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
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
It's not the eclipse duration that is the issue; it's the time between eclipses. — Banno
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
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.
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
In either case, there isn't any error in principle. — Michael Ossipoff
On this point, I am afraid, you have not changed my mind. — Hachem
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
, 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
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