psybrain wrote:b) Leave all bodies unchanged and just correct the time display for light time delay. This is the simple realization I chose in the code. It is meant mainly as a simple help if people want to reproduce mutual precision events among various moons, for example.
When mooving quickly away from earth now, you will see the /clock changing/continuously, but not earth's position. I simply calculate and display the different time at which a remote observer would correctly have the displayed view, given the finite speed of light.
wouldnt it be correct to ADD light time then, instead of subtract it? the current view of earth for example will be visible for the sun in 8 mins not 8 mins ago.
greets!
This is a matter of the frame or in other words, depends how the
displayed time is to be interpreted!
The main purpose of the '-' key is best illustrated with a concrete example:
Suppose an observer /located on earth/ wants to reproduce from an astronomical document a
tabulated mutual event among jupiter moons, say. The given times
usually refer to Universal time on earth and have the LT-delay already
incorporated. The Celestia simulation time that you must enter to
reproduce that event, however, corresponds to the time for that event
in jupiter's local frame, which is 36-38 mins /earlier/ than the
/corresponding/ UT on earth, since it takes the light 36-38 mins to
travel from jupi to earth.
In summary, what is displayed if the '-' key is hit, is the time in the local
frame of the /selected/ object that corresponds to /exactly the same
view/ of the object which the far-away observer (e.g. on earth) has.
The use of the '-' key avoids tediously calculating the LT-delay and
then subtracting it from the given time of an event to obtain the time
that has to be keyed into Celestia to simulate that event.
Incidentally:
When the far-away observer (on earth) now starts travelling towards
the selected jupiter (hit G), the time display will /increase/ again
quickly and continuously, until it corresponds to the actual time on
earth, precisely when the observer has reached jupiter. Note, by
definition, anywhere in the universe, the observer measures time with
a clock showing always earth time (local or UT)!
Since special relativity is not yet incorporated, an observer (unlike
light!) can even travel /infinitely/ fast e.g. from earth to jupiter. He is
kind of a "superman";-).
The best way of understanding these somewhat complex matters might be to try this
all out with Celestia while reading...
Bye Fridger
