Orbits: Newsletter Challenge (11/28/06)

Light from Jupiter has to travel extra distance when the Earth is on the far side of the sun.

I was going to post a comment saying this answer is crap...

But... going from memory, the Earth is about 90 million miles from the Sun. At 186,000 miles per second, it takes light 8 minutes to travel from the sun to us. And since observations must be made at night, the earth is either between the Sun and Jupiter (closest) or off to the side, like a right triangle with the Sun and Jupiter as the other vertices. So the change in distance is about equal to the distance between the Earth and the Sun. About 8 light-minutes.

So it's the difference between a midnight observation and a twilight observation.

Just to be a smart tush - you for got the Diameter of the Sun is 4.6774193548387096774193548387097 Seconds

It is the Tidal Drag of the bodies that is the cause of the change in orbits. Certainly any scientist / astronomer who is capable of calculating the change is smart enough to take the time difference into account. The 8 minutes was there to remind you of the 8 light minutes to the Sun from the Earth and throw you off.

Wait a minute here. The position of earth in its orbit -- therefore its relative closeness to Jupiter and the periodic change of that distance over time -- is at the heart of the original question, is it not? Tidal drag and relativistic considerations are many orders of magnitude less important, right? Especially relativistic considerations. While everything in our solar system is speeding very fast relative to earth-bound engineering experience, those velocities are negligible compared to the speed of light. I would think relative motion within our solar system can shift photon wavelength only ever so slightly, and is not directly involved in the 8 minute anomaly. Help me out! You don't have to be an astrophysicist to desire to understand the big picture.

I think that it is due to the doppler effect. Travelling towards Jupiter and moons means that the wavefronts of light pass quicker (i.e. is blue shifted), whilst when going away, they are red-shifted. This isn't quite the same thing as a reltavistic effect, although the two get close because the apparent speed of the light doesn't change.

Bingo! I'm with you on this one. The big clue is the 12-month cycle, which is relevant only to Earth's orbit. The puzzle doesn't say how big the time differential is, but I'll bet it's about 16 minutes -- the diameter of the Earth's orbit is about 16 light-minutes.

-- AstroNut

I suspect mass bulges, or tides in Jupiter and/or its atmosphere. As I recall, Jupiter is mostly gas, but may have liquid or even solids near its core. Mass gradients within the non-solid portion of Jupiter could be shifting and may result in constructive and destructive interference (transient "standing" waves?) of higher and lower mass concentrations. This would exert varying tugs on the moons. While this is plausible, it may be insignificant in magnitude. I will yield to any knowledgeable astrophysicist.

I'm sure Jorrie should be answering this one but since he has not chimed in yet here goes:

Ole Romer was the first to suggest that light travels at a finite speed. He noticed that the moons lost time as the earth moved away from Jupiter and gained time as Earth traveled towards Jupiter. With this observation he concluded that light does not travel from one point to another instantaneously. The farther we are from Jupiter the longer it will take light to reach us. The closier we are the less time it takes for the light to reach us. Today we would refer to this as red shift and blue shift.

KasterKid, you wrote: "The farther we are from Jupiter the longer it will take light to reach us. The closer we are the less time it takes for the light to reach us. Today we would refer to this as red shift and blue shift."

You are pretty correct, although many will argue that to call it red shift and blue shift is stretching the definition a bit! (I actually agree with you.)

Romer timed the orbits of the Jovian moon Io by observing and timing it moving into and out of Jupiter's shadow. While Earth is moving away from Jupiter, the moon's 'black-out' periods are "stretched" due to the extra distance that accrued between the entry and exit times. This is essentially a red shift in period, as you said. The opposite happens when the Earth is moving closer to Jupiter.

Romer was able to crudely* calculate the speed of light from his measurements, the first person to do so, if I remember correctly.

*The primary uncertainty at the time of Romer was the diameter of Earth's orbit around the Sun.

I will never tired to repeat: in order to avoid carelessness and confusion in the mind of the participants, one must always specify the "type" of speed: the TWO-way speed. Also the Ole Roemers measure was the (first effective, after Galileo) determination of the two-way speed of light. The ONE-way speed was never measured (up to now, perhaps will never), because requires TWO clocks, and their synchronization is matter of convention (or postulation, as Einstein did).

Camillo, you said: "Also the Ole Roemers measure was the (first effective, after Galileo) determination of the two-way speed of light."

How do you suggest Romer's measurement was "two-way"? He measured the arrival time of a one-way signal and then the arrival time of another one-way signal. From the variation in the difference between arrival times over a year, he roughly calculated the speed of light.

To me, two or more one-way timings do not constitute a two-way light speed measurement!

With today's accurate knowledge of the orbits of Earth, Jupiter and it's moons, plus accurate atomic clocks, do you not think we can make a good case for an accurate one-way measurement of the speed of light using a single clock?

Were are the basis for the (more or less) accurate knowledge of the orbits? In some TACIT stipulation on the one-way speed. So, it seems to me evident the circularity of the argument. On the convention on the one-way (apart of Einstein himself, with the phrase: "....BUT a stipulation wich I can make of my own free will"), refer to:

H. Reichenbach The Philosophy of Space and Time (1935 !!!!!)

Mansouri - Sexl General Relativity and Gravitation (1977)

M. Jammer Some fundamental problems in the special theory o relativity (1979)

Refer also: Conventionality of Simultaneity - Stanford Encyclopedia of Philosophy.

Et cetera, et cetera, et cetera, ...

Hi Camillo, you said: "Were are the basis for the (more or less) accurate knowledge of the orbits?"

You must be joking! We only need Newton's theory to very, very accurately determine the orbits of the whole solar system for now and for some fair time into the future.

Where does "TACIT stipulation on the one-way speed" [of light, I presume] come into the knowledge of the orbits? In case you don't know, NASA has sent more than one probe out there to watch them from close-up, and guess what, the moons agreed with Newton!

My question is, Camillo, are you on an "anti-relativity crusade"? If so, consider all the evidence of a century or more and refute them one by one - a formidable task!

If not, learn all you can about relativity - the next 'quantum step' is about to happen and leave you further behind...

Regards, Jorrie

Another my interest is history (also epistemology and philosophy of science, as any physicist should do). Just one thing I have learned: the dogmatism starts with the prehistory and probably will go on a lot of time.

Do you remember: "Ipse dixit" ?

In german there is a phrase: "Alles was ist, endet".

"Alles" means also the theories, in any field.

It never ceases to amaze me how seemingly intelligent people can make so many spelling and grammar mistakes.

I wuz just about to right that same thin.

Regarding one-way measurement of the speed of light: we do have to make some assumptions in order to do this, but they are similar to assumptions that we make for all sorts of other physical measurements. In the end, the problem does indeed come down to knowledge of the Earth's velocity and synchronisation of clocks. The present situation is that independent man-made clocks can provide frequency consistency below 1E-15, and that some milisecond pulsars also exhibit consistencies in the 1E-15 region. So we should be able to use doppler and the earth's velocity to calculate the speed of light. It would probably be best to make these measurements with the earth in near-equivalent gravitational positions. Now we need to make corrections for gravity and motion of our earthly clock. Then the accuracy of our measurement will be in the order of the accuracy of our knowledge of the Earth's orbit. This is effectively the inverse of the methods used to verify the relative medium-term stability of the astronomical and Earthly clocks - so we could say that at least part of the experiment has been performed. I don't have the detailed results before me, but my recollection is that the measurements were consistent with the one-way velocity of light being the same as the two-way velocity 'to within experimental error' (whatever level that was). But at least the theories are self-consistent...

http://tycho.usno.navy.mil/ptti/1995/Vol%2027_36.pdf is an old (1995) paper on Pulsar timing that is freely accessible. Since that time, the anticipated stabilities in the pulsars were confirmed, and Earthly atomic clocks have been further improved.

fyz

I forgot to say - not being close to these measurements, I'm not certain what dominated the 'experimental error'. N.B. that if we devoted the resources to measure pulsars in opposing locations oin the ecliptic, we would be able to confirm that the speed of light is independent of direction to an accuracy of parts in 10^11. However, resources are scarce, so I doubt this will happen

Hi Physicist, OK, you won the one on Doppler shift vs. red/blue shift - I suppose nobody would call a Doppler shift in sound a red shift or blue shift, but then, it's only a semantical convention...

On to more technical stuff - you mentioned the directions along the ecliptic as good for one-way-speed-of-light 'measurements'. Why not rather along the directions of the dipole in the cosmic microwave background radiation? The observed velocities are at least ten times larger, so would that not be a better 'reference plane'?

Further, I'm not quite clear how measuring pulsars in opposite directions could yield a confirmation of the independence of the speed of light on our movement relative to any reference frame. Could you elaborate on that?

This has been a fascinating discussion. I know that you can calculate the speed of a train if you stand by the track and measure the change of whistle frequency as it goes by. But frequency shift for astronomical bodies this is a more complex problem. Did the ancient Babylonians know the elliptical equation of the earth's orbit? Of Jupiters? The Egyptians? The Chinese? Galileo Galilei? I am not asking for specific answers just now: what I'd like is someone to reference a book that approaches astronomy on the basis of who knew what and when (like a good investigative reporter).

AstroNut comments that he believes the time should be 16 minutes, rather than 8. I think that he is right (I suspect the question was reworded from an original article that gave the timing relative to the average). But it's conceivable that I've missed something. Please would you put us right. Thanks in advance

TSPG

Jorrie wrote:

"KasterKid, you wrote: "The farther we are from Jupiter the longer it will take light to reach us. The closer we are the less time it takes for the light to reach us. Today we would refer to this as red shift and blue shift."

You are pretty correct, although many will argue that to call it red shift and blue shift is stretching the definition a bit! (I actually agree with you.)"

Wait a minute, Jorrie. Arent we confusing relative distance and relative velocity (direction)? Doppler shift (to red or to blue) is based on a velocity difference, approaching or departing, not a difference in distance. Is it really the "farther away or closer to Jupiter" that we are, or the fact that we are going toward or away from Jupiter?

Can you really have your cake and eat it too? Please explain.

I think considering red-shift-blue-shift is overcomplicationg things in this case (before anyone else points this out - perhaps I'm in no position to complain about overcomplicating things...); however:
If we make measurements at the extrema of the earth's position relative to Jupiter, the earth will be travelling perpendicular to the line of intersection at the measurement times - so there is no related colour shift. We could of course regard the time-shift as the integration of the shifts in the 1/f of the light - but I can't see what it adds.

Physicist, you wrote: "If we make measurements at the extrema of the earth's position relative to Jupiter, the earth will be travelling perpendicular to the line of intersection at the measurement times - so there is no related colour shift."

Right. But if we could somehow make a number of time measurements of discrete events at the extrema, we would also find no change in intervals. So in a way, the differences in intervals that Romer measured were average radial velocity related - i.e., red-shifts-blue-shifts, sort-of...

OK, I know this looks like arguing for the sake of arguing! I agree that the simplest way is to view it as a difference in distance between the events and their observers.

My comment was that it complicated matters in a way that I felt were unhelpful in terms of the question - quite the reverse of what nearly everyone did for the ant. Whether it is accurate comes down to an exercise in pedantry. Red-shift applies to any electromagnetic radiation that we could sense leaving the moons. The measurements we are making are of periods exceeding an earth-day. I don't believe that we can detect any radiation of this low a frequency from these moons - so in that sense we are not measuring a red shift. That is not to say that no radiation at that frequency (or any other) experiences a related doppler shift, as I implied in the statement about integrating varying wavenumber. BTW, I think the peak variations for the moons (and other observations of light in the ecliptic) due to the earth's orbit would be ~ +/- 0.01%. But I could have dropped orders of two (even orders of magnitude) on the way.

STL, you wrote: "Arent we confusing relative distance and relative velocity (direction)? Doppler shift (to red or to blue) is based on a velocity difference, approaching or departing, not a difference in distance."

Well, deep down, it is the slightly longer/shorter distance that the 'leading edge' and the 'trailing edge' (if one can say that) of a single full wave must travel that causes red shift and blue shift if there is relative movement between the transmitter and the receiver.

As "physicist" has pointed out, this perhaps over complicate things, but I think he/she will agree that it is not a wrong interpretation. As far as I know, one can talk about Doppler shift of any time interval, no matter how long it is.

Our second largest planet Saturn influences Jupiter's orbit and was alternately pulling and holding it back. This is how they knew where to look for Saturn.

Hey, Guest --

We've known where to look for Saturn since ancient times! Mayan and Roman and Chinese astronomers all knew where Saturn would be on any given night.

Maybe you were thinking of Uranus or Neptune?

-- AstroNut

Any backyard astronomer can tell you, we never needed to find out where to look for Saturn, it is a big bright obvious light in the sky and as soon as the simplest telescope was invented, the rings became obvious. It doesn't take much of a scope to see the rings. when it is at it's closest point Saturn is very bright and stands out to the point if you were not aware of its current position, one might mistake it for Jupitor. Thousands of years before anyone could measure the effect of the gravity of Saturn on Jupiter, man was well aware of Saturn.

More likely the orbits are not symetical around jupiter hence on the first half it travels further around the second half... People assume that orbits are perfect but most heavenly bodies have wobbles tilts and even reversed rotatons !!!

This was due to the Doppler's effect.

Since both the Earth & Jupiter orbit the sun in the same direction and approximately in the same plane in roughly circles orbits. The period of the earth's orbit is 1 year & Jupiter's orbit is approximately 12 years. Roughly 6 months of the year the Earth is speeding away from Jupiter & the other 6 months it's speeding towards Jupiter.

Ole Romer made the first quantitative measurements of the speed of light in 1676.

If this had been due to Doppler's effect, then a daily change also would have been noticeable, as the circumferential speed of the observer on the Earth's surface must be added to or subtracted from the orbital speeds of the two planets! Besides, Doppler's effect changes (shifts) the frequency the observer notices from an approaching or diverging source, so Mr. Roamer would have noticed a reddish or blueish tint on the moons of Jupiter. ;-) In fact, the speed difference is so small, it has no noticeable effect on the color.

I really should quit while I am behind (rather than get further behind), but here goes. My comment #3 was wrong. I concede that the time differential described in the puzzle is due to the light reflected from Jupiter's moons having to travel a greater distance to reach an earth-bound observer, depending upon where earth is in its orbit -- Jupiter's position and vector from the sun don't change a great deal over a six earth-month span. My comment is that the red-shift/blue-shift of a photon's wavelength is observed with respect to the relative speed between two bodies, not the distance between them. Time of transit is based upon distance and the speed of light: time of transit is not wavelength dependent. So, for the original question, red-shift/blue shift is irrelevant.

I'm pretty sure that when KasterKid said 'red shift blue shift' he meant Doppler shift and that he was referring to the frequency of the orbits, not the frequency of the light. When Tera was approaching Jupiter the orbits would get ahead because they were of a higher frequency or 'bluer' and when Tera was drawing away they got behind because they were of a lower frequency or 'redder'.

It's the same as police radar. They don't measure the change in the frequency of the microwaves, they send out a series of pulses and measure the shift in the frequency of (or period between) the pulses.

All that aside, if he had the equipment and knowledge to measure the colour of the light, I'm pretty sure he would detect the colour shift too because Tera doesn't stop in her orbit just because someone happens to be looking at a distant moon through their telescope.

Later,

Gordie.

Hi Gordie, and Bingo!

I'm relieved not to be the only one defending Kasterkid's Doppler shift view!

Jorrie

It seems to me that someone here is shifting their ground - many objections were to "red-blue" - I certainly wouldn't object to describing the observations as integrations of a Doppler effect

My hats off to the great thinkers of the past, and the ones continuing the quest to better understand our universe. This to me seems like a lonely undertaking, that almost never produces concrete, in hand proof. I love seeing pictures of the universe's treasures. I lack the forsight to recognize the benifits we recieve from studying the stars. But I know they are vast. So once again, Thank you folks for you deep thinking, and studying. This saves "Average Joe's" like me from getting alot of headaches!

My guess is that the doppler effect, in essence, of the planet Jupiter and its moons causes this phenomena. While Jupiter is moving away from Earth, time becomes distorted and light took longer to reach Romer's telescope. While Jupiter is moving towards Earth, less time appeared to be required for a moon to orbit Jupiter. Einstein's Theory of Relativity shows that time is a relative thing. The speed of light is a constant. Romer used this test to determine the speed of light, albeit not too accurately.

Dan Barbis

I would think that it is simply an eliptical orbit of the moons that is causing the appearance of the orbit slowing down or losing time. It would seem odd however that all of the moons (visible at that time) would have the same orbital path. Now, I'll see if someone has come up with a better answer and maybe I'm completely off base.

Reply 1 is correct and 2 explains it.

It is not red and blue shift - these terms explain ony the changes in spectra caused by a relative speed between source and viewer. (i.e. when the light source is moving toward you the spectrum you see is moved one way and when the source is moving away from you the spectrum is shifted the other way.)

"Back then" using the eye as the basic instrument, the shifts would only be noted as a colour differentiation.

Distance is the difference. Or maybe there are hills and valleys.

The orbits are eliptical.

Because earth is the only planet with life on it in our galaxy, it is the only planet that has any chicks to scope out. The moons slow down on the "earth side" of Jupiter so that they can watch the girls go by for a longer period of time. They then speed up on the far side so that they can get back to doing what they like to do.

Maybe they slow down to watch the girls in summer, but speed up to keep warm in winter...

Jupiter is very gradually slowing down due to the tidal drag produced by the Galilean satellites. Also, the same tidal forces are changing the orbits of the moons, very slowly forcing them farther from Jupiter. The period of 8 minutes would certainly bring to mind the distance of the Earth to the Sun, probably as it was ment to do. Being an backyard astronomer is not the reason I was able to come up with this answer, being a computer nut and about 30 seconds with Google led me to this site:

http://www.seds.org/nineplanets/nineplanets/jupiter.html

Don't forget that the question indicates that the moons apparently slow down AND speed up. As the moons are forced further out in their orbits, they will only slow down.

Courtesy of Wikipedia, note that it mentions Ole Romer by name...

The first quantitative estimate of the speed of light was made in 1676 by Ole Rømer, who was studying the motions of Jupiter's satellite Io with a telescope. It is possible to time the orbital revolution of Io because it enters and exits Jupiter's shadow at regular intervals (at C or D). Rømer observed that Io revolved around Jupiter once every 42.5 hours when Earth was closest to Jupiter. He also observed that, as Earth and Jupiter moved apart (as from L to K), Io's exit from the shadow would begin progressively later than predicted. It was clear that these exit "signals" took longer to reach Earth, as Earth and Jupiter moved further apart. As a result of the extra time it took for light to cross the extra distance between the planets, which had accumulated in the interval between one signal and the next. The opposite is the case when they are approaching (as from F (not shown but opposite of K) to G). Quite as in the familiar Doppler effect. On the basis of his observations, Rømer estimated that it would take light 22 minutes to cross the diameter of the orbit of the Earth (that is, twice the astronomical unit); the modern estimate is closer to 16 minutes and 40 seconds.

http://en.wikipedia.org/wiki/Speed_of_light#Measurement_of_the_speed_of_light

You are obviously more informed than I on these matters related to early estimates of the speed of light, but I am informed enough to keep advocating for the fact that the Doppler Effect is about a change of frequency (wavelength) induced by relative velocity between source and target. If you are going to relate the frequency shift to the speed of light, it seems that you'd need to know a lot more about the absolute value of the distances and velocities of Earth and Jupiter than I suspect Romer knew in 1676. Now, with enough measurements over enough different vectors (orbital positions) you might determine the speed of light and the distances and the orbital dimenisons (n equations in n unknowns). Is that how old Romer did it?

I am wondering as a layman if they had the technical accuracy to make the measurements as precisely as indicated.

Going back to those times of Kepler, Huygen, Galileo, Hook etc. many things were just being discovered and accuracy determination or cross-comparisons etc. did not exist the same way as we know them today. These scientists were spread out and the means of communication and dissemination of information were very slow.

Even mechanical clocks were just being developed, the minute hand had just been recently added. I just checked a few webpages with the words clocks, timekeeping, accuracy timeline etc. for the period from 1600-1700. As aresult I am not sure if the time period accuracy indicated can be independently ascertained.

And how did he make his calculation is it documented somewhere, during his time period or is it based upon some speculation afterwards.

www.clocksonly.com

www.ahsoc.demon.co.uk/timeline_clks.html

www.swissworld.org/eng/swissworld/html

etc.

Exactly! Time is important. I'm not talking about relativistic issues. Just plain old common earth time. Navigation was limited in accuracy for centuries by the lack of reliable chronometers. If an ancient Phoenician had a cell phone and called back to headquarters and said, "Orion is in the south, bearing 175 degrees, at 40 degrees above the horizon, where am I.", part of the answer would include the questions, "What date is it, and what time do you have?" An alternate twist on my previous entry, "Who knew what and when?"! What about my request for a book to read (#47, give or take one)?

I don't know what Roemer actually did. But: time can be measured reasonably accurately at night by reference to the Earth's rotation and the positions of stars. The average periods of Jupiter's moons could be measured over a year, and the timing errors could be measured near the extrema of Earth's orbits (nearest and furthest* from Jupiter). Six minutes should be easy. I'm not clear what would have been the cause of errors - timing variations in the periods of the moons should have been apparent to Roemer if he could see more than one of them, and changes due Jupiter's position should have been relatively small (and also calculable). Does anyone know?

*As six_bits says, not too near the point when we are furthest, because Roemer could only see Jupiter's moons when the sky was not too bright. But a few degrees (less than one month) position either side should be enough, and the calculations from there are quite straightforward.

Physicist, you have stuck very close to physics (physics is a good thing) and I have been curious about the historical path. Thinkers were making progress in navigation on our planet at the same time they were learning about the solar system. Some of my remarks were made thinking on the keyboard without adequately placing them in history. For example the Babylonians couldn't have known the true orbit of Jupiter, because several centuries transpired before Copernicus shocked the world by proposing that Earth was not the center of the universe.

In the terrestrial realm, my hypothetical Phoenician sailor would need only to be concerned with Greenwich Mean Time, but alas, it was not yet invented. The European Admiralty must have truly felt like "Kings of the world" when they could leave port with a chronometer set on GMT and armed with confidence to sail a year or more with a time error measured only in minutes. And there we go, our measurement of the speed of light can never be more precise than our measurement of time -- but it can be a lot worse, if we don't accurately know the orbits. Perhaps that is why you physicists like to look for really big things, really far away. At infinity, orbits are less consequential to relative movement.

Speaking of minutes, it is likely that X degrees, Y minutes, Z seconds ultimately refers to how the sky "moves" in time, and ultimately how far you are displaced on the globe from the prime meridian. These conventions and their origin are hiding in the history of science. I guess I need an encyclopedia, not just a book, but if anyone makes a book recommendation, I would consider reading it.

It's a pity, but angle is divided 360/60/60, whereas the day is 24/60/60. It's almost according to the Babylonian numeric system (too complex for me, but duodecimal and mixed number systems are for another day), but not quite close enough to be useful. BTW, I'm not certain if this was clarified, but local time has long been marked quite accurately - it's using differences in local time to determine loongitude that prompted the need for a precise chronometer.

I spent a little time considering the 360 vs. 24 anomaly, then kind of shrugged it off after considering that a dietitian's calorie is not a physicist's calorie and an Imperial gallon is not a US Gallon. I may have overreached to even imply a direct association. Different intellectual communities use terms in ways that are only related in the most general of ways (1st degree murder for example). We engineers tend to hope for Y=mX+B and if the relationship is factually linear, we are almost as relieved as if m=1 and B=0. My college Physics Professors took absolutely nothing for granted, especially when it came to units. They would not approve of my shrugging.

The reason is the speed of light and I would suspect the time difference was more like 16.66 minutes since it is 8.33 minutes one way to the sun.

Fred Johnson

I don't think you could ever observe the 16.66 minute delay, the full orbit "diameter" differential of time of flight of light from a moon of Jupiter because for 6 months of the year, you are on the opposite side of the sun from Jupiter. Jupiter and its moons are still right where they are supposed to be, the time of flight differential still exists, but you are looking away from Jupiter when darkness comes to your location on Earth. For some portion of that 6 months, you may see Jupiter briefly in the morning or evening, but at those Earth positions light's time of flight differential will be less than the maximum of 16.66 minutes (relative to the time of flight when Earth's orbit is nearest Jupiter).

This time shift is due to the position of the Earth.
For a given position in the plane of the ecliptic, the Earth will move 16 light minuttes from it's closest, to its furthest distance. This is a real experience of the theory of relativity, and give a proov for the speed of light.

-Lars-

What theory of relativity are you referring to? Roemer made these measurements in the 17th century, not the 20th. As they were at that time the only measurements of the speed of light, the relevence of Einstein's special theory of relativity (that the the speed of light appears the same to all observers) is at best dubious.

Fyz