It takes 2,549 minutes for Io to complete one orbit around Jupiter.

Let us start from the moment Io disappears behind Jupiter. We will consider the exact minute when Io goes behind Jupiter as minute 1.

We have a telescope pointed at Jupiter, and it takes one photo every minute.

In the first 120 photos, Io is not visible because Io is behind Jupiter (we know this).

When Earth is close to Jupiter , Io starts becoming visible again from the 130th photo onward.

That is, out of the total 2,549 photos taken in one full orbit, Io is recorded in 2,419 photos.

Six months later, when Earth is far from Jupiter , Io starts becoming visible only from the 141st minute.

That is, out of the total 2,549 photos, Io is recorded in only 2,408 photos.

11 minutes of Io were visible — we ourselves are the witness.

11 minutes of Io were missing — we ourselves are the witness again.

(Pay close attention to the 11 photos in this place. I'm not saying they arrived late, I'm saying they disappeared.)

If you say that those same 11 minutes that were visible when Earth was close to Jupiter appear delayed when Earth is far away, you are effectively claiming that Io’s orbital period increases by 11 minutes every single orbit. That is impossible.

When Io does not change its orbital period due to the delay of light.

When Earth is close to Jupiter → Io reappears at position 2.

When Earth is far from Jupiter → Io reappears at position 3.

We call them light waves, we call them photons.

Using these, how will you explain this phenomenon?

14 hours ago, KJW said:

According to the Wikipedia article, "Rømer's determination of the speed of light", it wasn't the occultation of Io that was used for the time measurements, but the eclipse of Io by Jupiter's shadow. And it was both immersion, when Io suddenly disappears into Jupiter's shadow, and emergence, when Io suddenly reappears from Jupiter's shadow, that was used (immersion and emergence cannot be observed from Earth for the same eclipse because one or the other will be hidden by Jupiter).

@jalaldn , did you not see this? You keep mentioning "the moment Io disappears behind Jupiter", whereas this is not the moment that interested Ole Rømer.

5 hours ago, jalaldn said:

When Io does not change its orbital period due to the delay of light.

No of course it doesn't.

But the very fact that you have bothered to state this shows that you have not understood the method of deduction.

5 hours ago, jalaldn said:

That is, out of the total 2,549 photos taken in one full orbit, Io is recorded in 2,419 photos.

Six months later, when Earth is far from Jupiter , Io starts becoming visible only from the 141st minute.

That is, out of the total 2,549 photos, Io is recorded in only 2,408 photos.

Again of course, but so what ?

You are not considering the appropriate information.

Today we know that the Earth is moving at an average speed of 2.575 million km/earthday

In Roemer's day they knew the average period of Io's orbit of Jupiter to be around about 1.8 earthdays from Cassini's observations, taken over a complete earthyear.

In that time, the Earth will have travelled about 4.5 million km.

They did not know this because they did not know Earth's speed accurately enough.

But Roemer realised that there was a big difference depending upon which direction that 4.5 million km were measured.

Along the line of the Earth's path or perpendicular to it.

This in turn makes a difference, not to the Io's period of obit, but to when Io appears and disappears.

Perpendicular earth travel makes no difference to when Io appears or disappears.

But when Earth is travelling ( as near as possible ) directly towards Jupiter that period appears to be shortened by 15 seconds and

When the Earth is travelling directly away from Jupites (six months later) the period appears lengthened by 15 seconds

So there is a variation in the apparent period of 30 (+/-15) seconds over a full year.

Now it is very important to understand what this information will tell you and what it won't.

The time variation from one Io cycle to the next will be nothing like this number because the Earth is still travelling towards or away from Jupiter.
It is the comparison with the average that matters and where it shows up.

So Roemer did not have a good radius for the Earth's orbit and records that have survived do not show an actual lightspeed calculation.

He estimated that when the Earth was travelling directly towards or away from Jupiter the light had a corresponding shorter or longer distance to travel roughly equal to the radius of the Earth's obit, because of the geometry of the situation.
So the peak to peak variation corresponds to the diameter of the Earth's orbit and the deviation to the radius.

Does this help ?

19 minutes ago, studiot said:

Along the line of the Earth's path or perpendicular to it.

Actually I realise that I didn't put that very well.

I should have said along the direct line between Earth and Jupiter or perpendicular to it.
In other words when earthspeed is perpendicular to that line it make no difference to the transit time for light as the distance between the two does not vary over a couple of days.
But when earthspeed is along the direct EJ line the distance Earth travels will be additive or subtractive from the EJ distance, that light has to travel, therby changing its travel time (slightly).

11 hours ago, jalaldn said:

It takes 2,549 minutes for Io to complete one orbit around Jupiter.

Let us start from the moment Io disappears behind Jupiter. We will consider the exact minute when Io goes behind Jupiter as minute 1.

We have a telescope pointed at Jupiter, and it takes one photo every minute.

In the first 120 photos, Io is not visible because Io is behind Jupiter (we know this).

No “we” don’t know this. If, by “disappears” you mean “we lose sight” then yes, it’s true. But if you mean its location is behind Jupiter, then it’s not. Io will be visible to our eyes when it’s behind Jupiter, or in its shadow, for an additional ~10 minutes after it’s physically there, because light from it takes time get to us. In the same way we notice a lag between thunder and lightning, because sound travels slower; we see the light almost immediately (of order a microsecond) but sound takes of order a second if we’re a few kilometers away.

You need to be precise in your descriptions

If the light from Io is blocked at 12:00, we will see it on earth until 12:10

@jalaldn here is a crude animation, maybe this helps.

• The blue line illustrates line of sight between a moon and a remote observer.

• The moving dots illustrate photons in transit.

Photons hitting the back side of the planet are not included. Note that the blue line is for illustration; there is of course no real time "connection" between observer and moon.

In case you missed it:

On 11/24/2025 at 5:14 PM, Ghideon said:

I think your description of light and of Ole Rømer’s observations deviates significantly from the established sources. Could you explain what personal idea or hypothesis you are pursuing that leads you to this different interpretation?

Edited November 25Nov 25 by Ghideon

@jalaldn here is a crude animation, maybe this helps.

• The blue line illustrates line of sight between a moon and a remote observer.

• The moving dots illustrate photons in transit.

Photons hitting the back side of the planet are not included. Note that the blue line is for illustration; there is of course no real time "connection" between observer and moon.

In case you missed it:

You can even go as far as 50 horizontally, but the photons stopped by that moon will have the same time, whether it's number 4 or number 50.

How could ole rømer have measured the speed of light when this is the case?

No “we” don’t know this. If, by “disappears” you mean “we lose sight” then yes, it’s true. But if you mean its location is behind Jupiter, then it’s not. Io will be visible to our eyes when it’s behind Jupiter, or in its shadow, for an additional ~10 minutes after it’s physically there, because light from it takes time get to us. In the same way we notice a lag between thunder and lightning, because sound travels slower; we see the light almost immediately (of order a microsecond) but sound takes of order a second if we’re a few kilometers away.

You need to be precise in your descriptions

If the light from Io is blocked at 12:00, we will see it on earth until 12:10

Okay, now let's talk in a different way. Instead of talking about how much time is delayed, let's talk about how much time it was visible.

When Earth is close to Jupiter, for each full orbit of Io, Io was visible for 2,419 minutes.

When Earth was far from Jupiter, for each full orbit of Io, Io was visible for 2,408 minutes.

If, as you say, there is a delay, then the amount of time Io is visible should not change.

The actual time for Io to complete one full orbit is 2,549 minutes.

If, as you say, we measure using the first minute when Io becomes visible:

When Earth is close to Jupiter, the time for Io to complete one full orbit becomes 2,549 minutes.

When Earth is far from Jupiter, the time for Io to complete one full orbit becomes 2,559 minutes.

8 hours ago, jalaldn said:

Okay, now let's talk in a different way. Instead of talking about how much time is delayed, let's talk about how much time it was visible.

When Earth is close to Jupiter, for each full orbit of Io, Io was visible for 2,419 minutes.

When Earth was far from Jupiter, for each full orbit of Io, Io was visible for 2,408 minutes.

If, as you say, there is a delay, then the amount of time Io is visible should not change.

The actual time for Io to complete one full orbit is 2,549 minutes.

If, as you say, we measure using the first minute when Io becomes visible:

When Earth is close to Jupiter, the time for Io to complete one full orbit becomes 2,549 minutes.

When Earth is far from Jupiter, the time for Io to complete one full orbit becomes 2,559 minutes.

Where are you getting these numbers? From actual observation, or are you basing them on your own ideas?

The idea behind Rømer’s observations is not that the duration that Io is visible changes. It’s when it becomes visible, or disappears. If the orbit is determined to sufficient precision, those times should be predictable, but observation didn’t match up - there was a discrepancy when the earth was farther away. Whatever event was being observed happened later than expected. That applied to its disappearance and its reappearance

11 hours ago, jalaldn said:

How could ole rømer have measured the speed of light when this is the case?

Note the time (on a clock) that the photons stop arriving. It happens when you take the time the light is blocked/unblocked and add the time it takes for the light to reach earth (Ttravel)

Notice that this happens on a regular cycle, so you can predict the time you expect the photons to stop/start.

When the earth moves farther away, the clock time when the photons stop/start later than expected because of the increase in Ttravel

In the animation, the light is blocked at ~ 6 seconds and the photons stop arriving at 8 second so Ttravel is 2 seconds. If you double the distance, the light will be blocked at the same time, but the photons would not stop arriving until the timer reads 10 seconds, because Ttravel is now 4 seconds. The event is observed later.

Where are you getting these numbers? From actual observation, or are you basing them on your own ideas?

The idea behind Rømer’s observations is not that the duration that Io is visible changes. It’s when it becomes visible, or disappears. If the orbit is determined to sufficient precision, those times should be predictable, but observation didn’t match up - there was a discrepancy when the earth was farther away. Whatever event was being observed happened later than expected. That applied to its disappearance and its reappearance

Note the time (on a clock) that the photons stop arriving. It happens when you take the time the light is blocked/unblocked and add the time it takes for the light to reach earth (Ttravel)

Notice that this happens on a regular cycle, so you can predict the time you expect the photons to stop/start.

When the earth moves farther away, the clock time when the photons stop/start later than expected because of the increase in Ttravel

In the animation, the light is blocked at ~ 6 seconds and the photons stop arriving at 8 second so Ttravel is 2 seconds. If you double the distance, the light will be blocked at the same time, but the photons would not stop arriving until the timer reads 10 seconds, because Ttravel is now 4 seconds. The event is observed later.

If light had the property you think it has, this is exactly how it would behave.

This graph ends at 8; extend it to the right up to 100. No matter which number you pick between 4 and 100, the amount of time Jupiter blocks the light at that point will always be the same, because distance does not change the duration of the blocking.

But what Ole Rømer observed was different. He started counting time from the moment Io disappeared. When Earth was very close to Jupiter, it took 2 hours and 10 minutes for Io to disappear and reappear. Six months later, when he recorded it again (with Earth on the opposite side of the Sun, very far from Jupiter), it took Io 2 hours and 21 minutes to disappear and reappear. He subtracted the first measurement from the second and said the remaining 11 minutes was the delay.

Edited November 26Nov 26 by jalaldn

1 hour ago, swansont said:

Where are you getting these numbers? From actual observation, or are you basing them on your own ideas?

1 hour ago, jalaldn said:

If light had the property you think it has, this is exactly how it would behave.

This graph ends at 8; extend it to the right up to 100. No matter which number you pick between 4 and 100, the amount of time Jupiter blocks the light at that point will always be the same, because distance does not change the duration of the blocking.

But what Ole Rømer observed was different. He started counting time from the moment Io disappeared. When Earth was very close to Jupiter, it took 2 hours and 10 minutes for Io to disappear and reappear. Six months later, when he recorded it again (with Earth on the opposite side of the Sun, very far from Jupiter), it took Io 2 hours and 21 minutes to disappear and reappear. He subtracted the first measurement from the second and said the remaining 11 minutes was the delay.

I note once again you are avoiding questions that have been asked.

I asked these (and others) several times before and you appear to have blanked me.

Do I smell bad or something ?

1 hour ago, jalaldn said:

If light had the property you think it has, this is exactly how it would behave.

This graph ends at 8; extend it to the right up to 100. No matter which number you pick between 4 and 100, the amount of time Jupiter blocks the light at that point will always be the same, because distance does not change the duration of the blocking.

But what Ole Rømer observed was different. He started counting time from the moment Io disappeared. When Earth was very close to Jupiter, it took 2 hours and 10 minutes for Io to disappear and reappear. Six months later, when he recorded it again (with Earth on the opposite side of the Sun, very far from Jupiter), it took Io 2 hours and 21 minutes to disappear and reappear. He subtracted the first measurement from the second and said the remaining 11 minutes was the delay.

Yes, that’s because the distance between Jupiter and the observer changed. So the light path got longer.Your diagram fails to show this basic point.

@jalaldn I have added the observer movement back and forth for you:

32 minutes ago, exchemist said:

Your diagram fails to show this basic point.

Fair point; I kept it simple. New version posted.

4 minutes ago, Ghideon said:

@jalaldn I have added the observer movement back and forth for you:

Fair point; I kept it simple. New version posted.

Very nice. Let’s hope that at last he gets it now. But I am not optimistic.

5 hours ago, jalaldn said:

But what Ole Rømer observed was different. He started counting time from the moment Io disappeared. When Earth was very close to Jupiter, it took 2 hours and 10 minutes for Io to disappear and reappear. Six months later, when he recorded it again (with Earth on the opposite side of the Sun, very far from Jupiter), it took Io 2 hours and 21 minutes to disappear and reappear. He subtracted the first measurement from the second and said the remaining 11 minutes was the delay.

No, it’s not what he did. As KJW noted earlier, he looked at eclipses, and you only see one per orbit, either entering or leaving shadow. A link to the Wikipedia page was provided. from that article: “The key phenomenon that Rømer observed was that the time between eclipses was not constant, but varied slightly over the year.” (bold added)

IOW, the time of day was not consistent with the occurrences on a regular cycle. He was looking at the time between the eclipses, not the duration of them. The time of day of observed vs time of day expected is what showed the discrepancy.

I note once again you are avoiding questions that have been asked.

I asked these (and others) several times before and you appear to have blanked me.

Do I smell bad or something ?

Your question doesn't seem to point out my mistake,

I'll avoid repeating the answer I've already given. Nothing else.

I have given you a map. That map is right in front of you. There are many features in it that you can study. Before you answer me, will you check whether what I have said is true or not? There are lines from 2 to 8 on it. There is a stopwatch on your phone. Use it to measure and tell me what difference you see.

No, it’s not what he did. As KJW noted earlier, he looked at eclipses, and you only see one per orbit, either entering or leaving shadow. A link to the Wikipedia page was provided. from that article: “The key phenomenon that Rømer observed was that the time between eclipses was not constant, but varied slightly over the year.” (bold added)

IOW, the time of day was not consistent with the occurrences on a regular cycle. He was looking at the time between the eclipses, not the duration of them. The time of day of observed vs time of day expected is what showed the discrepancy.

Okay, now let’s imagine you are placed exactly in Ole Rømer’s position.

Suppose you have been given the responsibility of discovering the travel time of light between Jupiter and Earth.

Tell me step by step how you would find it. Let’s discuss it.

(Anyone who believes light is delayed can do this too.)

1 hour ago, jalaldn said:

Okay, now let’s imagine you are placed exactly in Ole Rømer’s position.

Using your words: "I have given you a map. That map is right in front of you."

Look at the timestamps when IO moves behind Jupiter; the moment when there is no line of sight:

The timestamps have a consistent interval, IO moves in behind Jupiter every 9nth second.

Let's look at time stamps and interval from Rømer's position: These frames are from the point where he observes that IO disappears; no light reaches him from IO.

The timestamps does not have a consistent interval, the interval between 1st and 2nd time is 8 seconds and between 2nd and 3rd time it is 9.9 seconds.

This illustrates what @swansont and others already said:

7 hours ago, swansont said:

“The key phenomenon that Rømer observed was that the time between eclipses was not constant, but varied slightly over the year.”

Edited November 27Nov 27 by Ghideon

Using your words: "I have given you a map. That map is right in front of you."

Look at the timestamps when IO moves behind Jupiter; the moment when there is no line of sight:

The timestamps have a consistent interval, IO moves in behind Jupiter every 9nth second.

Let's look at time stamps and interval from Rømer's position: These frames are from the point where he observes that IO disappears; no light reaches him from IO.

The timestamps does not have a consistent interval, the interval between 1st and 2nd time is 8 seconds and between 2nd and 3rd time it is 9.9 seconds.

This illustrates what @swansont and others already said:

The time interval between when light is stopped at a point and when it is received again

23 minutes ago, jalaldn said:

The time interval between when light is stopped at a point and when it is received again

You need to be more specific. Also: your description of light and of Ole Rømer’s observations deviates significantly from the established sources. Could you explain what personal idea or hypothesis you are pursuing that leads you to this different interpretation?

In this picture, you know the time when the light started, but Rama didn't know that. He was in a place where he could receive the light. He could only see the light that was stopped by him and the light that was received again.

You need to be more specific. Also: your description of light and of Ole Rømer’s observations deviates significantly from the established sources. Could you explain what personal idea or hypothesis you are pursuing that leads you to this different interpretation?

Okay, now let’s imagine you are placed exactly in Ole Rømer’s position.

Suppose you have been given the responsibility of discovering the travel time of light between Jupiter and Earth.

Tell me step by step how you would find it. Let’s discuss it.

(Anyone who believes light is delayed can do this too.)

10 hours ago, jalaldn said:

20 hours ago, studiot said:

I note once again you are avoiding questions that have been asked.

I asked these (and others) several times before and you appear to have blanked me.

Do I smell bad or something ?

Your question doesn't seem to point out my mistake,

I'll avoid repeating the answer I've already given. Nothing else.

Really ???

I mentioned no mistake.
I neither agreed with you nor disagreed.

On 11/23/2025 at 2:17 PM, studiot said:

This thread seems to have got bogged down in a dispute over Roemer's actual reading.

Thus only the first of thye fout claims have been discussed.

I am interested in the other three, particularly the last two.

Please expand on both of these.

Your made no answer whatsoever.

Do you normally go about ignoring people who don't disagree with you ?

My apologies for the spelling mistake, but I think most would realise that I meant 'four' not fout.

Edited November 27Nov 27 by studiot

9 hours ago, jalaldn said:

Suppose you have been given the responsibility of discovering the travel time of light between Jupiter and Earth.

Establish the timing of a cyclical process, which in this case is the eclipse of Io. You notice it happens regularly (either the disappearance or re-emergence) so you can predict when it will happen in the future, which you confirm over the course of several days or weeks.

Predict when they will happen several months from now, and also the following year when they are again at the same distance. Check when they actually do, and note the discrepancy in time, t, between predicted and actual eclipses. Confirm that the measurement at closest position is correct but measurements spaced by several months are off, which tells you it’s not an issue with Io’s orbit.

Using orbital mechanics, estimate the difference in distance (d) between earth and Io for these events. Light speed is d/t

This thread seems to have got bogged down in a dispute over Roemer's actual reading.

Thus only the first of thye fout claims have been discussed.

I am interested in the other three, particularly the last two.

Please expand on both of these.

Really ???

I mentioned no mistake.
I neither agreed with you nor disagreed.

Your made no answer whatsoever.

Do you normally go about ignoring people who don't disagree with you ?

My apologies for the spelling mistake, but I think most would realise that I meant 'four' not fout.

The second one is not finished yet. Let's not get distracted now. Let's finish the second thing first.

Establish the timing of a cyclical process, which in this case is the eclipse of Io. You notice it happens regularly (either the disappearance or re-emergence) so you can predict when it will happen in the future, which you confirm over the course of several days or weeks.

Predict when they will happen several months from now, and also the following year when they are again at the same distance. Check when they actually do, and note the discrepancy in time, t, between predicted and actual eclipses. Confirm that the measurement at closest position is correct but measurements spaced by several months are off, which tells you it’s not an issue with Io’s orbit.

Using orbital mechanics, estimate the difference in distance (d) between earth and Io for these events. Light speed is d/t

Give everything a scale with a numerical score. It doesn't have to be exact. Just an average is enough. Only then can we make a decision about where to discuss.

Edited November 27Nov 27 by jalaldn

37 minutes ago, jalaldn said:

The second one is not finished yet. Let's not get distracted now. Let's finish the second thing first.

You are the one that made 4 very specific claims.

So either put up or retract them and shut up.

The rules here are very very clear that if you make claims the onus is entirely on you to support them.

Are you refusing to do this ?

Establish the timing of a cyclical process, which in this case is the eclipse of Io. You notice it happens regularly (either the disappearance or re-emergence) so you can predict when it will happen in the future, which you confirm over the course of several days or weeks.

Predict when they will happen several months from now, and also the following year when they are again at the same distance. Check when they actually do, and note the discrepancy in time, t, between predicted and actual eclipses. Confirm that the measurement at closest position is correct but measurements spaced by several months are off, which tells you it’s not an issue with Io’s orbit.

Using orbital mechanics, estimate the difference in distance (d) between earth and Io for these events. Light speed is d/t

(~4.2 AU) When Earth is close to Jupiter → SD

(~6.2 AU) When Earth is far from Jupiter ) → BD

The time is yours, there must be an 11-minute difference.

Now tell me your idea.

15 minutes ago, jalaldn said:

(~4.2 AU) When Earth is close to Jupiter → SD

(~6.2 AU) When Earth is far from Jupiter ) → BD

The time is yours, there must be an 11-minute difference.

Now tell me your idea.

Light takes a little over 8 minutes to travel 1 AU.