What delta T and sticking to UT

Asked by R catchpole on 2010-11-17

I am using Stellarium (0.10.5) on Macbook pro (OSX X 10.6.5) to look at planetary configurations in the past (1000 to 3000yrs before present) and would like to know how you are correcting for the slowing down in the rate of the earth's daily rotation. This is usually expressed as delta T = dynamical time - Universal time. (DT=2h 53 at around 1AD and shifts eclipse tracks by 43 degrees)

It would be very useful to have more control over the time setting. It seems that whatever longitude I use, the time is the local time at the longitude of my computer (Cambridge UK). This can be very confusing because it jumps from UTC to UTC+1 hour, when the day crosses into daylight saving time. It would be nice to be able to suppress this if required and stick to UTC (corrected for DT) or set to another local time corresponding to the longitude at which one has chosen to look at the sky.

Stellarium is superb!

best wishes
Robin Catchpole

Question information

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Status:
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Stellarium Edit question
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Last query:
2010-11-17
Last reply:
2010-12-14

There is a new plugin which will be available with the next release, for more control over the timezone.

I'm not sure about the rotation of the Earth issue. Before you worry about that though, you should take note of the periods of time over which the positions of the various planetary bodies are considered accurate. There is an appendix in the user guide which describes this in detail.

Bogdan Marinov (daggerstab) said : #2

The time zone can be overridden from a setting in the configuration file. In the next version, there will be a convenient GUI for this. If you can't wait, open the configuration file, scroll down to the "[localization]" section and change the value of "time_zone" to "UTC+0" for UTC.

It uses the format of the POSIX TZ variable:
http://www.gnu.org/s/libc/manual/html_node/TZ-Variable.html

According to the user guide, Stellarium uses the "ELP2000-82B" model of the Moon's orbital movement, and it is only accurate for about a century or so: "Unsure about interval of validity or precision at time of writing. Possibly valid from 1828 A.D. to 2047 A.D."

So I'm not sure if an implementation of delta-T would do any good. :)

R catchpole (catchpol) said : #3

Dear Matthew and Bogdan,

Thanks, I am pleased to hear about the time issue and how to change it.

I did read the manual and noted that it is uncertain about the ephemeris
for the Moon before 1800AD with the implication this would be sorted out
at some stage. Accurate ephemerides are available earlier as these are
used in NASA publications (2000BC to 3000AD) see below. Planet positions
on the other hand are good to seconds of arc.

The Delta T issue is pretty important and if it is not implemented then it
can be applied by hand. What it ammounts to in practice is whether a
conjunction or occultation will take place above or below the local
horizon. Of course it applies into the future as well as into the past.

If it is not implemented as seems to be the case then thats fine I know
what to do. Many thanks.

If you are interested in bringin Stellarium up to a high precission and if
you want to know more, hhave a look at these references.

You may like to look at

http://eclipse.gsfc.nasa.gov/SEpath/ve82-predictions.html

It gives details for the moon but the link to delta T is broken but here
it is

http://eclipse.gsfc.nasa.gov/SEhelp/deltaT.html

Sorry I dont have the skill to implement myself.

Best wishes
Robin Catchpole

----------------------------------------------------------------------------
Robin Catchpole (Dr)

Phone 01223 337 553 (Cambridge) Institute of Astronomy
SwitcB 01223 337 548 (Cambridge) Madingley Road
                                             CAMBRIDGE CB3 OHA
mobile 077 1421 8670 (cellnet) U.K.

email <email address hidden>
web http://www.ast.cam.ac.uk/~catchpol/
----------------------------------------------------------------------------

On Wed, 17 Nov 2010, Matthew Gates wrote:

> Your question #134432 on Stellarium changed:
> https://answers.launchpad.net/stellarium/+question/134432
>
> Status: Open => Answered
>
> Matthew Gates proposed the following answer:
> There is a new plugin which will be available with the next release, for
> more control over the timezone.
>
> I'm not sure about the rotation of the Earth issue. Before you worry
> about that though, you should take note of the periods of time over
> which the positions of the various planetary bodies are considered
> accurate. There is an appendix in the user guide which describes this
> in detail.
>
> --
> If this answers your question, please go to the following page to let us
> know that it is solved:
> https://answers.launchpad.net/stellarium/+question/134432/+confirm?answer_id=0
>
> If you still need help, you can reply to this email or go to the
> following page to enter your feedback:
> https://answers.launchpad.net/stellarium/+question/134432
>
> You received this question notification because you are a direct
> subscriber of the question.
>

I don't think an alternative calculation method for the Moon is very likely in the short term. Is there a particular method you had in mind?

R catchpole (catchpol) said : #5

Dear Matthew,

Thanks for email, the first web reference in my last email gives a
reference and
brief discussion of the method used by NASA for their eclipse predictions.
I guess a truncated version of that would be the way to do it. Fred
Espenak discusses the extent to which they have truncated it for their
eclipse work.

Best wishes
Robin
----------------------------------------------------------------------------
Robin Catchpole (Dr)

Phone 01223 337 553 (Cambridge) Institute of Astronomy
SwitcB 01223 337 548 (Cambridge) Madingley Road
                                             CAMBRIDGE CB3 OHA
mobile 077 1421 8670 (cellnet) U.K.

email <email address hidden>
web http://www.ast.cam.ac.uk/~catchpol/
----------------------------------------------------------------------------

I'm working on delta-t integration. For more information and progress reports, please follow this blueprint:

https://blueprints.launchpad.net/stellarium/+spec/stellarium-deltat

Matthew

Allan Johnson (snofriacus) said : #7

This morning in searching for info on the status of this Delta T fix, I encountered a related bug report, so I've linked it to this question.

Quick status update: I've not had time to work on this for a while, but I might be able to find time soon... stay posted.

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