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Since: Nov 04, 2007
Posts: 311
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(Msg. 16) Posted: Mon Dec 17, 2007 9:49 am
Post subject: Re: Google mash-up I'd like to see [Login to view extended thread Info.]
Archived from groups: rec>photo>digital (more info?)
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Don Wiss <donwiss.DeleteThis@no_spam.com> wrote:
> On Sat, 15 Dec 2007, The Good Doctor <nospam.DeleteThis@googlemail.com> wrote:
>>Neil Ellwood <cral.elllwood.DeleteThis@btinternet.com> wrote:
>>
>>>Don Wiss wrote:
>>>
>>>> I'd like to see someone create a Google mash-up that tells me when
>>>> the sun is in the direction I want. On the map you would put two
>>>> points. One for where you would be standing. The second would be off
>>>> in the direction you want to be looking. Then you would input a
>>>> date. The program would return the time on that date that the sun
>>>> would be directly behind you.
You don't need the date. The time at which the sun will be directly
behind you is always the same, all that changes is the summer time
adjustment and the height in the sky. Otherwise sundials wouldn't
work.
>>>Use a compass and a watch.
>>
>>Learn to use your watch as a compass, and a watch is all you need.
>>It is all I need, and it gets plenty of use.
> I'm sorry but I don't follow. How can that tell me at what time I should go
> visit a site to take a picture with the sun exactly in the direction I want
> it to be?
The earth spins 360 degrees in 24 hours. An analogue watch face is
conveniently graduated in units of 1/60th of a circle. It also tells
you the time now. So if you can see the sun now, it's easy to use the
watch face to show you where it will be at any later or earlier time
in the day. Or when it will be in any particular required direction.
Since the earth revolves in 24 hours and the watch hour hand in 12 the
hour hand moves twice as fast as the sun. At 6am and 6pm local solar
time the sun is due east or west, although sometimes, depending on
latitude, it will be below the horizon. You could make yourself a sun
position protractor graduated in time you can just drop over your
local map.
I was taught this kind of thing when I was taught map reading in
school. But then I'm so old that when I was at school they still
believed in educating children 
--
Chris Malcolm cam.DeleteThis@infirmatics.ed.ac.uk DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/] |
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Since: Aug 22, 2005
Posts: 292
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(Msg. 17) Posted: Mon Dec 17, 2007 10:06 pm
Post subject: Re: Google mash-up I'd like to see [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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[A complimentary Cc of this posting was sent to
Chris Malcolm
<cam RemoveThis @holyrood.ed.ac.uk>], who wrote in article <5smv1pF1a78oqU1 RemoveThis @mid.individual.net>:
> >>>> date. The program would return the time on that date that the sun
> >>>> would be directly behind you.
>
> You don't need the date. The time at which the sun will be directly
> behind you is always the same, all that changes is the summer time
> adjustment and the height in the sky. Otherwise sundials wouldn't
> work.
Did you see a curve somewhat like "figure 8" drawn near equator on
some globes? My conjecture is that it shows the position of sun at
(local) midday relative to earth.
If so, the width of this curve is how much your conjecture errs.
(Hint: the orbit of Earth about Sun is not circular; thus in some
parts the orbital speed is quickier, in some slower. Effectively, the
1/365.25day [+-1] [sp? ;-] movement of Sun about Earth is not with
constant angular velocity).
Hope this helps,
Ilya |
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Since: Nov 04, 2007
Posts: 311
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(Msg. 18) Posted: Tue Dec 18, 2007 1:04 am
Post subject: Re: Google mash-up I'd like to see [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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Ilya Zakharevich <nospam-abuse.TakeThisOut@ilyaz.org> wrote:
> [A complimentary Cc of this posting was sent to
> Chris Malcolm
> <cam.TakeThisOut@holyrood.ed.ac.uk>], who wrote in article <5smv1pF1a78oqU1.TakeThisOut@mid.individual.net>:
>> >>>> date. The program would return the time on that date that the sun
>> >>>> would be directly behind you.
>>
>> You don't need the date. The time at which the sun will be directly
>> behind you is always the same, all that changes is the summer time
>> adjustment and the height in the sky. Otherwise sundials wouldn't
>> work.
> Did you see a curve somewhat like "figure 8" drawn near equator on
> some globes? My conjecture is that it shows the position of sun at
> (local) midday relative to earth.
> If so, the width of this curve is how much your conjecture errs.
> (Hint: the orbit of Earth about Sun is not circular; thus in some
> parts the orbital speed is quickier, in some slower. Effectively, the
> 1/365.25day [+-1] [sp? ;-] movement of Sun about Earth is not with
> constant angular velocity).
THat's true, and important if you're navigating with a sextant or
doing astronomy, but for the purposes of working out when the sun will
be at your back when you're photographing the front of a building it's
irrelevant.
--
Chris Malcolm cam.TakeThisOut@infirmatics.ed.ac.uk DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/] >> Stay informed about: Google mash-up I'd like to see |
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Since: Aug 22, 2005
Posts: 292
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(Msg. 19) Posted: Tue Dec 18, 2007 9:44 am
Post subject: Re: Google mash-up I'd like to see [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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[A complimentary Cc of this posting was sent to
Chris Malcolm
<cam.DeleteThis@holyrood.ed.ac.uk>], who wrote in article <5soklaF1ae7bpU1.DeleteThis@mid.individual.net>:
> > (Hint: the orbit of Earth about Sun is not circular; thus in some
> > parts the orbital speed is quickier, in some slower. Effectively, the
> > 1/365.25day [+-1] [sp? ;-] movement of Sun about Earth is not with
> > constant angular velocity).
>
> THat's true, and important if you're navigating with a sextant or
> doing astronomy, but for the purposes of working out when the sun will
> be at your back when you're photographing the front of a building it's
> irrelevant.
The width of analemma (thanks for the word, BTW!) is 8 degrees. So if
30min error is negligible, then one does not need to know anything -
just come in the morning, and some time soon the sun will be at your
back. 
Yours,
Ilya >> Stay informed about: Google mash-up I'd like to see |
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Since: Nov 04, 2007
Posts: 311
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(Msg. 20) Posted: Tue Dec 18, 2007 10:26 am
Post subject: Re: Google mash-up I'd like to see [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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Ilya Zakharevich <nospam-abuse RemoveThis @ilyaz.org> wrote:
> [A complimentary Cc of this posting was sent to
> Chris Malcolm
> <cam RemoveThis @holyrood.ed.ac.uk>], who wrote in article <5soklaF1ae7bpU1 RemoveThis @mid.individual.net>:
>> > (Hint: the orbit of Earth about Sun is not circular; thus in some
>> > parts the orbital speed is quickier, in some slower. Effectively, the
>> > 1/365.25day [+-1] [sp? ;-] movement of Sun about Earth is not with
>> > constant angular velocity).
>>
>> THat's true, and important if you're navigating with a sextant or
>> doing astronomy, but for the purposes of working out when the sun will
>> be at your back when you're photographing the front of a building it's
>> irrelevant.
> The width of analemma (thanks for the word, BTW!) is 8 degrees. So if
> 30min error is negligible, then one does not need to know anything -
> just come in the morning, and some time soon the sun will be at your
> back.
Thirty minutes sounds like a lot, but the point is that the real
estate photographer's general rule of "sun at your back" has a lot
more more than eight degrees of tolerance in sun position in it. And
if you do happen to be fussier than that, such as wanting the sun to be
just grazing a wall to highlight texture, then a simple recent
sighting of the sun's noon position will enable you to cut down that
error by a factor of at least ten which should be plenty.
What interests me about this thread is how much cheap computing power
and web access has taken over from simple common sense. It wouldn't
surprise me these days to hear the following conversation from two
tourists standing in a street in the sun:
"Which direction is north?"
"Wait I minute while I get Google Earth up on this thing and I'll
tell you."
--
Chris Malcolm cam RemoveThis @infirmatics.ed.ac.uk DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/] >> Stay informed about: Google mash-up I'd like to see |
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Since: Apr 25, 2007
Posts: 38
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(Msg. 21) Posted: Tue Dec 18, 2007 6:54 pm
Post subject: Re: Google mash-up I'd like to see [Login to view extended thread Info.]
Archived from groups: rec>photo>digital, others (more info?)
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On Dec 14, 7:06 pm, Don Wiss <donwiss.DeleteThis@no_spam.com> wrote:
> I'd like to see someone create a Google mash-up that tells me when the sun
> is in the direction I want. On the map you would put two points. One for
> where you would be standing. The second would be off in the direction you
> want to be looking. Then you would input a date. The program would return
> the time on that date that the sun would be directly behind you.
>
> Don <www.donwiss.com/pictures/> (e-mail link at page bottoms).
I am doing this off of the top of my head, but the process goes
something like this:
Draw your line between your location and your subject. Get a compass
direction in degrees. Say it is 5' west of north, or 355'. Well,
then the line is also 5' east of south, or 175'.
At solar noon, the sun is due south (180'). That's not quite the same
as clock time because we have time zones, but it's within 15
degrees.
IIRC, the sun moves through a 360' arc over 24 hours, so it moves 15'
per hour. So it moves 5' in 20 minutes. So in this case, your time
would be 20 minutes before solar noon.
I hope this makes sense and hope it helps. It's not perfect but it'll
get you close. >> Stay informed about: Google mash-up I'd like to see |
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Since: Aug 22, 2005
Posts: 292
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(Msg. 22) Posted: Tue Dec 18, 2007 11:52 pm
Post subject: Re: Google mash-up I'd like to see [Login to view extended thread Info.]
Archived from groups: rec>photo>digital (more info?)
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[A complimentary Cc of this posting was sent to
Chris Malcolm
<cam.TakeThisOut@holyrood.ed.ac.uk>], who wrote in article <5spli8F19vgebU1.TakeThisOut@mid.individual.net>:
> > The width of analemma (thanks for the word, BTW!) is 8 degrees. So if
> > 30min error is negligible, then one does not need to know anything -
> > just come in the morning, and some time soon the sun will be at your
> > back.
>
> Thirty minutes sounds like a lot, but the point is that the real
> estate photographer's general rule of "sun at your back" has a lot
> more more than eight degrees of tolerance in sun position in it.
Then the same argument applies: if 8degrees error is negligible, it is
not a problem to come at any time. Just come before the noon if the
shot direction is to the west of north, and in afternoon if it is to
the east of north - and most of the bases are covered.
> What interests me about this thread is how much cheap computing power
> and web access has taken over from simple common sense.
What interests me about this thread is how much error the "simple
common sense" would bring. [I saw the analemma drawn on maps; I
puzzled about it, then deduced what it should mean; but I never paid
attention on HOW WIDE it is! Only yesterday, looking on Wikipedia, I
found that error might be up to half an hour; my guts feeling would
bring it to an order of magnitude of a minute.]
Hope this helps,
Ilya >> Stay informed about: Google mash-up I'd like to see |
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Since: Nov 04, 2007
Posts: 311
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(Msg. 23) Posted: Wed Dec 19, 2007 1:43 am
Post subject: Re: Google mash-up I'd like to see [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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Ilya Zakharevich <nospam-abuse RemoveThis @ilyaz.org> wrote:
> [A complimentary Cc of this posting was sent to
> Chris Malcolm
> <cam RemoveThis @holyrood.ed.ac.uk>], who wrote in article <5spli8F19vgebU1 RemoveThis @mid.individual.net>:
>> > The width of analemma (thanks for the word, BTW!) is 8 degrees. So if
>> > 30min error is negligible, then one does not need to know anything -
>> > just come in the morning, and some time soon the sun will be at your
>> > back.
>>
>> Thirty minutes sounds like a lot, but the point is that the real
>> estate photographer's general rule of "sun at your back" has a lot
>> more more than eight degrees of tolerance in sun position in it.
> Then the same argument applies: if 8degrees error is negligible, it is
> not a problem to come at any time. Just come before the noon if the
> shot direction is to the west of north, and in afternoon if it is to
> the east of north - and most of the bases are covered.
>> What interests me about this thread is how much cheap computing power
>> and web access has taken over from simple common sense.
> What interests me about this thread is how much error the "simple
> common sense" would bring. [I saw the analemma drawn on maps; I
> puzzled about it, then deduced what it should mean; but I never paid
> attention on HOW WIDE it is! Only yesterday, looking on Wikipedia, I
> found that error might be up to half an hour; my guts feeling would
> bring it to an order of magnitude of a minute.]
Your gut feeling was right, because that's about the size of the error
you'd be dealing with if you simply calibrated your local time by
observing the difference between your predicted and actual sun
direction now and then. The analemma describes the variation over a
whole year. Common sense would suggest you simply do a bit of local
calibration now and then, which makes the analemma problem
disappear. And if your common sense didn't know about the analemma,
you'd very quickly both both discover it and how to get round it if
you were doing these calculations and predictions regularly.
--
Chris Malcolm cam RemoveThis @infirmatics.ed.ac.uk DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/] >> Stay informed about: Google mash-up I'd like to see |
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Since: Aug 22, 2005
Posts: 292
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(Msg. 24) Posted: Wed Dec 19, 2007 1:43 am
Post subject: Re: Google mash-up I'd like to see [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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[A complimentary Cc of this posting was sent to
Chris Malcolm
<cam.RemoveThis@holyrood.ed.ac.uk>], who wrote in article <5srd8hF1a3750U1.RemoveThis@mid.individual.net>:
> > What interests me about this thread is how much error the "simple
> > common sense" would bring. [I saw the analemma drawn on maps; I
> > puzzled about it, then deduced what it should mean; but I never paid
> > attention on HOW WIDE it is! Only yesterday, looking on Wikipedia, I
> > found that error might be up to half an hour; my guts feeling would
> > bring it to an order of magnitude of a minute.]
>
> Your gut feeling was right
No, it was not.
> because that's about the size of the error you'd be dealing with if
> you simply calibrated your local time by observing the difference
> between your predicted and actual sun direction now and then.
My gut feeling was not about this.
Summing up: your argument looks like this: one needs to recalibrate
the analemma error "now and then"; then recalibrate your recalibrarion
whenever you move to a different meridian inside your time zone, then
recalibrate it when DST changes, then recalibrate it when you move
across timezone boundary, then take into account magnetic declination
(if you use compass), then ... (whatever else happens to matter).
(All these errors are of the same order of magnitude; and if you
ignore them ALL, you get comparable precision to "come in the
morning", and "come in the afternoon".) And you assume that all this
is "common sense". Your common sense looks pretty stretchable...
Yours,
Ilya >> Stay informed about: Google mash-up I'd like to see |
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Since: Dec 19, 2007
Posts: 2
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(Msg. 25) Posted: Wed Dec 19, 2007 3:23 am
Post subject: Re: Google mash-up I'd like to see [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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Ilya Zakharevich wrote:
> [A complimentary Cc of this posting was sent to
> Chris Malcolm
> <cam RemoveThis @holyrood.ed.ac.uk>], who wrote in article <5srd8hF1a3750U1 RemoveThis @mid.individual.net>:
>>> What interests me about this thread is how much error the "simple
>>> common sense" would bring. [I saw the analemma drawn on maps; I
>>> puzzled about it, then deduced what it should mean; but I never paid
>>> attention on HOW WIDE it is! Only yesterday, looking on Wikipedia, I
>>> found that error might be up to half an hour; my guts feeling would
>>> bring it to an order of magnitude of a minute.]
>> Your gut feeling was right
>
> No, it was not.
>
>> because that's about the size of the error you'd be dealing with if
>> you simply calibrated your local time by observing the difference
>> between your predicted and actual sun direction now and then.
>
> My gut feeling was not about this.
>
> Summing up: your argument looks like this: one needs to recalibrate
> the analemma error "now and then"; then recalibrate your recalibrarion
> whenever you move to a different meridian inside your time zone, then
> recalibrate it when DST changes, then recalibrate it when you move
> across timezone boundary, then take into account magnetic declination
> (if you use compass), then ... (whatever else happens to matter).
>
> (All these errors are of the same order of magnitude; and if you
> ignore them ALL, you get comparable precision to "come in the
> morning", and "come in the afternoon".) And you assume that all this
> is "common sense". Your common sense looks pretty stretchable...
I saw your message containing:
" I do not understand the difference in precision of semiaxis and orbit
period of outer plants on solarsystem.nasa.gov): "
but it's not on the news-server I use to reply ...
What you say about solar eclipse tracks, lunar eclipse visibility, seems
completely reasonable. Figuring out how Delta T, and the uncertainty in
Delta T influence these predictions is quite complicated ...
solarsystem.nasa.gov doesn't seem to have accurate orbital
elements, from which not so bad positions can be computed.
I don't know why they don't have a link to the
JPL solar system dynamics web site, i.e.
< http://ssd.jpl.nasa.gov/ >
It seems to me some people might like the S.S.D. web site.
The semi-major axis element 'a' for Neptune, according to
< http://ssd.jpl.nasa.gov/txt/p_elem_t1.txt >
is: 30.06992276 (astronomical units) at J2000 (~ 1/1/2000 ? ).
They also have variation rates in AU per century.
Regards,
David Bernier >> Stay informed about: Google mash-up I'd like to see |
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Since: Nov 04, 2007
Posts: 311
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(Msg. 26) Posted: Wed Dec 19, 2007 9:25 am
Post subject: Re: Google mash-up I'd like to see [Login to view extended thread Info.]
Archived from groups: rec>photo>digital, others (more info?)
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In rec.photo.digital.slr-systems Pat <groups RemoveThis @artisticphotography.us> wrote:
> On Dec 14, 7:06 pm, Don Wiss <donwiss RemoveThis @no_spam.com> wrote:
>> I'd like to see someone create a Google mash-up that tells me when the sun
>> is in the direction I want. On the map you would put two points. One for
>> where you would be standing. The second would be off in the direction you
>> want to be looking. Then you would input a date. The program would return
>> the time on that date that the sun would be directly behind you.
>>
>> Don <www.donwiss.com/pictures/> (e-mail link at page bottoms).
> I am doing this off of the top of my head, but the process goes
> something like this:
> Draw your line between your location and your subject. Get a compass
> direction in degrees. Say it is 5' west of north, or 355'. Well,
> then the line is also 5' east of south, or 175'.
Use geographic rather than magnetic north. Local maps should tell you
the correction factor.
> At solar noon, the sun is due south (180'). That's not quite the same
> as clock time because we have time zones, but it's within 15
> degrees.
Eliminate this factor by adding in the local time correction, i.e. the
observed difference between calculated sun position and observed sun
position. This takes care of the local solar time difference, and if
you repeat it every few weeks will take care of the annual cyclic
shift in local time (analemma).
> IIRC, the sun moves through a 360' arc over 24 hours, so it moves 15'
> per hour. So it moves 5' in 20 minutes. So in this case, your time
> would be 20 minutes before solar noon.
> I hope this makes sense and hope it helps. It's not perfect but it'll
> get you close.
Incorporate the above corrections and if your sightings are within a
degree then your time predictions should be within a few minutes.
--
Chris Malcolm cam RemoveThis @infirmatics.ed.ac.uk DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/] >> Stay informed about: Google mash-up I'd like to see |
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Since: Nov 04, 2007
Posts: 311
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(Msg. 27) Posted: Wed Dec 19, 2007 10:09 am
Post subject: Re: Google mash-up I'd like to see [Login to view extended thread Info.]
Archived from groups: rec>photo>digital (more info?)
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Ilya Zakharevich <nospam-abuse DeleteThis @ilyaz.org> wrote:
> [A complimentary Cc of this posting was sent to
> Chris Malcolm
> <cam DeleteThis @holyrood.ed.ac.uk>], who wrote in article <5srd8hF1a3750U1 DeleteThis @mid.individual.net>:
>> > What interests me about this thread is how much error the "simple
>> > common sense" would bring. [I saw the analemma drawn on maps; I
>> > puzzled about it, then deduced what it should mean; but I never paid
>> > attention on HOW WIDE it is! Only yesterday, looking on Wikipedia, I
>> > found that error might be up to half an hour; my guts feeling would
>> > bring it to an order of magnitude of a minute.]
>>
>> Your gut feeling was right
> No, it was not.
>> because that's about the size of the error you'd be dealing with if
>> you simply calibrated your local time by observing the difference
>> between your predicted and actual sun direction now and then.
> My gut feeling was not about this.
> Summing up: your argument looks like this: one needs to recalibrate
> the analemma error "now and then"; then recalibrate your recalibrarion
> whenever you move to a different meridian inside your time zone, then
> recalibrate it when DST changes, then recalibrate it when you move
> across timezone boundary, then take into account magnetic declination
> (if you use compass), then ... (whatever else happens to matter).
> (All these errors are of the same order of magnitude; and if you
> ignore them ALL, you get comparable precision to "come in the
> morning", and "come in the afternoon".) And you assume that all this
> is "common sense". Your common sense looks pretty stretchable...
It's pretty elementary common sense when you find your calculations
are off by the same amount in practice to add that amount in as a
correction factor. Doing that is all the "stretching" required to
calibrate out all those sources of error you mention.
Of course explaining why those errors happen (e.g. the analemma)
requires more than common sense, but you don't need to know why your
calculations were wrong. All you need to discover is that when you're
in the same general geographic area and time of year the correction
factor is constant. Having discovered that it's no big step to
discover that if you move a lot in time or space between calculations
that the correction factor changes. One local observation is
all you need to correct it.
--
Chris Malcolm cam DeleteThis @infirmatics.ed.ac.uk DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/] >> Stay informed about: Google mash-up I'd like to see |
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Since: Aug 22, 2005
Posts: 292
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(Msg. 28) Posted: Wed Dec 19, 2007 10:51 am
Post subject: Re: Google mash-up I'd like to see [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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[A complimentary Cc of this posting was sent to
David Bernier
<david250.RemoveThis@videotron.ca>], who wrote in article <ay4aj.7715$vz3.466010@wagner.videotron.net>:
> The semi-major axis element 'a' for Neptune, according to
> < http://ssd.jpl.nasa.gov/txt/p_elem_t1.txt >
>
> is: 30.06992276 (astronomical units) at J2000 (~ 1/1/2000 ? ).
> They also have variation rates in AU per century.
Thanks; but what they do NOT have is the estimated errors of these
measurements. And my initial question was about these errors...
Thanks for the reference,
Ilya >> Stay informed about: Google mash-up I'd like to see |
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Since: Dec 19, 2007
Posts: 2
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(Msg. 29) Posted: Wed Dec 19, 2007 10:06 pm
Post subject: Re: Google mash-up I'd like to see [Login to view extended thread Info.]
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Ilya Zakharevich wrote:
> [A complimentary Cc of this posting was sent to
> Chris Malcolm
> <cam DeleteThis @holyrood.ed.ac.uk>], who wrote in article <5srd8hF1a3750U1 DeleteThis @mid.individual.net>:
>>> What interests me about this thread is how much error the "simple
>>> common sense" would bring. [I saw the analemma drawn on maps; I
>>> puzzled about it, then deduced what it should mean; but I never paid
>>> attention on HOW WIDE it is! Only yesterday, looking on Wikipedia, I
>>> found that error might be up to half an hour; my guts feeling would
>>> bring it to an order of magnitude of a minute.]
>> Your gut feeling was right
>
> No, it was not.
>
>> because that's about the size of the error you'd be dealing with if
>> you simply calibrated your local time by observing the difference
>> between your predicted and actual sun direction now and then.
>
> My gut feeling was not about this.
>
> Summing up: your argument looks like this: one needs to recalibrate
> the analemma error "now and then"; then recalibrate your recalibrarion
> whenever you move to a different meridian inside your time zone, then
> recalibrate it when DST changes, then recalibrate it when you move
> across timezone boundary, then take into account magnetic declination
> (if you use compass), then ... (whatever else happens to matter).
>
> (All these errors are of the same order of magnitude; and if you
> ignore them ALL, you get comparable precision to "come in the
> morning", and "come in the afternoon".) And you assume that all this
> is "common sense". Your common sense looks pretty stretchable...
>
> Yours,
> Ilya
I read your message about the accuracy to which the 'a' for Neptune is
known, accuracy of ephemerids, etc. It's an interesting
question.
Currently, effects from general relativity are included by JPL and
others. Trying to understand solar dynamics formulated
in general relativity is beyond me.
Probably readers of sci.astro or sci.physics would know better.
Here's a link to a preprint by Sergei Klioner:
< http://arxiv.org/abs/astro-ph/0508292 >
Regards,
David Bernier >> Stay informed about: Google mash-up I'd like to see |
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Since: Aug 22, 2005
Posts: 292
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(Msg. 30) Posted: Thu Dec 20, 2007 1:47 am
Post subject: Re: Google mash-up I'd like to see [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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[A complimentary Cc of this posting was sent to
David Bernier
<david250.DeleteThis@videotron.ca>], who wrote in article <T_kaj.6427$Bw2.501387@weber.videotron.net>:
> Currently, effects from general relativity are included by JPL and
> others. Trying to understand solar dynamics formulated
> in general relativity is beyond me.
I'm very sure that (maybe with an exception of Mercury; but I think
even for it) the main term of the effects of GR can be expressed as
minor additions to Newton inverse square law (a term of degree not
-2, + a term depending on relative velocity). (I do not have a
reference at hand, but I think it is in the beginning of any book on GR...)
Of course, this assumes the observer far away from the system, and not
hopping from one body to another one. Which reminds me (an urban
legend?):
When first launched, GPS satellites had a (software) switch. In one
position, they would take into account the effects of GR; in the
other, they would not.
Engineers being naturally suspicious of "high brow" calculations,
the at-launch position was OFF. During the first day of testing,
the errors accumulated so much, that they grudgingly agreed to turn
it ON.
Yours,
Ilya >> Stay informed about: Google mash-up I'd like to see |
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