Considering light speed

[Mariusz Kuźmiński]

I have got the following example: when we look at Jupiter from short dinstace (using G key), its Great Red Spot is in the same place, such as we look at Jupiter from Earth with large magnification. Its moons are visible in the same places, too. Normally, we can see all that things later (by telescope) - light goes from Jupiter to us for about 35-40 minutes
Is it possible to attend this problem for the next version of program?

[t00fri]

I have got the following example: when we look at Jupiter from short dinstace (using G key), its Great Red Spot is in the same place, such as we look at Jupiter from Earth with large magnification. Its moons are visible in the same places, too. Normally, we can see all that things later (by telescope) - light goes from Jupiter to us for about 35-40 minutes
Is it possible to attend this problem for the next version of program?

I have implemented the LightTravel (LT) delay feature already ~10 years ago into Celestia.
You just have to switch it on. If activated you will see a green LT in the top right text block.
You toggle LT by pushing the '-' key.

LT is an excellent tool e.g. for looking at extraterrestial eclipses viewed by an Earth-based observer. Here is a neat example application of LT to Jovian moon eclipses:

The precise timing of the annular 60% shadow of Europa on Io (event 2e1A)
on Dec 27, 2002, start 7:20 UT end 7:30 UT

Here is what one gets spot on, at 7:23 UT with LT (LIght travel delay) activated


If LT is not activated, the eclipse takes place MUCH earlier, at 6:47 UT instead...

In 2007, I summarized a few more interesting timing examples that work very precisely with my LT feature: viewtopic.php?f=4&t=11410&p=92375&hilit=+light+travel#p92375

I have written many explanations of the LT action in this forum in the course of time. Just search for them. One such description of LT with an explicit example you find here:
viewtopic.php?f=2&t=2236&p=14413&hilit=+light+travel#p14413

In any case this post of yours is off-topic in the "Bugs" department . The LT key-shortcut
can be found in the Celestia file 'controls.txt' or in the Celestia Wiki book
http://en.wikibooks.org/wiki/Celestia/Time_Control since a long time.

Fridger

[VikingTechJPL]

Mariusz,

Your comments are correct, and Fridger is in error that this is "off-topic" in the "Bugs" department.

This issue, that "global light-time delay" does not exist in Celestia, has been discussed at length in this forum — in the "Bugs" department. Here's the thread:
http://www.shatters.net/forum/viewtopic.php?f=2&t=15817

In that thread, Chris mentions that he did not implement "global light-time delay" originally because the added computations necessary would hurt performance. The effect of not implementing "global light-time delay" is that objects in Celestia are not plotted in their "actual" locations in 3-D space at their proper times, but only in their positions as they appear from Earth. This difference is not great in most cases, and you'll find an analysis for the major planets in the thread above.

Fridger's addition of Light-Time Delay does a good job of correcting for the times of Solar and Lunar Eclipses on other planets as seen from Earth, though it is not valid for planet occultations. In addition, it currently ignores correcting for the time-vs-position of component stars of Celestia's multiple star systems.

I don't know whether Chris has decided to tackle the "global light-time delay" issue in future versions of Celestia, as he suggests that it may not be an easy task. Likewise Fridger suggests that, if it is implemented, it should also include relativistic effects. The question is whether this level of accuracy is needed, balanced against the effort required to include it.

In any event, your thinking in your post is sound, and it's always good to see Forum members asking questions that may ultimately improve future versions of Celestia.

-GW

[t00fri]

Mariusz,

Your comments are correct, and Fridger is in error that this is "off-topic" in the "Bugs" department.

Come on...

This issue, that "global light-time delay" does not exist in Celestia, has been discussed at length in this forum — in the "Bugs" department. Here's the thread:
http://www.shatters.net/forum/viewtopic.php?f=2&t=15817

In that thread, Chris mentions that he did not implement "global light-time delay" originally because the added computations necessary would hurt performance. The effect of not implementing "global light-time delay" is that objects in Celestia are not plotted in their "actual" locations in 3-D space at their proper times, but only in their positions as they appear from Earth. This difference is not great in most cases, and you'll find an analysis for the major planets in the thread above.

The whole discussion about Light Time delay and special relativity more generally, was initiated back in 2002 not by Chris but by me! Having decades of experience in scientific programming and being a professional expert about relativity (unlike Chris), I was aware from the beginning that an extended LT version as you and me would prefer it, cannot be realized in practice. That's why I implemented my "baby" version of LT about 10 years ago.

And that statement remains valid til today: Celestia is for normal people with normal computers. This means that an extended version of LT is "asking far too much".

It was my impression that the request of Mariusz Ku?mi?ski above would be well accounted for by the implemented version of LT. That's why I still feel that this discussion in the bug department is off-topic. Moreover, implementing simplifications or approximations for reasons of computer performance is unavoidable in general and thus can hardly be called a bug! In the course of Celestia development we had to make many conscious code & astrophysics simplifications for reasons of hardware limitations.

Fridger's addition of Light-Time Delay does a good job of correcting for the times of Solar and Lunar Eclipses on other planets as seen from Earth, though it is not valid for planet occultations.

That's what I wrote above. Yet you are surely aware that the observer does NOT need to be located on Earth. Could be anywhere far away from the event under observation.

I don't know whether Chris has decided to tackle the "global light-time delay" issue in future versions of Celestia, as he suggests that it may not be an easy task.

Celestia development is DEAD ( or at best in deep coma) since about ONE year. Just in case you missed that...

Fridger

[Mariusz Kuźmiński]

OK, thanks to all of You I found "Shift + ?" and "-" shortcuts in Celestia User's Guide. Really nice tool!
I'm still quite fresh user of Your great program

[PlutonianEmpire]

"Normal computers"? Come on, it's been ten years. Have "normal" computers REALLY stayed the exact same the ENTIRE ten years, with a "normal" computer having the EXACT SAME configurations and crap performances from ten years ago?

[John Van Vliet]

--- edit ---

[t00fri]

"Normal computers"? Come on, it's been ten years. Have "normal" computers REALLY stayed the exact same the ENTIRE ten years, with a "normal" computer having the EXACT SAME configurations and crap performances from ten years ago?

Of course, computers and graphics cards have become more powerful during the past 10 years... in principle. As to reality, John has already given some arguments above.

10 years ago, many people still used desktops, while today most use laptops. I still have a desktop with a Pentium 4, 3.2 GHz and lots of (fast) RAM (4GB) that is almost 10 years old. It can easily compete with today's popular (lower) middle class laptops with CPU <= core2Duo, i3-i5 (without Turbo features, say). Laptops are often quite weak as to graphics performance for good reasons (limited cooling abilities and power saving aspects). Many even have integrated on-board graphics chips. Notably so-called business computers often use such weak graphics hardware. All these aspects add up to form an average performance index.

Since Celestia was never meant to be only for hardware freaks, and many users are still young without too much money at hand...I can safely confirm that an extended LT approach would be far too much, even today.

There is another, probably most important argument:

Already the presently implemented LT approach is only used by few. Many apparently don't even know about its existence. Since it costs NO performance => so what!

But the extended LT approach (requiring also a special relativity framework for consistency), does cost lots of performance and will choke even todays average computers. Yet such an extended multi-body LT feature would address even fewer Celestians with special interests in precision timings of a special class of celestial events that are outside the validity of the present LT feature!

On the other hand, there are many advanced (graphics) features ahead that will make Celestia or more realistically celestia.Sci increasingly attractive to MANY. These also require performance reserves, such that a selection of features to be implemented is unavoidable. I clearly vote for the latter features, of which MANY will benefit.

According to my informations at least, there is little hope that ANY exciting new features will ever be integrated into a future Celestia distribution. Last time that ChrisL showed up with a 2-line mail in the developers list was in May of this year. The last Celestia-related post in this forum by him was in Dec. 2011! All other devs apart from myself are gone and I am busy with celestia.Sci...

As you can see from the ongoing discussions over at CelestialMatters, I am continuously working very hard towards making the ongoing celestia.Sci development public ASAP.

Fridger