm, gravity0.htm ; quotings.htm, pascal.htm,
etc.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail
News.

John

I do not know offhand, but perhaps you could open up Skymap or any similar
program, place your location at Lagos or any of the other places you
mention, plot the daylight time (sunset minus sunrise) at 10-day intervals
throughout the year, and let us know the answer. Maybe compare this with
the results of a similar exercise for a location just outside the Tropics.

Something you can do this one cloudy evening.

Best regards, Roger

John,

Not quite what you are after, but related, is some material on my website
looking at the length of twilight, rate of change of length of twilight (day
by day), and altitude of M27 at a time of constant sky brightness. Have a
look under "Astronomy and Cosmology" on the site below.

If it's of any interest, I'll dig out the equations I used ( if I can from
my well-commented model :-) ). From memory the Excel model I describe could
be used to answer your point of interest.

Cheers
--
Alan Robinson
http://homepage.ntlworld.com/robinsonfizz/Alansworld/

"Dr John Stockton" wrote in message
...
The Solstices are when the Sun, considered as a body moving round a
spinning but otherwise fixed Earth, is at its furthest north and south;
and the Equinoxes are when it is over the Equator (the formal
definitions may be a little different).

Those terms were introduced in or near Europe, where the latitude is,
say, +45+-15 deg. It is well-known that the Summer Solstice marks the
longest daylight of the year and the winter the shortest, and that the
Equinoxes are when daylight and darkness are of equal duration, give or
take twilight, refraction, ... ; and that the same occurs in
corresponding southern latitudes --- but that named events occur six
months displaced, and with any north & south in the descriptions
exchanged.

Clearly, for any day of the year, the length of the day must vary
smoothly along the length of the Greenwich Meridian; and at any point on
the meridian it must vary smoothly throughout the year.

But what happens in the Tropics, within ~23deg of the Equator?

At the equator, to first order day and night are always each 12 hours,
which just makes the word Equinox linguistically unjustified there; but
the word Solstice is still etymologically appropriate there as
everywhere.

But how does the day-length behave through the year at latitudes of,
say, 15deg? Near Senegal, Honduras, Vietnam, Mauritius, Darwin, Lima,
for example?

While the daylight duration will not vary much in such places, it will
vary; and ISTM that it will have two different minima at the Solstices,
with therefore maxima in between.

Does anyone know of a good description or graphical representation?

--
© John Stockton, Surrey, UK. Turnpike v4.00
MIME. ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms &
links;
some Astro stuff via astro.htm, gravity0.htm ; quotings.htm, pascal.htm,
etc.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail
News.

At a rough approximation Lagos is 4N 4E according to my atlas. Feeding
that into my web page
http://www.aphayes.pwp.blueyonder.co...ris/index.html
the sunrise gor the month of the winter solstice varies from 05:36 to
05:50 and sunset from 17:30 to 17:44, which is the same as my partners
experience of a holiday in Kenya, the day varies very little, sun up
between five and six and sets about 12 hours later. Remember the
midnight summer sun and extended winter darkness at the poles so
logically near the equator there is very little change. Meeus is the
'bible' for amateurs to program how to calculate these sort of
phenomona, http://www.willbell.com/math/mc1.htm

Pete H

"Dr John Stockton" wrote in message
...
But what happens in the Tropics, within ~23deg of the Equator?

Another thing to consider is the rapidity of the onset of darkness. With the
sun skimming less of the horizon twilight is very short.

Dr John Stockton wrote

At the equator, to first order day and night are always each 12 hours,
which just makes the word Equinox linguistically unjustified there; but
the word Solstice is still etymologically appropriate there as
everywhere.

Surely they are both equally justified or unjustified since every day is
arguably both an equinox and a solstice.

The words themselves are Latin-based, while observation of the events
clearly long predates the evolution of written Latin. It would be
interesting to know how different cultures described the events
linguistically.

--
Hil

One characteristic of a solstice is that the Sun's position in the sky is at
its furthest from the equator, and one characteristic of an equinox is that
the Sun's position in the sky is right on the equator. These happen at the
equator, and at the same time as on the rest of the planet. Therefore the
terms remain valid there.

"Hils" wrote in message
...
Dr John Stockton wrote

At the equator, to first order day and night are always each 12 hours,
which just makes the word Equinox linguistically unjustified there; but
the word Solstice is still etymologically appropriate there as
everywhere.

Surely they are both equally justified or unjustified since every day is
arguably both an equinox and a solstice.

The words themselves are Latin-based, while observation of the events
clearly long predates the evolution of written Latin. It would be
interesting to know how different cultures described the events
linguistically.

--
Hil

JRS: In article , dated Wed, 23
Feb 2005 18:14:32, seen in news:uk.sci.astronomy, Hils
posted :
Dr John Stockton wrote

At the equator, to first order day and night are always each 12 hours,
which just makes the word Equinox linguistically unjustified there; but
the word Solstice is still etymologically appropriate there as
everywhere.

Surely they are both equally justified or unjustified since every day is
arguably both an equinox and a solstice.


Solstices are well-defined everywhere on Earth (except for Winter/Summer
inversion/ambiguity) as they mark the north/south extremes of the
apparent motion of the Sun.

At the Equator, equinox is no longer appropriate as a description of the
crossing by the Sun of the equatorial plane, because to first order day-
length and night-length are always equal there.

By the way, to second order, since the length of (day+night) varies
everywhere (equation of time) there *must* everywhere be at least one
shortest/longest day *and/or* night per year, and in fact "and" applies.

Logically, the Spring Equinox occurs in Spring, and the Autumn Equinox
in Autumn. The Vernal Equinox, AIUI, occurs in March. Is there a good
specific name for the second Equinox of an ISO 8601 calendar year?

--
© John Stockton, Surrey, UK. Turnpike v4.00 MIME. ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.

Dr John Stockton wrote in message ...
JRS: In article , dated Wed, 23
Feb 2005 18:14:32, seen in news:uk.sci.astronomy, Hils
posted :
Dr John Stockton wrote

At the equator, to first order day and night are always each 12 hours,
which just makes the word Equinox linguistically unjustified there; but
the word Solstice is still etymologically appropriate there as
everywhere.

Surely they are both equally justified or unjustified since every day is
arguably both an equinox and a solstice.


Solstices are well-defined everywhere on Earth (except for Winter/Summer
inversion/ambiguity) as they mark the north/south extremes of the
apparent motion of the Sun.

At the Equator, equinox is no longer appropriate as a description of the
crossing by the Sun of the equatorial plane, because to first order day-
length and night-length are always equal there.

By the way, to second order, since the length of (day+night) varies
everywhere (equation of time) there *must* everywhere be at least one
shortest/longest day *and/or* night per year, and in fact "and" applies.

Logically, the Spring Equinox occurs in Spring, and the Autumn Equinox
in Autumn. The Vernal Equinox, AIUI, occurs in March. Is there a good
specific name for the second Equinox of an ISO 8601 calendar year?

You silly,stupid no good for nothing ignorant ****,whoever give you a
doctorate is an incompetent organisation.

How ****ing difficult is it to acknowledge that the Earth's axial
orientation remains constant -

http://ottawa.rasc.ca/pictures/pdelorme/polaris.jpg

As you are a ****ing indoctrinated dummy it is unlikely that you could
figure out that Equatorial orientation to the Sun plays no role to the
Equation of Time nor that the change is due to the constant axial
orientation of the Earth passing through the shifting orbital
orientation to the Sun.

As the tilt of the Earth and therefore the Equatorial orientation
plays no role in the Equation of Time correction ( SIMPLE LOGIC)I
strongly suggest that you take up stamp collecting or gardening as a
pursuit otherwise remain pig ignorant for the rest of your miserable
life.

Let me help you out.

Drop references of equatorial orientation to the Sun in terms of
Latitude and adopt longitudinal variations through orbital
orientation.If you have difficulties with this technical language then
I suggest that you fix the Sun as a constant positional reference
,axial rotation and orientation as a constant and consider the
variation along longitudinal changes in orbital orientation.THIS HAS
NEVER BEEN DONE BEFORE.

http://www.mhhe.com/physsci/astronom...ages/04f15.jpg



The Sun/Earth line by which diurnal rotation is designated
(orientation remains constant) runs parallel with planetary orbital
orientation (varies over an annual orbit) at the Equinoctal points or
what amounts to the same thing, varying orbital orientation intersects
the constant polar axis orientation.

Unfortunately cataloguers such as Maskelyne forced men into considering
equatorial orientation to the Sun via axial longitude coordinate as a
fudge to bring the Equation of Time in line with the celestial sphere
and the sidereal format.

In short,don't try to explain the solstice or equinox by way of
equatorial axial coordinates to the Sun for some people might imagine
that the Earth tilts towards and away from the Sun during the seasons
like these poor creatures from an obscure organisation called NASA.

http://imagine.gsfc.nasa.gov/docs/as...s/980116c.html