Light Time Delay: possible workaround

Calculus · Post #1by **Calculus** » 26.01.2003, 01:37

Since it is not easy to take into account the light time delay in Celestia (dixit Chris), I just thought of a very easy thing that can be done:
when an object is selected, one can read its distance from the viewer location. would it be possible to also display the corresponding light time ?
Even though it is not THE Solution, it would give a good hint.

Post #2by **t00fri** » 26.01.2003, 03:23

Calculus wrote:Since it is not easy to take into account the light time delay in Celestia (dixit Chris), I just thought of a very easy thing that can be done:
when an object is selected, one can read its distance from the viewer location. would it be possible to also display the corresponding light time ?
Even though it is not THE Solution, it would give a good hint.

I have already coded a nice and practical light travel delay scheme. Christophe has even put it into the KDE-GUI.

I have attached to the '?' key a flash message of the light travel delay between the observer and the selected object in hr:min:sec (if the delay is < 1day).

To the '-' key I have bound the subtraction of the LT-delay from the actual time along with a flash display again.

I found by comparison with horizons that for all bodies in the solar system the accuracy is essentially /perfect/.

It is very practical, indeed...

Bye Fridger

Post #3by **t00fri** » 26.01.2003, 03:24

Calculus wrote:Since it is not easy to take into account the light time delay in Celestia (dixit Chris), I just thought of a very easy thing that can be done:
when an object is selected, one can read its distance from the viewer location. would it be possible to also display the corresponding light time ?
Even though it is not THE Solution, it would give a good hint.

I have already coded a nice and practical light travel delay scheme. Christophe has even put it into the KDE-GUI.

The trick is to only incorporate LT-delay when it is needed for quantitative work, like mutual satelite events etc.

So, I have attached to the '?' key a flash message of the light travel delay between the observer and the selected object in hr:min:sec (if the delay is < 1day).

To the '-' key I have bound the /subtraction/ of this LT-delay from the actual (JD) time again with a flash display.

I found by comparison with horizons that for all bodies in the solar system the accuracy is essentially /perfect/.

It is very practical, simple and efficient, ...

Bye Fridger

Redfish · Post #4by **Redfish** » 26.01.2003, 20:39

That's quite cool. If you think about it. You're near the earth and watching the sun. Which has a delay of 8 minutes. So the light you see at the earth left the sun 8 minutes ago. You now start moving towards the sun with a given speed. The delay now becomes smaller and smaller until you are at the sun. But the thing i can't figure out is: are you travelling back or forth into the time then? As seen from a timepoint of the earth. This is really weird.

Post #5by **t00fri** » 26.01.2003, 21:50

Redfish wrote:That's quite cool. If you think about it. You're near the earth and watching the sun. Which has a delay of 8 minutes. So the light you see at the earth left the sun 8 minutes ago. You now start moving towards the sun with a given speed. The delay now becomes smaller and smaller until you are at the sun. But the thing i can't figure out is: are you travelling back or forth into the time then? As seen from a timepoint of the earth. This is really weird.

Christophe and I have been working together actively for much of today to get the Light-Travel (LT)-delay scheme incorporated OS-independently (me) and in the KDE-GUI (Christophe).

We shall check in the code into CVS still tonight. I advice people who are able to to their own Celestia compilation to upgrade from CVS, if they like to play with this. It's cool and works very well.

One may e.g. keep the '?'-key pressed down while travelling towards the Sun, watching the flash message. You then get a continuous output of the decreasing time delay of events on the Sun relative to you, the observer.

Bye Fridger

Post #6by **t00fri** » 26.01.2003, 22:41

After quite a bit of joint efforts by Christophe and myself today, this is how the new light-travel delay scheme is incorporated in the LINUX kde3 GUI 'set time...' dialog:

The current selection is displayed along with the light-travel delay time between the observer and the selection. Pushing simply the "Subtract" button, subtracts the LT-delay from the current simulation time. The menue structure has also been considerably simplified...

Bye Fridger