But I have seen such eclipses. They are instantaneous. You can do the very same thing.

I am afraid we cannot convince each other.

Well, you have not convinced me, to the extent that I don't see that you actually have an argument as opposed to some hand waving with the words "parallax" and "resolution" thrown in.

Cheers.

can't win'em all
edit: I would like to thank you though for the opportunity you offered me to clarify some points. You did not make me change my mind, as I did not make you change yours. But that is is the risk and also the benefit of an open discussion.

You're misunderstanding Roemer's observation. At any constant distance between the Earth and Jupiter, the observed time between a Jovian moon's disappearance and re-appearance would be the same.

But what if the distance between Earth and Jupiter is different when the times of disappearance and re-appearance are recorded? That's when you have information from which to calculate the speed of light.

So, your criticism of the validity of Roemer's determination of the speed of light is based on a misunderstanding of how he made that determination.

Michael Ossipoff

It's like watching a pendulum on Jupiter. The time at which an eclipse is seen varies from the predicted time at different time of the year. The difference is down to the distance between Earth and Jupiter.

It's like watching a pendulum on Jupiter. The time at which an eclipse is seen varies from the predicted time at different time of the year. The difference is down to the distance between Earth and Jupiter. — Banno

The duration of the observed time during which a Jovian moon is eclipsed depends on the difference in the distance between Earth and Jupiter. at the times of that moon's disappearance and re-appearance.

The change in the observed eclipse duration, when the disappearance and re-appearance are first observed from a certain two distances, and then observed again at a different pair of distances, is what enables calculation of the speed of light.

Michael Ossipoff

For accuracy, it would be good to measure one eclipse duration when the Earth is moving rapidly toward Jupiter, and measure another eclipse duration when the Earth is moving rapidly away from Jupiter.

...so that the two pairs of distances, for the two measurements, would give the greatest possible difference in observed eclipse-duration.

Michael Ossipoff

It's not the eclipse duration that is the issue; it's the time between eclipses.

But what if the distance between Earth and Jupiter is different when the times of disappearance and re-appearance are recorded? That's when you have information from which to calculate the speed of light. — Michael Ossipoff

The whole argumentation is based on the idea that we see the (dis)appearance at the moment it happens, and that distance does not have any effect on our perception and observations.

I am afraid that is a circular argument that cannot be proven.

Please note that I am not discussing the speed of light, only Rømer's argument how to calculate it.

It's not the eclipse duration that is the issue; it's the time between eclipses. — Banno

That would work too.

Michael Ossipoff

One advantage of using the duration between two disappearances, instead of between a disappearance and a re-appearance is that a you'd have observational information about when to expect a disappearance.

...whereas the time of an appearance could be predicted only by clock, and clocks weren't as accurate in Roemer's day.

Of course the clock's inaccuracy would affect the speed-determnation's accuracy anyway, but you still wouldn't want the added uncertainty about when the re-appearance happened. ...if you came back to the telescope some time after the re-appearance.

Michael Ossipoff.

The first version of my objection was completely different. I started doubting its validity and after some time settled for a different approach.

I was not, and I am still not sure whether the idea of different times vs different positions in space made any sense.

I am presenting it here in the hope that your critical comments will help me better make up my mind.
***
1) Rømer, and Huygens' drawing of Earth (p.8/22) moving on an orbit path is rather deceiving in its simplicity. What we have is a point on Earth, the observation point - and the location where the clock is situated- that continuously travels the circumference of the earth at the same time it is orbiting the sun. The path is more a spiral than an ellipse. We cannot therefore judge of the speed of light by taking two single points on the orbit path as landmarks. We have to take into account the whole distance traveled, and the time it took the observation point to move from A to B.
The only way to, justifiably, consider the straight line BC as the distance used to calculate the speed of light, would be to have two synchronized clocks, one at B, the other at C.

2) Furthermore, Rømer and Huygens assume that they are allowed to compare the times of observations. For them, observing Jupiter's moon at, say, 5 o'clock, when Earth is at position B, and then at, say, 5:10 when it is at position C, allows us to draw a conclusion about light speed. But 5 o'clock at B is not (necessarily) the same as 5 o'clock at C.
When timing the eclipse, the astronomer uses the same clock through the seasons. Even assuming a regular clock with no deviation whatsoever, 5 o'clock will indicate another position of earth relative to the sun at different points on the orbit path. The sun does not rise or set at the same time each day, due to the tilting of the earth. The two factors, appearance of Jupiter's moon, and the time indicated by the clock are only related to each other by the presence of an observer or a proxy device. The observer/machine relates a clock time to a physical event. Both events, the appearance and the clock time, are not causally related, so we need the mediation of the observer-machine.


3) Imagine you are Rømer, you have a 17th century atomic clock on your wrist, timing the appearance of Jupiter's moon each time it appears from behind its planet. You can now easily draw a graph with time and distance. That is in fact what Rømer's argumentation ultimately amounts to.
What is wrong with such a view?
Well, it is simply too... simple.

The whole point is the moment the observer (can be a machine) sees the moon reappearing from behind Jupiter. In our graph the observer is implicitly represented by the times shown on the clock, even though they are two absolutely distinct processes. In the case of a machine, programs and physical processes react to the detection of the moon, and then signal the clocking mechanism. As noted there are no causal links between what happens with the clock and the behavior of Jupiter's moon. Its appearance or disappearance have no effect on the functioning of the timing device.
Let us take an example where a causal link between two separate events can be clearly demonstrated. If a stone falls in a ponds, ripples are created on the surface of the water. It is impossible to have one without the other, while the (dis)appearance of the satellite is completely unrelated to the clocking mechanism, unless we create an artificial, technological, link between them. This link I call a proxy observer.
For Hume all events are in fact isolated events. What Hume, as far as I know, did not take into account is that we are able to control and manipulate some events better than others. We have no saying over the orbit of Jupiter's moons, or any other astronomic body, but we can decide what event we want detected and when a clocking mechanism should register it. So, even if we cannot prove the existence of causal links, and must be satisfied with empirical and statistical certainty, we can distinguish between events which we can control, and those we cannot.
Going back to our main theme, the speed of light, we can create correlations between events and timing processes. What we must realize is that we need at least two physical events and a timing procedure to obtain a meaningful set.
In the case in question, we need the appearance of Jupiter's moon linked to distance and time. And distance has to be the distance effectively traveled by the clock and the observer, and not a geometrical abstraction.


4) The conclusion that it takes longer for light to travel to C than to B is therefore not justified by Rømer-Huygen's argumentation. Even though more precise calculations might confirm their conclusion.

1) Rømer, and Huygens' drawing of Earth (p.8/22) moving on an orbit path is rather deceiving in its simplicity. What we have is a point on Earth, the observation point - and the location where the clock is situated- that continuously travels the circumference of the earth at the same time it is orbiting the sun. The path is more a spiral than an ellipse. We cannot therefore judge of the speed of light by taking two single points on the orbit path as landmarks. We have to take into account the whole distance traveled, and the time it took the observation point to move from A to B.
The only way to, justifiably, consider the straight line BC as the distance used to calculate the speed of light, would be to have two synchronized clocks, one at B, the other at C. — Hachem

The Earth's speed in orbit is about 18.5 miles per second.

At Denmark's latitude, the speed of the ground's movement with respect to the Earth's center is about 1/6 of a mile per second.

In fact, if the observation is made when Jupiter is on or near the meridian (a the observer's longitude), then of course the ground at that location would have even less speed with respect to Jupiter.

The limiting factor for Roemer's determination was the accuracy of clocks in his day.

For that matter, are you even sure that Roemer didn't take the Earth's rotational motion into account when determining the distances from him to Jupiter?

2) Furthermore, Rømer and Huygens assume that they are allowed to compare the times of observations. For them, observing Jupiter's moon at, say, 5 o'clock, when Earth is at position B, and then at, say, 5:10 when it is at position C, allows us to draw a conclusion about light speed. But 5 o'clock at B is not (necessarily) the same as 5 o'clock at C.

See above.

When timing the eclipse, the astronomer uses the same clock through the seasons. Even assuming a regular clock with no deviation whatsoever, 5 o'clock will indicate another position of earth relative to the sun at different points on the orbit path. The sun does not rise or set at the same time each day, due to the tilting of the earth.

See above.

The two factors, appearance of Jupiter's moon

It would be better to use disappearances, because there's observational information about exactly when to expect them.

, and the time indicated by the clock are only related to each other by the presence of an observer or a proxy device. The observer/machine relates a clock time to a physical event. Both events, the appearance and the clock time, are not causally related, so we need the mediation of the observer-machine.

If you're saying that it's necessary to record the time of a moon-disappearance (behind Jupiter), then you're right. That's what Roemer did.

3) Imagine you are Rømer, you have a 17th century atomic clock on your wrist, timing the appearance of Jupiter's moon each time it appears from behind its planet

As noted above, it would be better to record the time of disappearances, because it's easier to know when to expect them.

. You can now easily draw a graph with time and distance. That is in fact what Rømer's argumentation ultimately amounts to.
What is wrong with such a view?
Well, it is simply too... simple.

Earlier in Roemer's century, Isasc Newton solved the differential equations of the Earth's orbit (at least for a two-body model, disregarding perturbations by other planets).

That made it possible to calculate the Earth's distance from Jupiter at any time.

Most likely, using the orbital solution method originated by Newton, calculated the date on which the Earth would have the greatest speed-component toward Jupiter, and the date on which the Earth would have the greatest speed-component away from Jupiter.

On each those two dates, he'd record the times of two successive disappearances of a moon behind Jupiter.

Then, by the orbital solution introduced by Newton, Roemer would calculate the distances between Earth and Jupiter, at the times of the four observations, mentioned in the paragraph before this one.

When the Earth is moving toward Jupiter, the 2nd eclipse beginning will happen when the Earth is closer to Jupiter. Light won't have as far to travel as far, and so the event will be recorded a bit earlier. and so the observed time between successive disappearances will be shorter (than it would be if the Earth and Jupiter were stationaryi).

When the Earth is moving away from Jupiter, the effect is opposite, and the observed time between successive dissappearances of that moon will be longer.

According to Wikipedia, Roemer measured a 7-minute difference in the duration between successive disappearances on the two dates.

...resulting from the different light-transit-times at different distances.

From that he could calculate the speed of light.

The accuracy of his lightspeed determination was limited by the accuracy of the clocks of his day.

Michael Ossipoff
.

Okay. I am not really worried about the accuracy, but about the principle.

If I understand you right, it is possible and legitimate to compare times at different locations? That 5 o'clock at location A on the orbit path, is the same as 5 o'clock (or another time) at location B?

Again, I'm left with the impression that you really have not understood the calculations and observations involved in this experiment.

The maximum delay measured was sixteen and a half minutes. In comparison, the second or two that it would take light to cross the width of the earth is irrelevant.

The errors you describe are infinitesimal, and hence irrelevant to the outcome of the experiment.

Again you do not understand my question. I do not care whether it is 7 or 70 minutes.

Maybe you should think about Einstein's thought experiment involving the synchronization of clocks. I am not saying relativity is involved at this level, I am just asking ( and remember, this is a discarded version), how times and clocks relates.

If I understand you right, it is possible and legitimate to compare times at different locations? That 5 o'clock at location A on the orbit path, is the same as 5 o'clock (or another time) at location B? — Hachem

By his clock and calendar, and Newton's solution for planetary orbits, Roemer could calculate the Earth's position, and Jupiter's position, at each of the 4 observation-times...and thereby the distances between Earth and Jupiter at those times.

Roemer most likely would prefer to make his observations when Jupiter is at or near the meridian at his longitude. But of course it would have to also be times at which the sky is dark enough to observe Jupiter's moons.

The speed of the ground that Roemer stood on, with respect to the Earth's center, is about 100 times less than the Earth's orbital speed. The component of Roemer's ground's speed toward or away from Jupiter would be even less.

Roemer could have ignored the position of his ground, with respect to the Earth's center, due to Earth-rotation,.because his ground's rotational speed is only about 1/100 of the Earth's orbital speed, and his clock-inaccuracy was limiting his result-accuracy anyway.

It's common to disregard small errors when there are much bigger ones.

Or he could have taken into account the position of his ground with respect to the Earth's center. Maybes he did. ...unless he was sure that that error was swamped by the error caused by his clock's inaccuracy.

In either case, there isn't any error in principle.

Michael Ossipoff

In either case, there isn't any error in principle. — Michael Ossipoff

In this case I was right to discard my first argumentation, and concentrate on the perception of the (dis)appearance of the moon.

On this point, I am afraid, you have not changed my mind.

On this point, I am afraid, you have not changed my mind. — Hachem

I did my best.

I'll just suggest that you re-read my description of Roemer's determination method, from a fresh perspective, instead of in terms of beliefs that you've previously formed regarding what Roemer did.

When you're convinced that you already know, you're cheating yourself out of the opportunity to find out.

Other than that, there's nothing that I can say that would help.

As I said, i did my best.

Michael Ossipoff

As I said, i did my best. — Michael Ossipoff

and I appreciate it. Thank you.

Let's do the experiment.

Do you accept the accuracy of the following?

https://www.projectpluto.com/jevent.htm#oct

Io is the innermost moon, I.

You apparently have trouble grasping my objection. There is no reason for me to reject these observations, I am not an astronomer.

What we are talking about is the epistemological value of those observations.

To put it as clearly as I am able:

1) You see the moon appear at time t1 while you are at positon p1 of the orbit.
2) you see the moon appear at time t2 while you are at position p2 of the orbit.

@Michael Ossipoff made it clear to me that I was right in doubting the validity of my first version in which I doubted whether it was possible to assume a common clock. So, looking at the times and trying to falsify Rømer's argumentation was wrong.

My objection concerns the fact of the observations themselves.
When you see the moon at t1p1, that is an observation that is determined by the resolution of your device and the distance to the moon.
That distance determines the moment at which the moon becomes distinguishable from Jupiter as a singular object.

The longer the distance from Jupiter, the longer it will take for Jupiter and the moon to become distinguishable.

My objection may be be invalid concerning modern telescopes. Maybe modern optical technology has made distance irrelevant. I would not know and defer to the experts on that point.

What is certain is that this was not the case in the 17th century, and that is why I dare claim that Rømer's argument is not the proof everybody thinks it is.

And my reply is simply that the differences you are talking about are minuscule, and irrelevant to the observations.

You are talking about seconds, at the very most, in a measurement of minutes.

Anyhow, the period of Io is 1.769138 days - agreed?

You still do not understand my point.

If I am right, you are, in such a situation, not measuring the speed of light, but the moment the moon appears or disappears for you.

All you can say is that when you are at p1, you see it at t1, and when you are at p2, you see it at t2. You cannot draw any conclusion about light.

That is what, I suppose, makes my analysis so difficult to accept.

All you can say is that when you are at p1, you see it at t1, and when you are at p2, you see it at t2. You cannot draw any conclusion about light. — Hachem

You realise that this is not an argument?

You realise that this is not an argument? — Banno

I am saying that Rømer did not have an argument.

Roemer's main source of inaccuracy was the clocks of his day. I've said that several times already.

No one's claiming that Roemer's determination was dead-on accurate. He estimated 140,000 mi/sec. That estimate was low.

But it was pretty good anyway, especially when you remember that, before Roemer, the speed of light was completely unknown.

Is that your whole point, that Roemer didn't have the instrumentation to make a highly accurate determination? But you've been claiming that Roemer was wrong in principle.

...and your attempted justifications of that claim have been completely vague and without meaning. It isn't possible to answer your argument, because it isn't saying anything that means anything.

Look, in philosophy, the academics can be full of sh_ _, and I claim that they are.

But in the physical sciences, like astronomy, how likely is it that Roemer, an astronomer, and all the later astronomers and physicists, agree that Roemer's light-speed determination method is right in principle, but that you know better than all astronomers and physicists?

When you believe that you're right, and all the astronomers and physicists are wrong, about a matter of astronomy and physics, then could it be that you might want to reconsider your belief?

You'd be able to understand my description of Roemer's method, but you won't look past your pre-formed belief that you have the truth, and all the astronomers and physicists are wrong.

Michael Ossipoff

You'd be able to understand my description of Roemer's method, but you won't look past your pre-formed belief that you have the truth, and all the astronomers and physicists are wrong. — Michael Ossipoff

Too bad you have changed your style in favor of polemics. So be it.

Not, it is not a matter of how precise measurements are. In fact, it does not concern measurement at all, except in asking the question:

What are we exactly measuring?

You assume, like everybody else, that it is the speed of light. Once you accept this assumption any inexactitude becomes historically irrelevant. It becomes solely a matter of progress in the measurement process.

I am asking of you to do one step back, before you decide that it is light speed that it is being measured.

When you do that you can ask yourself again: what am I measuring?

What happens when I observe an event from position p1, and then observe the same event from p2?

If you take the way your observation takes place as irrelevant. That is, if you do not doubt an instant that when you see the event happening, it is happening not at that moment but because light needs time to reach you and make you see it. Then you have already assumed that which you wanted to prove, and the only thing that rests is count the seconds or the minutes between one event and the other.

So, in fact, I am saying that Rømer had already decided for himself that it takes time for events to reach us, and the matter of calculations was then easily resolved.

Apparently, I am unable to convince you that that was an illegitimate jump in his reasoning. That is why we keep going back and forth without anyone of us convincing the other.

For the last time, once you accept the main idea, that is now a given in modern science, then there is no reason to doubt the validity of Rømer's argument.

But you must remember that in his time, that was not obvious and had to be proven. Well, my claim is that he did not, and simply assumed it, then presented calculations and made it sound scientific.

Whether he was proven right afterwards, or not, is irrelevant. What matters is, was his argumentation valid when he made it?

My answer is NO!

, if you do not doubt an instant that when you see the event happening, it is happening not at that moment but because light needs time to reach you and make you see it. Then you have already assumed that which you wanted to prove, and the only thing that rests is count the seconds or the minutes between one event and the other. — Hachem

No, Roemer didn't necessarily assume that light had a finite speed. His experiment depended on no such assumption and implied no such assumption.

Roemer's measurement could have yielded an infinite speed of light. ...which would have told Roemer that either light's speed is infinite, maybe just too high to be measured by Roemer's method. But Roemer's measurements indicated a finite speed of light, and that measured speed was reasonably accurate.

In short--Roemer didn't assume, and his experiment didn't depend on an assumption of, a finite speed of light. But that's what it found.

If light-speed were infinite, the observed times between eclipse-beginnings would have been unchanged.

Before Roemer, there'd been no measurement to indicate that. Galileo had tried to measure light's speed, but, light was too fast for its speed to be measured by Galileo's method. From Galileo's measurement, either light's speed was infinite, or it was too fast to be measured by Galileo's method.

Roemer's measurements could have likewise indicated an infinite light-speed, which would have indicated that light's speed is infinite, or too fast for Roemer's method to measure.

So, no--Roemer didn't just determine what he'd assumed. He didn't assume a finite light-speed. But he found one.

Michael Ossipoff