NavList:

George H wrote (of my guess that he hadn't seen many occultations):

"Frank's guess is wrong here; I have."

 

Oh sorry then. My mistake. From previous posts I had the distinct impression that you had not done much actual observing of astronomical events outside a navigational context. Did you know there was a Jupiter occultation last month, too? Many people in the eastern US observed it... in broad daylight. Unfortunately, it was cloudy here.

 

And:

" However, I have not made any serious attempts to time the event. I wonder if Frank has ever made such timing comparisons"

 

Sure, I have. Planetary occultations are among the prettiest things you can see with a small telescope. Timing them is part of the game.

 

And:

" using different observers with differing optical aids, to back
up his assertion " etc.

 

LOL. Nope, George, I haven't wasted my time on that project... yet!

 

And:

"that they "are not likely to disagree by more than a
couple of seconds", or whether he can quote some authoritative reference.

Otherwise, it is indeed no more than assertion; with no higher status than
my own assertion that "different observers are kikely to disagree about the
moment"."

 

George, it is obviously a matter of degree. I felt that your previous reply was too dismissive of any and all possibility that this observation could serve any navigational purpose. So I offered my opinion based on my experience. As for "higher status" and all that... wow, that's just not a game I want to play.

 

And:

"And what would be the point of observing the slow decline of the light from
Jupiter, when star occultations (which are far more frequent) are
instantaneous events, presenting no timing problems at all?"

 

Ok. First things first. The point of observing *this* occultation is the simple fact that it is a very pretty astronomical event easily observed with the naked eye, or binoculars, or a sextant telescope, or any other telescope. An observer with an interest in navigation (like those on this list) might take some interest in timing the events of the occultation and contemplating whether they might have served some purpose if observed carefully in past occultations. As for stellar occultations, yes, for bright stars they can be timed more accurately than a Jupiter occultation. But so what? We don't have one of those. And besides,  NOBODY is using these observations for practical navigational purposes any longer. Observing them for fun and contemplation is worth getting up a little early (or staying up a little late, if you have hours like mine).

 

And:

"Well, you could, but do do so you would need a firmly-planted telescope on land,
with high magnification. And to time THAT event to a couple of seconds, you
would need to determine the moment when the first bit was shaved off
Jupiter's limb. How deep a shave? Just 1 part in 120 of Jupiter's
semidiameter, or about half an arc-second. Quite an observational feat,
that would be!"

 

I mentioned a "couple of seconds" for final disappearance. For initial contact, it's not as good since this is more like a lunar distance sight contact. An amateur telescope with 50x mag would be more than enough for a couple of seconds accuracy at initial contact. With a sextant telescope, you could expect perhaps 15 second accuracy (seconds of time) for a typical observer (at initial contact).

 

By the way, I think you doubled the apparent diameter of Jupiter somewhere along the way... It's about 0.6 arcminutes.

And George wrote:

"Here, he seems just to be picking a disagreement, where none exists. What
was unattainable, just as I said, was the "very elementary lunar
calculation" that the original enquiry had proposed."

 

I see. No problem then. You agree that the calculation could be done, but (at least with pencil and paper) it's tedious.

 

And added:

"The end result is that Jupiter has only about a fifth of the
surface brightness of the Moon."

 

Not really. When the Moon is past last-quarter, phase effects diminish its surface brightness considerably. Jupiter and the Moon will have very nearly equal average surface brightness for this occultation. In astronomers' units, the surface brightness of both will be about -3.2 magnitudes per square arcminute.

 

And:

"If Frank can make out a Jupiter DISC under those circumstances, against the
glare from the Moon, he's a much better observer than I am (which may well
be the case)"

 

This is, of course, very closely related to the issue of lunars accuracy. Can you make out a tenth of a minute of arc when you look through a sextant? How about two-tenths? If you believe that most observers can do this, then it is also possible to see the disk of Jupiter (and yes, I can see it clearly at 7x and better at 10x). The disk of Jupiter is about 0.6 minutes of arc across right now. I agree that *some* people can't see that small an angle through a telescope at these magnifications, but I think most people can and most people with sextant experience routinely do so. And if you *don't* agree that most people can see the disk of Jupiter, then you would have to conclude that lunars would have to have been much less accurate historically then people have supposed.