Dipping my oar in the water, Delta-T is absolutely required for accurate calculations. Almost all of the published polynomials for calculating orbital values are in the form of a polynomical based on the fraction of a Julian century for the epoch. This variable, T, is calculated from the Julian day number. The fraction of the Julian day number is the the fraction of the day since noon in dynamical time (which is now called Terrestrial Time, TT). To calculate values for a given UT, you have to adjust Terrestrial (i.e. dynamical) time by the factor Delta-T = TT-UT.
So, if you want to do a calculation at noon UT, it is not for Julian day number xxx.5, but at xxx.5 - delta T (currently about 72 sec.). Clearly, the adjustment is not a big deal for a contemporary sundial, but is very, very important for reproductions or studies for times far in the past. Note that almost all values in the Astronomical Almanac are for 12 hr TT. The only subject that uses UT is eclipses (I think).
I trust my program, "The Electric Astrolabe" which can be downloaded from my web site. I trust it because of the hundreds of hours I have spent confirming it gives good answers and I know how it works. It's hard to trust any program that doesn't tell you how the answers are calculated.
Best regards,
Jim
James E. Morrison
[email protected]
Astrolabe web site at http://astrolabes.org
------------ Forwarded message ------------
From: Aimo Niemi <[email protected]>
Date: Mar 3, 2010
Subject: Re: Re: Solar position calculator
To: [email protected] <[email protected]>
Hi
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2010/3/3 [email protected] <[email protected]>
I think that no comparison can be made between different programs results without knowing the value of DeltatT that has been used.
I again think that DeltaT is not needed if we calculate apparent places and use UT.
Most accurate link capable for that (what I know) is at
If refraction is not calculated, the link gives
azimuth=200.7239 and altitude = 39.1824 (39.2031 if refraction is included)
Regards
Aimo
Ephemeris tables usually declare the value of DeltaT that was used.
It is also worthy to note that for gnomonic applications all these results are more than correct :-)
Regards.
Gian----Messaggio originale----
Da: [email protected]
Data: 03/03/2010 20.51
A: "John Goodman"<[email protected]>
Cc: "Sundial List"<[email protected]>
Ogg: Re: Solar position calculatorHiI got following results from the linkwhich I believe is trustworthy
azimuth=200.7233 (20.7233 if south is the zeropoint)altitude= 39.1827I also tested the results with my own semi-accurate PcAllakkawith following resultsazimuth=200.715 and altitude=39.188Anyone interested can load the program fromor perhaps some of you is curious enough to visit my solar tracking sundial pages atGreetingsAimo Niemi
2010/3/3 John Goodman <[email protected]>If anyone needs to make precise solar position calculations, the following software may be useful. There are a few caveats. The software is not a stand-alone application, it's a software library. The software library needs to be used from the command line and it runs using the Python programming language.
I'm a Mac user and this is one of those rare occasions when an arcane set of requirements actually favors the Mac. While Macintosh users tend to be allergic to the command line, it has existed in the OS for the past 10 years. Python is there too, lurking under the surface.
I don't know enough about the PC, but clearly the command line is there and Python is available too. ( http://www.python.org/download/windows/ )
With all that preface, here's what the code can do. Once the necessary files are installed, you can type the following commands and see the following results:
>>> import datetime, solar
>>> d = datetime.datetime(2010, 3, 3, 18, 0, 0)
>>> lat = 42.0
>>> long = -71
>>> solar.GetAltitude(lat, long, d)
39.203509044804704
>>> solar.GetAzimuth(lat, long, d)
-20.717667802423591
>>>
Someone with an ephemeris can check these results. The format for datetime is "yyyy, mm, dd, hh, mm, ss", optionally followed by a number of microseconds. The time value entered is for UTC.
There are other functions for calculations that I haven't explored. You can read more about the software tools at http://pysolar.org/ Example uses are shown at http://wiki.github.com/pingswept/pysolar/examples The software itself is at http://github.com/pingswept/pysolar/downloads
All of this software is the work of Brandon Stafford, who has generously offered his code free of charge to anyone who may find it useful. I have little experience with this software but, as much as I can, I'll be happy to help anyone trying to use it.
If there are any questions, I can pass them on to Brandon. I hope this is helpful to some.
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