# Thread: Light speed and calculating spacecraft routes to other planets

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## Light speed and calculating spacecraft routes to other planets

In the course of discussing light physics and vision, the question has been asked how the actual location of a planet, say Mars, is determined and if the calculation uses the apparent location as a variable, and if the calculation corrects for the light speed time delay. If we can get an actual formula and brief explanation, that would be most helpful.

I think this question is fairly elementary, but our interlocutor at another board remains dissatisfied with our answers, and would like input from experts. Here is the actual question:

"When sending rockets to other planets, do scientists take into account that the image of a planet produced by a telescope shows a delayed true location, due to the finite speed of light?"

"does this delay factor into the equation which determines the actual location of the planet?"

Thanks in advance for any input.

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David,

I've been slightly interested in this question before. It has
been discussed previously, and I vaguely recall that it was
is that as far as practical navigation is concerned, the
difference in location is so small that it is well within the
range handled by midcourse corrections, which are always
assumed to be needed due to other uncertainties. If the
navigation is controlled from Earth, not autonomously by
the spacecraft, the course corrections could either be
calculated based on observations from the spacecraft or
on observations from Earth, and either way the light-travel
delay gets progressively more and more fully taken into
account as the spacecraft nears its target.

However, the timing of events is equally important, and I
know the timings take the light-travel delay into account.
When I watched coverage of a Mars lander a few years
ago, we were told at one point that it must have landed
just then, but the signal, with "live" pictures, wouldn't
reach Earth for another twenty minutes. Of course, the
landing sequence was probably triggered by changes in
an accelerometer as the spacecraft passed through the
atmosphere, not just by a timer.

-- Jeff, in Minneapolis

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Thanks for the reply, Jeff. Now, just to be more precise. Let us suppose, hypothetically, that we assumed that the apparent position of Mars in the sky, as seen on earth, was its actual position. IOW, we assume that we see Mars in real time, and not delayed time. If now we plotted a trajectory on that basis, without any course corrections either handled from Mission Control or automatically executed by the on-board computers, would our hypothetical spacecraft rendezvous with Mars, or miss its target?

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It would miss.

Suppose Mars is passing in front of five distant stars, one after
the other: A, B, C, D, E. You launch your rocket assuming that
Mars is where you see it now, in front of star A, and will be in
front of star C when the rocket arrives.

So you plan the trajectory to make the rocket end up where
you see star C when it reaches the distance of Mars.

But Mars is actually in front of star B when you launch, and
will actually be in front of star D when the rocket gets to the
vicinity. What you will see is the rocket moving away from
you slower than you expected, with signals from it red-shifted
by Doppler effect. You see it reach the vicinity of Mars, in
front of star C, later than expected, when you see Mars in
front of star D. At that moment, Mars will actually be in front
of star E, and your rocket will be well beyond the intended
rendezvous point.

-- Jeff, in Minneapolis

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You'd miss it. The image is delayed on average by about 15 minutes or 900 seconds, mars' orbital speed is 24 km/s so it would be a little over 20000 km ahead of where you thought it is, which is more than its radius (which is about 3400 km).

6. When you get coordinates of solar-system bodies from JPL'S Horizons web service, it explicitly says that light-travel delay is included (that is, the calculation tracks positions in a common time frame and corrects to the observer's location).

To see how big the difference is, take Mars as an example. At a typical distance of 1 AU, the light-travel delay is 8 minutes. Mars moves in its orbit at a mean velocity of about 24 km/second, so in that time it moves about 1900 km, almost 2/3 of the planet's radius. Furthermore, you'd have to take this into account because the light time differs with the relative positions of Earth and the target - for Mars, it ranges from about 3-21 minutes.

7. Originally Posted by caveman1917
... The image is delayed on average by about 15 minutes or 900 seconds...
... and at the closest, about 3 minutes (186 seconds) away.

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Thanks.

We are currently attempting to persuade an interlocutor at another board that we do not see in real time. She is laboring under the misapprehension that everything we see, including the most distant galaxies, is seen in real time and not delayed time. And yes, we have gone over (multiple times) every other argument you can imagine to show that what she claims to be true, is actually impossible. (Start with special relativity .... etc. etc. etc. I am sure you can imagine the list.) We thought the example of how we make spacecraft rendezvous with bodies like Mars would be a nice example disproving her claims, and we thought it would be interesting to hear from scientists on this matter (although she has already heard from an astrophysicist and a biologist on matters pertaining to light and sight, to no avail, alas!)

We are pretty sure your input won't change her mind, but it was nice to get it. We are pretty sure we are not going to change her mind because she has been arguing for real-time seeing for nearly 1,000 pages (yes, you read that right) at the message board where we are discussing this matter with her; and for probably tens of thousands of pages at various message boards strewn across the hinterlands of the World Wide Web dating for the last ten years. Nevertheless, we will give her this link! Hope springs eternal in the heart of Man!

Thanks again!

9. How about Romer's measurement of the speed of light?
http://en.wikipedia.org/wiki/Speed_o...l_measurements

10. Originally Posted by davidm
Thanks.

We are currently attempting to persuade an interlocutor at another board that we do not see in real time. She is laboring under the misapprehension that everything we see, including the most distant galaxies, is seen in real time and not delayed time.
Doesn't this imply that the speed of light is infinite? What about the audible delay when communicating with astronauts? (Or does she think they are faked as well?) Or even just satellite communications?

In a way, I can see the problem. We can "see" the speed of sound because we observe the difference between seeing a distant event and hearing it. Nothing travels faster ha light so we have no benchmark. Good luck!

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Originally Posted by Strange
How about Romer's measurement of the speed of light?
http://en.wikipedia.org/wiki/Speed_o...l_measurements
Yes, we have discussed this particular measurement with her. I believe that by now it has been raised on at least 127 separate occasions. But who's counting!

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Originally Posted by Strange
Doesn't this imply that the speed of light is infinite? What about the audible delay when communicating with astronauts? (Or does she think they are faked as well?) Or even just satellite communications?

In a way, I can see the problem. We can "see" the speed of sound because we observe the difference between seeing a distant event and hearing it. Nothing travels faster ha light so we have no benchmark. Good luck!
Yes, it would imply that the speed of light is infinite, but no, she does not think that the speed of light is infinite. She believes as follows: by hypothesis, if God turned on the sun at noon, people on earth would see the sun instantly (not have to wait some eight and a half minutes), BUT, they would not see their neighbors standing next to them for eight and a half minutes.

Good luck to us, indeed!

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Delete mystifying double post.

14. Since the future position of any of the potential targets within the Solar System can be calculated within a few meters for next few decades, it's pretty easy to target a spacecraft to where they will be.

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Originally Posted by swampyankee
Since the future position of any of the potential targets within the Solar System can be calculated within a few meters for next few decades, it's pretty easy to target a spacecraft to where they will be.
Right, of course, but we were trying to show our interlocutor is that if we mistook the apparent location of Mars as seen from earth for its real location, the spacecraft would not hit its mark even if all our other calculations were accurate. This by way of proof that we see Mars in delayed time, not in real time, as she maintains.

Our efforts to convince her of this fact, via this thread, have failed.

16. Originally Posted by davidm
Right, of course, but we were trying to show our interlocutor is that if we mistook the apparent location of Mars as seen from earth for its real location, the spacecraft would not hit its mark even if all our other calculations were accurate. This by way of proof that we see Mars in delayed time, not in real time, as she maintains.

Our efforts to convince her of this fact, via this thread, have failed.
If she is a "true believer" in some line of thought that is contrary to valid physics, I would not be optimistic about changing her mind with any reasoning.

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Originally Posted by Hornblower
If she is a "true believer" in some line of thought that is contrary to valid physics, I would not be optimistic about changing her mind with any reasoning.
Believe me, if only you knew. But trust me, you don't want to know.

18. Originally Posted by davidm
She believes as follows: by hypothesis, if God turned on the sun at noon, people on earth would see the sun instantly (not have to wait some eight and a half minutes), BUT, they would not see their neighbors standing next to them for eight and a half minutes.
No ... But ... That's ...

Good luck to us, indeed!
I think you need a miracle, mate!

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Originally Posted by Strange
No ... But ... That's ...

I think you need a miracle, mate!

20. I wonder if this person was around when international phone calls across the atlantic ocean would go through satellites, and if she remembers the awkward delays that made conversations difficult.

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I may well regret this, but anyway ...

Is it only light? Or is it (almost) all of physics?

And is it a case of someone who is, in a specific and narrow sense, innumerate? By this I mean, as an illustration, demonstrating that feathers fall as fast as stones, in a vacuum is accepted if a feather and a stone are put in a transparent, evacuated cylinder and released together (something you can see with your own eyes), but not if you timed the fall of each, separately, and showed that the time to fall is the same (within some, clearly defined, error)? By extension, can this person follow - and accept - a presentation of Newtonian gravity, showing that apples and the Moon 'fall' the same way (for example)? If not (i.e. she is, in this narrow sense, innumerate), I submit that you have no common basis for a discussion; indeed, none of your 'discussions' have been anything more than two people talking past each other (no communication has been taking place).

On the other hand, ...

22. Originally Posted by davidm
She is laboring under the misapprehension that everything we see, including the most distant galaxies, is seen in real time and not delayed time. And yes, we have gone over (multiple times) every other argument you can imagine to show that what she claims to be true, is actually impossible. (Start with special relativity .... etc. etc. etc. I am sure you can imagine the list.)
Perhaps some kind of entanglement vs light propagation ATM confusion ?
Regards

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She believes that we see in real time (even though she simultaneously and contradictorily believes that light travels at finite speed) because her father wrote a book saying this was so. Her father was not wrong, because, had he ever been wrong, he would have said so; since he did not say so, he was not wrong. Hence he was right. Real-time seeing, coupled with her father's contention that "man's will is not free" are propositions that, once mutually accepted, will immediately lead to total world peace. When her father asked for an audience with President Carter to show him how to achieve world peace and the president refused to grant it, he sued President Carter.

I told you probably didn't want to hear this.

I think myself and a small doughty band of others have spent considerable time with her because we are simply gobsmacked at the obduracy of her irrationality. It's a case study like no others. She makes young earth creationists look like paragons of rationality.

24. Hi davidm;

Thanks for background. IMHO, sounds like it could be good let this one get away (?)
Everyone has the right to their beliefs.

My humble opinion is, if it comes down to her trusting her father, or adopting a rational perspective, one has to question the morality of forcing the issue further (??) (Sounds like its already gone beyond several boundaries).

A reasonable position would be that she goes forth acknowledging that there is a scientific view, which differs from her personal view, and if she can go forth without distorting this view, or attempting to misrepresent it, it would be kindly appreciated.

Regards

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Originally Posted by davidm
She believes that we see in real time (even though she simultaneously and contradictorily believes that light travels at finite speed) because her father wrote a book saying this was so. Her father was not wrong, because, had he ever been wrong, he would have said so; since he did not say so, he was not wrong. Hence he was right. Real-time seeing, coupled with her father's contention that "man's will is not free" are propositions that, once mutually accepted, will immediately lead to total world peace. When her father asked for an audience with President Carter to show him how to achieve world peace and the president refused to grant it, he sued President Carter.

I told you probably didn't want to hear this.

I think myself and a small doughty band of others have spent considerable time with her because we are simply gobsmacked at the obduracy of her irrationality. It's a case study like no others. She makes young earth creationists look like paragons of rationality.

If this is so, then I can't imagine how one could ever reach sufficiently broad mutual agreement - a foundation, if you will - on which to start having a meaningful discussion.

As Selfsim said, in this case the most you (collectively, including 'her') could expect to do is agree that there is no basis for a meaningful discussion (insufficient mutual consent on 'the ground rules', perhaps).

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Originally Posted by davidm
She believes that we see in real time (even though she simultaneously and contradictorily believes that light travels at finite speed) because her father wrote a book saying this was so. Her father was not wrong, because, had he ever been wrong, he would have said so; since he did not say so, he was not wrong. Hence he was right. Real-time seeing, coupled with her father's contention that "man's will is not free" are propositions that, once mutually accepted, will immediately lead to total world peace. When her father asked for an audience with President Carter to show him how to achieve world peace and the president refused to grant it, he sued President Carter.

I told you probably didn't want to hear this.

I think myself and a small doughty band of others have spent considerable time with her because we are simply gobsmacked at the obduracy of her irrationality. It's a case study like no others. She makes young earth creationists look like paragons of rationality.
With the greatest of respect, and recognising we're straying off topic, but why are you bothering trying to change her mind? If a lone person is as mistaken and stubborn as you describe, what benefit is there in continuing to engage with her?

In my experience it's less frustrating to politely agree to disagree and walk away. The only people you need to worry about are onlookers who may be convinced by her arguments. But if you've discussed the issues this clearly for this long, I find it hard to believe many would accept her arguments.

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I think good advice is being given here, and walking away is the right course of action. A small band of us has stayed with it for a very long time because I think we are like rubber neckers at a spectacular wereck, unable to believe our eyes. The latest from our interloctuor: the light-speed correction for sending craft to Mars and other heavenly bodies only means that "something else is going on," because, after all, we definitely see in real time; likewise, she has said, the fact that we see the moons of Jupiter in delayed time is "a coincidence," because we definitely seem them in real time. That level of irrationality is beyond remediation. Thanks for the responses.

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Hello Davidm, since this thread contains evidence that Lessans is incorrect, do you really think she will look at it?
In the mean time I'm going to try to address a few points here.

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Originally Posted by Strange
Doesn't this imply that the speed of light is infinite? What about the audible delay when communicating with astronauts? (Or does she think they are faked as well?) Or even just satellite communications?

In a way, I can see the problem. We can "see" the speed of sound because we observe the difference between seeing a distant event and hearing it. Nothing travels faster ha light so we have no benchmark. Good luck!
I believe that I know the material in the book and her thought process (and that really scares me) well enough to speculate that she will probably say that because seeing is efferent we do it instantly but radio communications is different and still is limited to the speed of light, which she acknowledges for the entire electromagnetic spectrum. The idea that we see in real time means that the light/photons involved in seeing do not travel because they are at the object then instantly at the retina, somehow?

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Originally Posted by Strange
No ... But ... That's ...

I think you need a miracle, mate!
And that is what many of the responders on the other forum have attributed to her explination of vision, but she claims that everything is mathematical and scientific, but her definitions of these terms are really hard to pin down.

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