Celestia Forums

Hi guys,

I hope you can help me clarify some things concerning the handling of the differnt time concepts used in celestia.

Let me list up, what I understood:

Celestia uses UTC to display on screen

Celestia uses TDB internally for all SSC and trajectory calculations

and tDB is around 32 seconds off from UTC (at the moment)

But what i don't understand now is, as what does Celestia see for example time declaration in an SSC?

For Expamle: Beginning "2009 04 03 04:42:51"

Is this now UTC or TDB or something else? And what happens with other time related commands in what format must i enter them in the SSC? Do i have to change those values over time?

I think there is some good and clear explanation needed (here in the forum and in the documentation!)

Or wouldn'T it be better to use only UTC in the SSC and let Celestia automatically convert those values into the right format for whatever it needs?

Or let the User add a Flagg like UTC to the time so that Celestia konws what is meant.
Otherwise a lot of addons with exact positioning would have to updated i think.

Just my 2 cents and questions on this.

Regards,

Guckytos

SSC files are supposed to be using TDB.
If they aren't, it's a bug.

Documentation is at
http://en.wikibooks.org/wiki/Celestia/Time_Scales

In particular, see the last paragraph:

UTC is used in the Celestia's Set Time dialog and it's also the time displayed in the upper right of the screen. Unless you're creating scripts or add-ons for Celestia, UTC is all that you will see. But everywhere else, the time scale is TDB: xyz tractory files, attitude files, beginning, ending, and epoch times in .ssc files.

Guckytos wrote:Or wouldn'T it be better to use only UTC in the SSC and let Celestia automatically convert those values into the right format for whatever it needs?

Or let the User add a Flagg like UTC to the time so that Celestia konws what is meant.
Otherwise a lot of addons with exact positioning would have to updated i think.

TDB is related to the fundamental physics of motion. So it has universal applicability. UTC is tied to the rotation of the earth, which is irregular and unpredictable. UTC can be converted to TDB only for events in the recent past where the conversion factor has been determined. It can't be used to accurately specify times for future events, or for events in the more distant past.

- Hank

Perhaps i wasn't to clear on what i really wanted to work out. (Probably because i myself wasn't too sure what i wnated.)

Well I am not too sure if i now know what i want to ask, but anyway ...

So, someone who wants to create an SSC of let's say some spacecraft, he has to take the date of the spacecraft (probably in UTC) and the put up the
Beginning statement in the SSC in TDB (which is at the moment UTC - 1 minute and 5.184 seconds?).

Okay, i can understand that. But what is the correction factor if the time is some decades in the past of hundreds of years in the future?
What happens then to an object that is supposed to be at an exact location and orientation in space and time?
Do i have to recalculate the values every other year? Is that then sensible?

And what happens to all the time entries in julian timescale?

With no doubt, from a scientific point of view it is totally correct and sensible to use now TDB.

But what happens to those of us that never knew of that change? (for whatever reason)

Well, before i get once more into babbling and confusing myself:

Wouldn't it be better to enter dates in the catalogue files (SSC, STC) in UTC and let Celestia do the work of converting UTC to TDB? So you could always be sure that the right time is used and everything shows up where it should. Or am i going now again amiss?

Regards,

Guckytos

Guckytos,

All of the professional ephemerides use TDB now: time is measured linearly with no arbitrary steps to compensate for the variations in the rotation of the Earth. As a result, it is important for Celestia to use TDB, too. Otherwise it will place the planets and spacecraft in the wrong locations.

For example, when Spice kernels are used with Celestia v1.5.0, Celestia now places spaceprobes at the same positions relative to their targets that NASA and ESO report. JPL's DE ephemerides place objects at the correct locations in Celestia, too. There used to be offsets which weren't understood.

All of the orbital parameters and xyz trajectories obtained from Horizons use TDB. However, one does have to be very careful to determine what time scale was used when using data from other sources for spacecraft trajectories and astronomical events.

A more complate description of TDB than is in Celestia's WikiBook is available in
http://aa.springer.de/papers/8336001/2300381.pdf
which describes its use by JPL and CfA (the Harvard-Smithsonian Center for Astrophysics).

An inconsistency is that Celestia does not actually display any variations in the rotation of the Earth. If you look in solarsys.ssc, you'll find that it is specified to rotate with a constant period of 23.9344694 hours (one sidereal day). Hopefully this will be corrected in a future version, perhaps using a ScriptedRotation.

Okay,

thank you Selden for explaining it for me. I will take a look at the PDF once i find some free time.

If everyone scientifig is using TDB it is clear that Celestia is also using it.

Regards,

Guckytos

Uh...
If i need to find the difference between TDB and UTC for a given date, is there a converter somewhere?

If the difference is 32.5 seconds today, how can i find what it was in november 1988?

If i must factor in leap seconds that have been added during the years, is there a table somewhere that lets me count them?

Spesificially, i want to generate julian dates that conform to the Celestia displayed clock for a series of events.

- rthorvald

rthorvald wrote:If i need to find the difference between TDB and UTC for a given date, is there a converter somewhere?

Runar,

You can use the following Celx script that displays the simulation TDB Julian date:

Vincent wrote:You can use the following Celx script

Thanks!

- rthorvald