Brain Burp re Noon and the Sun!
I recently read in an internet article:-
"Since solar time depends on the longitude, solar noon occurs at exactly the
same moment in all locations that share your local meridian."
Is that correct? I thought that the Latitude of the locations also comes
into this?
My reason for doubt, based on reading elsewhere, is Axial Tilt, also known
as obliquity, which is the angle between an objects rotational axis and its
orbital axis, which is the line perpendicular to its orbital plane. The
rotational axis of Earth, for example, is the imaginary line that passes
through both the North Pole and South Pole, whereas the Earth's orbital axis
is the line perpendicular to the imaginary plane through which the Earth
moves as it revolves around the Sun; the Earth's obliquity or axial tilt is
the angle between these two lines and is about 23 degrees.
Thus to me, looking at the Earth from a distance, a Meridian will not be in
line with the Earth's Axis (other than twice each year) and this would cause
noon at two or more different Latitudes on the same Meridian to be at
different times.
I realise that there are other complex variables involved, ie The Equation
of Time.
I am in the UK so tend to think in terms of the Greenwich Meridian and am
considering the case of two locations on the Greenwich Meridian, but could
be any other Meridian.
(For twenty years as a Merchant Navy RO, I supplied the Radio Time Signal
for the Navigators to keep the chronometer accurate; I wish now that I had
taken more notice of their sextant and tables.)
Many thanks.
Clive
On Wed 2023-11-22T22:28:36-0000 Clive S Carver via time-nuts hath writ:
I recently read in an internet article:-
"Since solar time depends on the longitude, solar noon occurs at exactly the
same moment in all locations that share your local meridian."
Is that correct?
Yes, mostly.
I thought that the Latitude of the locations also comes into this?
No, unless you are worried about polar motion at the level of centiseconds.
--
Steve Allen sla@ucolick.org WGS-84 (GPS)
UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855
1156 High Street Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 https://www.ucolick.org/~sla/ Hgt +250 m
On 2023-11-22 05:28 PM, Clive S Carver via time-nuts wrote:
"Since solar time depends on the longitude, solar noon occurs at
exactly the same moment in all locations that share your local meridian."
That's pretty much the definition of solar noon - when the sun crosses
your local meridian. The other factors change things like sun rise and
set, but not local noon.
Steve Hendrix
Have I misunderstood "Local Meridian"?
I was thinking that it was a line of Longitude (passing through the North &
South Poles of the Earth, (as does the Greenwich Meridian), with the
locations of interest on it. But Googling "Local Meridian" gives me
something else involving the Celestial Sphere. Ie The Local Meridian is an
imaginary Great Circle on the Celestial Sphere that is perpendicular to the
local Horizon. It passes through the North point on the Horizon, through the
Celestial Pole, up to the Zenith, and through the South point on the
Horizon.
In other words what I am trying to find out is whether solar noon would be
at the same instant at say, 50.0N 3.0W and 55.0N 3.0W?
Many thanks.
Clive
-----Original Message-----
From: Steve Allen via time-nuts [mailto:time-nuts@lists.febo.com]
Sent: 23 November 2023 00:18
To: Discussion of precise time and frequency measurement
time-nuts@lists.febo.com
Cc: Steve Allen sla@ucolick.org
Subject: [time-nuts] Re: Brain Burp re Noon and the Sun!
On Wed 2023-11-22T22:28:36-0000 Clive S Carver via time-nuts hath writ:
I recently read in an internet article:-
"Since solar time depends on the longitude, solar noon occurs at
exactly the same moment in all locations that share your local meridian."
Is that correct?
Yes, mostly.
I thought that the Latitude of the locations also comes into this?
No, unless you are worried about polar motion at the level of centiseconds.
--
Steve Allen sla@ucolick.org WGS-84 (GPS)
UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855
1156 High Street Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 https://www.ucolick.org/~sla/ Hgt +250 m
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an
email to time-nuts-leave@lists.febo.com
I can see how you are thinking Clive, so I wonder if the (hopefully!)
attached image will help. (A picture is worth a thousand words!) I found
an image online with lines of latitude and longitude, shown at a tilt,
similar to the Earth at the Spring or Autumn equinoxes, as viewed from the
orbital plane. I've added a red line which is perpendicular to that plane,
which I believe is what you are thinking would represent solar noon.
However, the blue line shows the axis of rotation, and even at extreme
latitudes. places along the same line of longitude will be closest to the
sun at the same moment. (If I have understood things correctly?)
Regards,
Peter Vince
On Thu, 23 Nov 2023 at 12:16, Clive S Carver via time-nuts <
time-nuts@lists.febo.com> wrote:
Have I misunderstood "Local Meridian"?
I was thinking that it was a line of Longitude (passing through the North
&
South Poles of the Earth, (as does the Greenwich Meridian), with the
locations of interest on it. But Googling "Local Meridian" gives me
something else involving the Celestial Sphere. Ie The Local Meridian is an
imaginary Great Circle on the Celestial Sphere that is perpendicular to
the
local Horizon. It passes through the North point on the Horizon, through
the
Celestial Pole, up to the Zenith, and through the South point on the
Horizon.
In other words what I am trying to find out is whether solar noon would be
at the same instant at say, 50.0N 3.0W and 55.0N 3.0W?
Many thanks.
Clive
Hi
Since we’re not confused enough already :) :) :).
Let’s say you are really picky about this. You are at the example location of 55.0 N 3.0W. You are standing on dry land. You don’t have a solid view of the horizon. You want to do this to find local noon and set your watch. What do you use to find “straight up”? ( ….noon is when the sun is straight overhead).
You might say that a plumb bob will tell you straight down and go from there. The gotcha is that local geology will skew that ever so slightly. If you want to set your watch to nanoseconds of “correct", that’s going to get in the way. At the precision most folks set their watch …. not a big deal.
Bob
On Nov 23, 2023, at 5:17 AM, Clive S Carver via time-nuts time-nuts@lists.febo.com wrote:
Have I misunderstood "Local Meridian"?
I was thinking that it was a line of Longitude (passing through the North &
South Poles of the Earth, (as does the Greenwich Meridian), with the
locations of interest on it. But Googling "Local Meridian" gives me
something else involving the Celestial Sphere. Ie The Local Meridian is an
imaginary Great Circle on the Celestial Sphere that is perpendicular to the
local Horizon. It passes through the North point on the Horizon, through the
Celestial Pole, up to the Zenith, and through the South point on the
Horizon.
In other words what I am trying to find out is whether solar noon would be
at the same instant at say, 50.0N 3.0W and 55.0N 3.0W?
Many thanks.
Clive
-----Original Message-----
From: Steve Allen via time-nuts [mailto:time-nuts@lists.febo.com]
Sent: 23 November 2023 00:18
To: Discussion of precise time and frequency measurement
time-nuts@lists.febo.com
Cc: Steve Allen sla@ucolick.org
Subject: [time-nuts] Re: Brain Burp re Noon and the Sun!
On Wed 2023-11-22T22:28:36-0000 Clive S Carver via time-nuts hath writ:
I recently read in an internet article:-
"Since solar time depends on the longitude, solar noon occurs at
exactly the same moment in all locations that share your local meridian."
Is that correct?
Yes, mostly.
I thought that the Latitude of the locations also comes into this?
No, unless you are worried about polar motion at the level of centiseconds.
--
Steve Allen sla@ucolick.org WGS-84 (GPS)
UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855
1156 High Street Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 https://www.ucolick.org/~sla/ Hgt +250 m
time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an
email to time-nuts-leave@lists.febo.com
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
I ran this scenario through the sundial calculator Sun_56.xls (found at
https://www.mysundial.ca/tsp/sun.html)
For 55 N, 3 W, Solar noon is at 11:58:19 UTC.
For 50 N, 3 W, Solar noon is at 11:58:19 UTC.
For 20 N, 3 W, Solar noon is at 11:58:19 UTC.
At the equator and 3 W, Solar noon is at 11:58:19 UTC.
Keith E. Brandt, MD, MPH
/keith.brandt@gmail.com/
///Christianity and science are opposed… but only in the same sense as
my thumb
and forefinger are opposed- and between them I can grasp everything.
/—Sir William Bragg (Nobel Prize for Physics- 1915)
*This message transmitted with 100% recycled electrons
/xb /
On 23-Nov-23 08:54, Bob kb8tq via time-nuts wrote:
Hi
Since we’re not confused enough already :) :) :).
Let’s say you are really picky about this. You are at the example location of 55.0 N 3.0W. You are standing on dry land. You don’t have a solid view of the horizon. You want to do this to find local noon and set your watch. What do you use to find “straight up”? ( ….noon is when the sun is straight overhead).
You might say that a plumb bob will tell you straight down and go from there. The gotcha is that local geology will skew that ever so slightly. If you want to set your watch to nanoseconds of “correct", that’s going to get in the way. At the precision most folks set their watch …. not a big deal.
Bob
On Nov 23, 2023, at 5:17 AM, Clive S Carver via time-nutstime-nuts@lists.febo.com wrote:
Have I misunderstood "Local Meridian"?
I was thinking that it was a line of Longitude (passing through the North &
South Poles of the Earth, (as does the Greenwich Meridian), with the
locations of interest on it. But Googling "Local Meridian" gives me
something else involving the Celestial Sphere. Ie The Local Meridian is an
imaginary Great Circle on the Celestial Sphere that is perpendicular to the
local Horizon. It passes through the North point on the Horizon, through the
Celestial Pole, up to the Zenith, and through the South point on the
Horizon.
In other words what I am trying to find out is whether solar noon would be
at the same instant at say, 50.0N 3.0W and 55.0N 3.0W?
Many thanks.
Clive
-----Original Message-----
From: Steve Allen via time-nuts [mailto:time-nuts@lists.febo.com]
Sent: 23 November 2023 00:18
To: Discussion of precise time and frequency measurement
time-nuts@lists.febo.com
Cc: Steve Allensla@ucolick.org
Subject: [time-nuts] Re: Brain Burp re Noon and the Sun!
On Wed 2023-11-22T22:28:36-0000 Clive S Carver via time-nuts hath writ:
I recently read in an internet article:-
"Since solar time depends on the longitude, solar noon occurs at
exactly the same moment in all locations that share your local meridian."
Is that correct?
Yes, mostly.
I thought that the Latitude of the locations also comes into this?
No, unless you are worried about polar motion at the level of centiseconds.
--
Steve Allensla@ucolick.org WGS-84 (GPS)
UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855
1156 High Street Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064https://www.ucolick.org/~sla/ Hgt +250 m
time-nuts mailing list --time-nuts@lists.febo.com To unsubscribe send an
email totime-nuts-leave@lists.febo.com
time-nuts mailing list --time-nuts@lists.febo.com
To unsubscribe send an email totime-nuts-leave@lists.febo.com
time-nuts mailing list --time-nuts@lists.febo.com
To unsubscribe send an email totime-nuts-leave@lists.febo.com
On Thu 2023-11-23T09:54:46-0500 Bob kb8tq via time-nuts hath writ:
Since we’re not confused enough already :) :) :).
Let’s say you are really picky about this.
In that case we need to define the notions of "pole" and "meridian".
Since 1968 90 degrees north and south latitude are almost universally
intended to mean points in a conventional reference frame.
The actual poles wander around not at those conventional points,
and because of that the actual meridian is not the one on a map,
and the actual meridian today is different than it was a month ago.
Various observatories tasked with timekeeping were making corrections
for this starting in the 1950s. This was UT1 vs. UT0.
Now UT1 is based on an entirely different convention that is vaguely
related to the classical pole and meridian model.
--
Steve Allen sla@ucolick.org WGS-84 (GPS)
UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855
1156 High Street Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 https://www.ucolick.org/~sla/ Hgt +250 m
You might say that a plumb bob will tell you straight down and
go from there.
The shadows cast by the sun are surprisingly fuzzy and that makes
it very hard to precisely time the shortest shadow.
I once tried photographing the shadow of a vertical pencil on a
horizontal sheet of graph-paper once per second, using a 6MP DSLR,
triggered by electronic timer.
Even with +/- 15 minutes of pictures and quite advanced imageprocessing,
it was not possible for me to nail the moment of shortest shadow better
than approx 40 seconds.
I was at the AO4RTC workshop at ESO a couple of weeks ago, presenting
the prototype RTC cluster we built for the ESO/ELT telescope.
Surprising, at least to me, was a couple of presentations from solar
observatories about how they use AO to cancel out atmospheric
turbulence, in order to get sharper pictures.
However, I have not been able to figure out any way that could be
used to improve the measurement of time. For one thing, with AO
you also have to figure out, where your telescope is actually
looking at any one moment in time.
Timing the stars at night is just /so/ much easier and precise.
(Not to mention much more aesthetically pleasing :-)
With the new GAIA catalogue as reference, and a moderately modern
digital camera, firmly bolted to a steady monument, it should be a
trivial matter of computing to determine both precise longitude,
latitude and time.
(Unless you live somewhere like Denmark, with only 50 clear nights
per year.)
Poul-Henning
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
On Thu 2023-11-23T22:59:53+0000 Poul-Henning Kamp via time-nuts hath writ:
Even with +/- 15 minutes of pictures and quite advanced imageprocessing,
it was not possible for me to nail the moment of shortest shadow better
than approx 40 seconds.
Looking at the azimuth of the sun visually yields about the same level
of uncertainty. Using the azimuth of a long sun shadow allows a
little better on any given day, but either way this is telling us why
astronomers shifted to measuring sidereal time to calculate mean solar
time at subsecond precision.
Worse for the sundial worshipper, from year to year the Gregorian
calendar plus lunar and planetary perturbations still mean that raw
measurement leaves a range of around 30 s in the determination of
noon. See the zoomed-in plots of all the analammata during my
lifetime and the span of meridian crossings for all of them
https://www.ucolick.org/~sla/temporary/myanalemma.pdf
It is imperative to have worldwide agreement on the value for the
basis of civil time to a microsecond or better. There is no point in
having the value of civil time try to track earth rotation to better
than 30 s.
--
Steve Allen sla@ucolick.org WGS-84 (GPS)
UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855
1156 High Street Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 https://www.ucolick.org/~sla/ Hgt +250 m