Saturn and speed of light

Ole Christiansen Roemer proposed in 1676 that light travels at an finite
speed. He reached this conclusion by observing the moons of Jupiter when
Jupiter was at different distances from earth.
I have read this in several places, amongst others in "A Brief History of
Time" by Stephen Hawking.
I really don't understand this.
Exactly how did he reach this conclusion?

Thanks

Wasn't it Iordani who wrote:
Ole Christiansen Roemer proposed in 1676 that light travels at an finite
speed. He reached this conclusion by observing the moons of Jupiter when
Jupiter was at different distances from earth.
I have read this in several places, amongst others in "A Brief History of
Time" by Stephen Hawking.
I really don't understand this.
Exactly how did he reach this conclusion?

He assumed that each orbit took exactly the same amount of time, but the
observations didn't agree with that. What happens is that things like transits
and occultations of Io do occur in a regular pattern, but observations of those
events are delayed by different amounts depending on how long it takes the
light to get from Jupiter to Earth.

Detailed explanation he

http://www.colorado.edu/physics/2000..._evidence.html

--
Mike Williams
Gentleman of Leisure

Iordani nous a donc écrit :

Ole Christiansen Roemer proposed in 1676 that light travels at an
finite speed. He reached this conclusion by observing the moons of
Jupiter when Jupiter was at different distances from earth.
I have read this in several places, amongst others in "A Brief
History of Time" by Stephen Hawking.
I really don't understand this.
Exactly how did he reach this conclusion?

Roemer tried to calculate the length of the revolution of Io around Jupiter
by measuring how long Io was in the dark cone of Jupiter.
He found that this value was not constant, but varied depending upon the
position of the Earth.
As this value must be constant (Kepler's law), he deduced that the speed of
light (for the Jupiter-Earth distance) was finite and had to be taken into
account.

--
Norbert. (no X for the answer)
======================================
knowing the universe - stellar and galaxies evolution
http://nrumiano.free.fr
images of the sky http://images.ciel.free.fr
======================================

Norbert wrote:

Iordani nous a donc écrit :

Ole Christiansen Roemer proposed in 1676 that light travels at an
finite speed. He reached this conclusion by observing the moons of
Jupiter when Jupiter was at different distances from earth.
I have read this in several places, amongst others in "A Brief
History of Time" by Stephen Hawking.
I really don't understand this.
Exactly how did he reach this conclusion?

Roemer tried to calculate the length of the revolution of Io around
Jupiter by measuring how long Io was in the dark cone of Jupiter.
He found that this value was not constant, but varied depending upon the
position of the Earth.
As this value must be constant (Kepler's law), he deduced that the speed
of light (for the Jupiter-Earth distance) was finite and had to be taken
into account.

Well, this can't be the way he did it. Or, if this is true, you'll have
to explain it in more detail.

The time for any moon to be in the dark cone of Jupiter would be the same,
regardless of the distance between Earth and Jupiter. As observed from
Earth, that is.

By my way of understanding things, that is.

Thanks

In ,
Iordani typed:

The time for any moon to be in the dark cone of Jupiter would be the
same, regardless of the distance between Earth and Jupiter. As
observed from Earth, that is.

By my way of understanding things, that is.

Which is wrong.

It is not the time in the dark cone time that varies, it is the time between
each dark cone event. As the Earth-Jupiter distance increases the events are
seen later and as the distance decreases they are seen earlier.
Jo

Jo wrote:

In ,
Iordani typed:

The time for any moon to be in the dark cone of Jupiter would be the
same, regardless of the distance between Earth and Jupiter. As
observed from Earth, that is.

By my way of understanding things, that is.

Which is wrong.

It is not the time in the dark cone time that varies, it is the time
between each dark cone event. As the Earth-Jupiter distance increases the
events are seen later and as the distance decreases they are seen earlier.

Right, so the ability to measure time in 1676 was that accurate?
If this is the case, then it's all quite clear.
(In fact, I did rule out that possibility)

Thanks you

Yep, don't forget that it was some time around then that the transit of
Venus has been not only observed but the next one had been predicted. The
method needed to calculate the distance to the Sun has also been calculated,
but they lacked the necessary data. It was about 130'ish years later that
the data was gathered and the math done. The distance first calculated, was
to all intents and purposes very accurate, and only relatively minor
corrections to that initial figure have been made.

All that you really need to do the experiment is an accurate set of
predictions, and a good clock. Both of these were available at the time.

Regards

Colin Dawson
www.cjdawson.com

"Iordani" wrote in message
...
Jo wrote:

In ,
Iordani typed:

The time for any moon to be in the dark cone of Jupiter would be the
same, regardless of the distance between Earth and Jupiter. As
observed from Earth, that is.

By my way of understanding things, that is.

Which is wrong.

It is not the time in the dark cone time that varies, it is the time
between each dark cone event. As the Earth-Jupiter distance increases the
events are seen later and as the distance decreases they are seen
earlier.

Right, so the ability to measure time in 1676 was that accurate?
If this is the case, then it's all quite clear.
(In fact, I did rule out that possibility)

Thanks you

Colin Dawson wrote:

Yep, don't forget that it was some time around then that the transit of
Venus has been not only observed but the next one had been predicted. The
method needed to calculate the distance to the Sun has also been
calculated,
but they lacked the necessary data. It was about 130'ish years later
that
the data was gathered and the math done. The distance first calculated,
was to all intents and purposes very accurate, and only relatively minor
corrections to that initial figure have been made.

All that you really need to do the experiment is an accurate set of
predictions, and a good clock. Both of these were available at the time.


Thanks for your input.

In ,
Iordani typed:

Right, so the ability to measure time in 1676 was that accurate?
If this is the case, then it's all quite clear.
(In fact, I did rule out that possibility)


Yes, the ability to measure time to the required accuracy was available in
1676.

Jo

In message , Mike Williams
writes
Wasn't it Iordani who wrote:
Ole Christiansen Roemer proposed in 1676 that light travels at an finite
speed. He reached this conclusion by observing the moons of Jupiter when
Jupiter was at different distances from earth.
I have read this in several places, amongst others in "A Brief History of
Time" by Stephen Hawking.
I really don't understand this.
Exactly how did he reach this conclusion?

He assumed that each orbit took exactly the same amount of time, but the
observations didn't agree with that. What happens is that things like transits
and occultations of Io do occur in a regular pattern, but observations of those
events are delayed by different amounts depending on how long it takes the
light to get from Jupiter to Earth.

Detailed explanation he

http://www.colorado.edu/physics/2000..._evidence.html


FWIW, in "The Xi Effect" by "Philip Latham" an astronomer finds that the
delay isn't there any more :-) I think reading that story at the age of
about 12 is one of the things that got me into astronomy.
There's also a guy who believes that Roemer was mistaken about the speed
of light, but with any luck the OP's putting Saturn and not Jupiter in
the subject line will make him miss this thread. Regulars will know who
I'm talking about :-)