[Physics] Mutual events of Galilean moons

[Thomas Goodey]

On 25 Jul 2017 at 12:00, Carl Johnson wrote:

> Ganymede and Callisto are generally much farther away
> from the other and much larger.  Yes, the math indicated
> a rare (solar) eclipse of one of the smalller ones by
> Ganymede, but that seemed to be extremely rare.  Io and
> Europa are often much closer together, and of roughly the
> same diameter, so most of the eclipses and occultations
> that I had calculated involved them, one way or the
> other. 

The discussion has confused two quite different types of 
occultation event:

(A) a solar eclipse of one of the GMs by another, in which 
one moon blocks the light from the Sun falling on another. 
For example, one case would be when Io, Europa, and the Sun 
come into exact line. 

That type of event is equally visible from anywhere on 
Earth. It appears as a (relatively) abrupt disappearance of 
the eclipsed moon (if the eclipse is total).

(B) an event in which, from the point of view of an 
observer on the Earth, one of the GMs moves in front of 
another. For example, a case would be when Europa moves in 
front of Io.

That type of event is seen differently from different 
places on the Earth. It appears as a coalescence of the two 
moons in question, and a reduction of their optical 
brightness to about half.

But neither case really could give any meaningful 
information about the one-way speed of light. The events 
simply don't happen quickly enough, definitely enough. 

For example, Wikipedia gives the orbital speed of Europa at 
around 14 km/sec, and that of Io at around 18 km/sec. So 
their relative speed can only be about 4 km/sec at maximum. 
Since Io is about 3600 km in diameter, you see that such an 
event, either of type A or B, must take place over a 
minimum of 900 seconds. And it happens gradually and 
progressively. To get data accurate enough to discuss the 
one-way speed of light, you would need a predictable event 
that can be localized in time to less than a second. Such 
events do not occur in the Jupiter system.

By the way, I don't see why you wouldn't have just as many 
events involving Ganymede and Callisto (which are bigger). 
The distance between the moons really wouldn't come into 
it. There is very little parallax at that distance.

Thomas Goodey

*****************************
Anne's search for security 
holes in the localizer network 
software was close to 
impossible. Every year her 
zipheads pushed back their 
deadline for certainty another 
year or two. But the quagmire 
of Qeng Ho fleet software 
was almost eight thousand 
years deep.
--------- Vernor Vinge
----------'A Deepness in the Sky'