| Local solar time [message #39904] |
Mon, 28 June 2004 02:01  |
David Oesch
Messages: 31
Registered: October 2000
|
Member |
|
|
Hi folks...
Has anyone already written a program to convert Julian to "local solar
time" ? Just before I re-invent the wheel..
Cheers, Dave
|
|
|
|
| Re: Local solar time [message #39963 is a reply to message #39904] |
Wed, 30 June 2004 06:29  |
David Fanning
Messages: 11724
Registered: August 2001
|
Senior Member |
|
|
James Kuyper writes:
> I don't believe that this definition of "Julian Day" is specific to
> the USNO. I believe that they're merely reporting on an international
> standard definition. As such, anyone who's using the term correctly is
> inherently implying use of UTC, not local time.
Boys! How about this as a Solomonic definition we can all
live with?
Julian Day has a specific meaning spelled out on the
USNO web page. But since no one reads it or remembers
what it is, and uses it ten ways to Sunday differently,
you assume that definition in your programming at your
own peril.
Does that help? :-)
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
|
|
|
|
| Re: Local solar time [message #39964 is a reply to message #39904] |
Wed, 30 June 2004 05:59 |
kuyper
Messages: 5
Registered: June 2004
|
Junior Member |
|
|
Craig Markwardt <craigmnet@REMOVEcow.physics.wisc.edu> wrote in message news:<on1xjyrl7b.fsf@cow.physics.wisc.edu>...
> James Kuyper <kuyper@saicmodis.com> writes:
>
>> Craig Markwardt wrote:
> ...
>>> However, I will take issue on your claim that Julian days is a "time
>>> system." In fact, Julian day numbers are simply a means of counting
>>> whole and fractional days, referred to some offset, but it is the
>>> definition of those days that are important!
>>
>> According to the US Naval Observatory,
>> <http://aa.usno.navy.mil/data/docs/JulianDate.html>, Julian days are
>> defined in terms of Universal Time, and each one starts at Noon GMT.
>
> Uhh, I'm sorry this is spilling out into the newsgroup. I still
> disagree. Whether or not the USNO "definition" includes mention of
> GMT or Universal Time, there are in practice enough different ways to
> assign days that one needs to explicitly name the time system being
> used. My biggest concern for the original poster was whether he meant
> Julian days expressed in local time or in UTC, which obviously makes
> all the difference in the world for his application.
I don't believe that this definition of "Julian Day" is specific to
the USNO. I believe that they're merely reporting on an international
standard definition. As such, anyone who's using the term correctly is
inherently implying use of UTC, not local time.
|
|
|
|
| Re: Local solar time [message #39969 is a reply to message #39904] |
Tue, 29 June 2004 16:36 |
Craig Markwardt
Messages: 1869
Registered: November 1996
|
Senior Member |
|
|
James Kuyper <kuyper@saicmodis.com> writes:
> Craig Markwardt wrote:
...
>> However, I will take issue on your claim that Julian days is a "time
>> system." In fact, Julian day numbers are simply a means of counting
>> whole and fractional days, referred to some offset, but it is the
>> definition of those days that are important!
>
> According to the US Naval Observatory,
> <http://aa.usno.navy.mil/data/docs/JulianDate.html>, Julian days are
> defined in terms of Universal Time, and each one starts at Noon GMT.
Uhh, I'm sorry this is spilling out into the newsgroup. I still
disagree. Whether or not the USNO "definition" includes mention of
GMT or Universal Time, there are in practice enough different ways to
assign days that one needs to explicitly name the time system being
used. My biggest concern for the original poster was whether he meant
Julian days expressed in local time or in UTC, which obviously makes
all the difference in the world for his application.
Craig
--
------------------------------------------------------------ --------------
Craig B. Markwardt, Ph.D. EMAIL: craigmnet@REMOVEcow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
------------------------------------------------------------ --------------
|
|
|
|
| Re: Local solar time [message #39972 is a reply to message #39904] |
Tue, 29 June 2004 13:19 |
James Kuyper
Messages: 425
Registered: March 2000
|
Senior Member |
|
|
Craig Markwardt wrote:
> James Kuyper <kuyper@saicmodis.com> writes:
>
>> I know of at least two different things that are referred to as
>> Julian days. The first of these is indeed a time system; it is a
>> single number for every single day, starting at Jan. 1, 4713 B.C.E.
>> at 12:00:00, a time when three different cycles associated with
>> three ancient calendar systems were all synchronized. Nobody was
>> using any of those three
>
>
> James, as a mini-tutorial on julian day numbers, your post was great.
>
>
> However, I will take issue on your claim that Julian days is a "time
> system." In fact, Julian day numbers are simply a means of counting
> whole and fractional days, referred to some offset, but it is the
> definition of those days that are important!
According to the US Naval Observatory,
<http://aa.usno.navy.mil/data/docs/JulianDate.html>, Julian days are
defined in terms of Universal Time, and each one starts at Noon GMT.
|
|
|
|
| Re: Local solar time [message #39973 is a reply to message #39904] |
Tue, 29 June 2004 12:13 |
Craig Markwardt
Messages: 1869
Registered: November 1996
|
Senior Member |
|
|
James Kuyper <kuyper@saicmodis.com> writes:
>
> I know of at least two different things that are referred to as Julian
> days. The first of these is indeed a time system; it is a single number
> for every single day, starting at Jan. 1, 4713 B.C.E. at 12:00:00, a
> time when three different cycles associated with three ancient calendar
> systems were all synchronized. Nobody was using any of those three
James, as a mini-tutorial on julian day numbers, your post was great.
However, I will take issue on your claim that Julian days is a "time
system." In fact, Julian day numbers are simply a means of counting
whole and fractional days, referred to some offset, but it is the
definition of those days that are important!
For example, we could define a day as
number of solar meridian crossings + (local solar time)/(24 hr) [LAST]
or
number of solar meridian crossings + (Greenwich solar time)/(24 hr) [GAST]
Both have meaning, and both are Julian day numbers, but one is a local
solar time, one is a the Greenwich solar time. [and both will have
irregular lengths]
But why stop there? "Day" could be mean solar days. UT1 is close to
this, with the exception of leap seconds, and the fact that the length
of day varies.
"Day" could also be 86400 seconds, as defined by atomic clock
standards [TAI], in which case every day is a fixed length. In fact
there are systems where the clock is placed at different locations
(barycenter time, TDB), or even ticks at slightly different rates
(coordinate times TCG, TCB).
So, in addition to saying "Julian days," which provides the unit of
time we also need to specify the time system in use, such as UTC, TAI,
solar time [local vs. Greenwich], etc.
Why is this important? The original poster did not specify which
system he was converting from, so there was some ambiguity at the time
of what conversions would need to be done.
Craig
--
------------------------------------------------------------ --------------
Craig B. Markwardt, Ph.D. EMAIL: craigmnet@REMOVEcow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
------------------------------------------------------------ --------------
|
|
|
|
| Re: Local solar time [message #39974 is a reply to message #39904] |
Tue, 29 June 2004 11:58 |
Craig Markwardt
Messages: 1869
Registered: November 1996
|
Senior Member |
|
|
"Haje Korth" <haje.korth@jhuapl.edu> writes:
> Craig,
> never mind my other message, you gave a definition. But does "RA" in "RA of
> sun" stand for?
>
Haje, RA is "Right Ascension," a common astronomical term. RA
measures the longitude of a point on the sky in the celestial
(equatorial) system. It is commonly measured in units of hours by
convention, and since it is useful to astronomical observatories (360
degrees = 24 h).
Craig
--
------------------------------------------------------------ --------------
Craig B. Markwardt, Ph.D. EMAIL: craigmnet@REMOVEcow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
------------------------------------------------------------ --------------
|
|
|
|
| Re: Local solar time [message #39975 is a reply to message #39904] |
Tue, 29 June 2004 11:54 |
Craig Markwardt
Messages: 1869
Registered: November 1996
|
Senior Member |
|
|
David Oesch <oesch@giub.nospam.unibe.ch> writes:
>> With some massaging, I think you can get what you want by using the
>> IDL Astronomy Library routines SUNPOS to compute the (RA, DEC)
>> position of the sun, and CT2LST, to compute the local mean sidereal
>> time, and then use the definition,
>>
>>
>>
> I got the Sun Azimuth and Sun zenith already, and use the LMST routine
> from the Johns Hopkins University/Applied Physics Laboratory to
> calculate the local mean sidereal time. Is the "local mean sideral time"
> the same as "local solar time"?
No, that's why I supplied the formula below. The local hour angle of
the sun is effectively the local solar time, plus a constant:
>> local hour angle of the sun = local sidereal time[1] - RA of sun[2]
and local apparent solar time = local hour angle + 12 hours
[1] is calculated with CT2LST
[2] is calculated with SUNPOS
Craig
--
------------------------------------------------------------ --------------
Craig B. Markwardt, Ph.D. EMAIL: craigmnet@REMOVEcow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
------------------------------------------------------------ --------------
|
|
|
|
| Re: Local solar time [message #39982 is a reply to message #39904] |
Tue, 29 June 2004 08:28 |
Michael Wallace
Messages: 409
Registered: December 2003
|
Senior Member |
|
|
>>> I didn't find any with a quick search. Just a word of caution if you
>>> are new to time systems. You need to be clear what time system you
>>> are starting in. Julian days is just a way of counting days, not a
>>> time system. I am quite certain that one could express local solar
>
>
> I know of at least two different things that are referred to as Julian
> days. The first of these is indeed a time system; it is a single number
> for every single day, starting at Jan. 1, 4713 B.C.E. at 12:00:00, a
> time when three different cycles associated with three ancient calendar
> systems were all synchronized. Nobody was using any of those three
> calendar systems at that synchronization time, it's just an arbitrary
> starting point. However, using that date as a starting point made it
> simple to convert dates in any of those calendar systems into Julian
> days, allowing those dates to be compared with each other. It's named
> the Julian date because the Julian calendar established by Julius Caesar
> was one of those three systems. It was designed for use by historians of
> astronomy, to allow them to correlate observations recorded by ancient
> peoples using those calendar systems. The fact that it starts at Noon
> Greenwich time was a reflection of the fact that for European
> astronomers, noon GMT was a time when they were usually making very few
> astronomical observations, because there's usually only one or two
> objects (the Sun and sometimes the Moon) that you can be observing at
> that time.
Just to throw another two cents on this, there is also Modified Julian
Date. In fact, I'd say that I use MJD more than JD. The conversion
between the two is simple: MJD = JD - 2400000.5
The differences between MJD and JD should be obvious in the formula.
MJD begins at midnight rather than noon which is more in line with our
current timekeeping systems. Also, the first two digits of the JD are
dropped. For ~300 years after 17 November 1858, the JD is between
2400000 and 2500000.
I should say that for processing of data, JD is used since that's what
all of IDL's routines use. For display, MJD tends to be what people
like to see.
> The other way I've seen the term used, is as a term for any system that
> simply counts consecutive days from a fixed starting point; the most
> popular starting point seems to be January 1st of the current year.
> However, as a pendant I'm pretty sure that this is a mis-use of the
> term, or at least a confusingly over-generalized extension of it.
It is a misuse of the term, but I have seen many cases where the term is
misused and abused. A more appropriate phrase would be something like
"days past the such-and-such epoch," but I guess "Julian Date" has a
better (although incorrect) ring to it. ;-)
-Mike
|
|
|
|
| Re: Local solar time [message #39983 is a reply to message #39904] |
Tue, 29 June 2004 08:00 |
Haje Korth
Messages: 651
Registered: May 1997
|
Senior Member |
|
|
well, according to Craig's definition it is not; the difference is this RA
of sun thingy that I wanted to know what it is.
Cheers,
Haje
"David Oesch" <oesch@giub.nospam.unibe.ch> wrote in message
news:40e15caf$1@news.unibe.ch...
>
>> With some massaging, I think you can get what you want by using the
>> IDL Astronomy Library routines SUNPOS to compute the (RA, DEC)
>> position of the sun, and CT2LST, to compute the local mean sidereal
>> time, and then use the definition,
>>
>>
>>
> I got the Sun Azimuth and Sun zenith already, and use the LMST routine
> from the Johns Hopkins University/Applied Physics Laboratory to
> calculate the local mean sidereal time. Is the "local mean sideral time"
> the same as "local solar time"?
>
>> local hour angle of the sun = local sidereal time - RA of sun
>>
>>
>
>> [ in the proper units of course ]
>>
>> Craig
>>
>>
>>
|
|
|
|
| Re: Local solar time [message #39984 is a reply to message #39904] |
Tue, 29 June 2004 05:12 |
Haje Korth
Messages: 651
Registered: May 1997
|
Senior Member |
|
|
Craig,
never mind my other message, you gave a definition. But does "RA" in "RA of
sun" stand for?
Haje
"Craig Markwardt" <craigmnet@REMOVEcow.physics.wisc.edu> wrote in message
news:onk6xq1yqz.fsf@cow.physics.wisc.edu...
>
> David Oesch <oesch@giub.nospam.unibe.ch> writes:
>
>> Craig Markwardt wrote:
>>
>>> "Haje Korth" <haje.korth@jhuapl.edu> writes:
>>>
>>>
>>>> Check the ESRG IDL library. Maybe zensun.pro does what you want.
>>>>
>>>>
>>>
>>> I don't think this is the right thing, but it's close. Local solar
>>> time would be the hour angle (=longitude) of the sun, not the zenith
>>> angle.
>>>
>>>
>> Yes.. I stummbled over zensun in ESRG already..it ain't that what I'm
>> looking for...
>
> With some massaging, I think you can get what you want by using the
> IDL Astronomy Library routines SUNPOS to compute the (RA, DEC)
> position of the sun, and CT2LST, to compute the local mean sidereal
> time, and then use the definition,
>
> local hour angle of the sun = local sidereal time - RA of sun
>
> [ in the proper units of course ]
>
> Craig
>
> --
> ------------------------------------------------------------ --------------
> Craig B. Markwardt, Ph.D. EMAIL: craigmnet@REMOVEcow.physics.wisc.edu
> Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
> ------------------------------------------------------------ --------------
|
|
|
|
| Re: Local solar time [message #39985 is a reply to message #39904] |
Tue, 29 June 2004 05:12 |
David Oesch
Messages: 31
Registered: October 2000
|
Member |
|
|
> With some massaging, I think you can get what you want by using the
> IDL Astronomy Library routines SUNPOS to compute the (RA, DEC)
> position of the sun, and CT2LST, to compute the local mean sidereal
> time, and then use the definition,
>
>
>
I got the Sun Azimuth and Sun zenith already, and use the LMST routine
from the Johns Hopkins University/Applied Physics Laboratory to
calculate the local mean sidereal time. Is the "local mean sideral time"
the same as "local solar time"?
> local hour angle of the sun = local sidereal time - RA of sun
>
>
> [ in the proper units of course ]
>
> Craig
>
>
>
|
|
|
|
| Re: Local solar time [message #39986 is a reply to message #39904] |
Tue, 29 June 2004 05:09 |
Haje Korth
Messages: 651
Registered: May 1997
|
Senior Member |
|
|
Craig,
Zensun.pro was a pure guess and there was that "local" keyword that I
thought might help. To be honest, I wasn't sure what David was looking for.
Maybe we can start with a definition!?
Cheers,
Haje
--
Dr. Haje Korth
Space Physics Group
The Johns Hopkins University
Applied Physics Laboratory
MS MP3-E128
11100 Johns Hopkins Road
Laurel, MD 20723-6099
USA
Phone: 240-228-4033 (Washington), 443-778-4033 (Baltimore)
Fax: 240-228-0386 (Washington), 443-778-0386 (Baltimore)
e-mail: haje.korth@jhuapl.edu
"Craig Markwardt" <craigmnet@REMOVEcow.physics.wisc.edu> wrote in message
news:oneknzrpow.fsf@cow.physics.wisc.edu...
>
> "Haje Korth" <haje.korth@jhuapl.edu> writes:
>> Check the ESRG IDL library. Maybe zensun.pro does what you want.
>
> I don't think this is the right thing, but it's close. Local solar
> time would be the hour angle (=longitude) of the sun, not the zenith
> angle.
>
>> "David Oesch" <oesch@giub.nospam.unibe.ch> wrote in message
>> news:40dfde52$1@news.unibe.ch...
>>> Hi folks...
>>>
>>> Has anyone already written a program to convert Julian to "local solar
>>> time" ? Just before I re-invent the wheel..
>
> I didn't find any with a quick search. Just a word of caution if you
> are new to time systems. You need to be clear what time system you
> are starting in. Julian days is just a way of counting days, not a
> time system. I am quite certain that one could express local solar
> time in Julian days. Presumably you mean that the starting time is a
> civil time like UTC?
>
> Craig
>
> --
> ------------------------------------------------------------ --------------
> Craig B. Markwardt, Ph.D. EMAIL: craigmnet@REMOVEcow.physics.wisc.edu
> Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
> ------------------------------------------------------------ --------------
|
|
|
|
| Re: Local solar time [message #39987 is a reply to message #39904] |
Tue, 29 June 2004 04:51 |
James Kuyper
Messages: 425
Registered: March 2000
|
Senior Member |
|
|
Craig Markwardt wrote:
>> "Haje Korth" <haje.korth@jhuapl.edu> writes:
>>
>
>>>>Check the ESRG IDL library. Maybe zensun.pro does what you want.
>
>>
>>
>> I don't think this is the right thing, but it's close. Local solar
>> time would be the hour angle (=longitude) of the sun, not the zenith
>> angle.
>>
>>
>
>>>>"David Oesch" <oesch@giub.nospam.unibe.ch> wrote in message
>>>>news:40dfde52$1@news.unibe.ch...
>>>>
>>
>>>>>>Hi folks...
>>>>>>
>>>>>>Has anyone already written a program to convert Julian to "local
solar
>>>>>>time" ? Just before I re-invent the wheel..
>
>>
>>
>> I didn't find any with a quick search. Just a word of caution if you
>> are new to time systems. You need to be clear what time system you
>> are starting in. Julian days is just a way of counting days, not a
>> time system. I am quite certain that one could express local solar
I know of at least two different things that are referred to as Julian
days. The first of these is indeed a time system; it is a single number
for every single day, starting at Jan. 1, 4713 B.C.E. at 12:00:00, a
time when three different cycles associated with three ancient calendar
systems were all synchronized. Nobody was using any of those three
calendar systems at that synchronization time, it's just an arbitrary
starting point. However, using that date as a starting point made it
simple to convert dates in any of those calendar systems into Julian
days, allowing those dates to be compared with each other. It's named
the Julian date because the Julian calendar established by Julius Caesar
was one of those three systems. It was designed for use by historians of
astronomy, to allow them to correlate observations recorded by ancient
peoples using those calendar systems. The fact that it starts at Noon
Greenwich time was a reflection of the fact that for European
astronomers, noon GMT was a time when they were usually making very few
astronomical observations, because there's usually only one or two
objects (the Sun and sometimes the Moon) that you can be observing at
that time.
This is the kind of Julian day that is used by many of IDL's
time-oriented functions such as JULDAT, CALDAT, and TIMEGEN, and
SYSTIME(/JULIAN), the "C()" format code, the LABEL_DATE function and the
"LABEL_DATE" option for [XYZ]TICKFORMAT, and idlGRaxis.
The other way I've seen the term used, is as a term for any system that
simply counts consecutive days from a fixed starting point; the most
popular starting point seems to be January 1st of the current year.
However, as a pendant I'm pretty sure that this is a mis-use of the
term, or at least a confusingly over-generalized extension of it.
|
|
|
|
| Re: Local solar time [message #39990 is a reply to message #39904] |
Tue, 29 June 2004 02:48 |
Craig Markwardt
Messages: 1869
Registered: November 1996
|
Senior Member |
|
|
David Oesch <oesch@giub.nospam.unibe.ch> writes:
> Craig Markwardt wrote:
>
>> "Haje Korth" <haje.korth@jhuapl.edu> writes:
>>
>>
>>> Check the ESRG IDL library. Maybe zensun.pro does what you want.
>>>
>>>
>>
>> I don't think this is the right thing, but it's close. Local solar
>> time would be the hour angle (=longitude) of the sun, not the zenith
>> angle.
>>
>>
> Yes.. I stummbled over zensun in ESRG already..it ain't that what I'm
> looking for...
With some massaging, I think you can get what you want by using the
IDL Astronomy Library routines SUNPOS to compute the (RA, DEC)
position of the sun, and CT2LST, to compute the local mean sidereal
time, and then use the definition,
local hour angle of the sun = local sidereal time - RA of sun
[ in the proper units of course ]
Craig
--
------------------------------------------------------------ --------------
Craig B. Markwardt, Ph.D. EMAIL: craigmnet@REMOVEcow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
------------------------------------------------------------ --------------
|
|
|
|
| Re: Local solar time [message #39993 is a reply to message #39904] |
Mon, 28 June 2004 23:31 |
David Oesch
Messages: 31
Registered: October 2000
|
Member |
|
|
Craig Markwardt wrote:
> "Haje Korth" <haje.korth@jhuapl.edu> writes:
>
>
>> Check the ESRG IDL library. Maybe zensun.pro does what you want.
>>
>>
>
> I don't think this is the right thing, but it's close. Local solar
> time would be the hour angle (=longitude) of the sun, not the zenith
> angle.
>
>
Yes.. I stummbled over zensun in ESRG already..it ain't that what I'm
looking for...
>
>
>> "David Oesch" <oesch@giub.nospam.unibe.ch> wrote in message
>> news:40dfde52$1@news.unibe.ch...
>>
>>
>>> Hi folks...
>>>
>>> Has anyone already written a program to convert Julian to "local solar
>>> time" ? Just before I re-invent the wheel..
>>>
>>>
>
> I didn't find any with a quick search. Just a word of caution if you
> are new to time systems. You need to be clear what time system you
> are starting in. Julian days is just a way of counting days, not a
> time system. I am quite certain that one could express local solar
> time in Julian days. Presumably you mean that the starting time is a
> civil time like UTC?
>
>
Yes I'm working with UTC...
> Craig
>
>
>
|
|
|
|
| Re: Local solar time [message #39995 is a reply to message #39904] |
Mon, 28 June 2004 20:46 |
Craig Markwardt
Messages: 1869
Registered: November 1996
|
Senior Member |
|
|
"Haje Korth" <haje.korth@jhuapl.edu> writes:
> Check the ESRG IDL library. Maybe zensun.pro does what you want.
I don't think this is the right thing, but it's close. Local solar
time would be the hour angle (=longitude) of the sun, not the zenith
angle.
> "David Oesch" <oesch@giub.nospam.unibe.ch> wrote in message
> news:40dfde52$1@news.unibe.ch...
>> Hi folks...
>>
>> Has anyone already written a program to convert Julian to "local solar
>> time" ? Just before I re-invent the wheel..
I didn't find any with a quick search. Just a word of caution if you
are new to time systems. You need to be clear what time system you
are starting in. Julian days is just a way of counting days, not a
time system. I am quite certain that one could express local solar
time in Julian days. Presumably you mean that the starting time is a
civil time like UTC?
Craig
--
------------------------------------------------------------ --------------
Craig B. Markwardt, Ph.D. EMAIL: craigmnet@REMOVEcow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
------------------------------------------------------------ --------------
|
|
|
|
| Re: Local solar time [message #40001 is a reply to message #39904] |
Mon, 28 June 2004 08:07 |
Haje Korth
Messages: 651
Registered: May 1997
|
Senior Member |
|
|
Check the ESRG IDL library. Maybe zensun.pro does what you want.
Haje
"David Oesch" <oesch@giub.nospam.unibe.ch> wrote in message
news:40dfde52$1@news.unibe.ch...
> Hi folks...
>
> Has anyone already written a program to convert Julian to "local solar
> time" ? Just before I re-invent the wheel..
>
> Cheers, Dave
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
