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Frank Reed wrote-

>George H wrote:
>"1. Jupiter doesn't extinguish all that suddenly. Jupiter has a  semidiameter
>of nearly an arc-minute, a diameter of nearly 2'. The Moon moves  with
>respect to the background of stars at about 30 minutes in an hour, so  it
>would take about 4 minutes in all to shut down Jupiter's light  altogether,
>and more if Jupiter wasn't crossing the midplane of the Moon. So,  you might
>suggest, just time the last-gasp of the light, which should be  pretty
>sudden. But then, that moment would depend on the light-gathering  power of
>your telescope, and different observers are likely to disagree about  the
>moment. For this reason, an occultation of a star would be better:  that
>happens instantaneously."
>
>I'm guessing that you've never watched an occultation of a planet by the
>Moon. The last-gasp is actually rather easy to time and different observers (at
>the same site) are not likely to disagree by more than a couple of seconds.
>You  could also time the initial contact.

=====================

Frank's guess is wrong here; I have. However, I have not made any serious
attempts to time the event. I wonder if Frank has ever made such timing
comparisons, using different observers with differing optical aids, to back
up his assertion 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".


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?

As for Frank's statement "You could also time the initial contact". 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 wrote-

>"2. The Moon, because it is so close to Earth, isn't in the same  direction
>when seen by different observers, at different places on Earth.  Because of
>parallax, the apparent Moon can be shifted by a whole degree from  the point
>in the stars where an observer exactly below the Moon would see it.  The
>Moon is only about 30 minutes across, so many observers wouldn't see  the
>occultation at all, and others would see Jupiter cross the Moon at  very
>different times and at different "levels", taking different times to  cross.
>An observer could allow for this parallax if he knew just where he was  on
>the Earth's surface. Unfortunately, that's just what he needs to find  out.
>So the "very elementary lunar calculation" that David describes  is
>unattainable."

and Frank responded-
>Unattainable? That's just not so. It's a fairly tedious calculation, that's
>for sure.

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. Of course, by a rather
complex process, it could indeed be calculated. But that wasn't what David
Edwards was asking for, in his original enquiry, and his question was
exactly what I was answering.

========

>And:
>"3. The extinguishing of Jupiter's light as it's overtaken by the  Moon is
>easy to see, if that part of the Moon's disc is dark, as it is around  first
>quarter. But when it happens at last quarter, that part of the Moon  is
>brightly lit, and then it's difficult (impossible?) to make out the  planet
>from the Moon as it's appproached and swallowed-up."

Frank responded-
>Jupiter is very bright.

Against the dark background of the sky, that may be true, but it's all
relative. It's illumination by the Sun is only about 1/30 of that of the
Moon, because it is so much further away. This is made up for, to some
extent, by its much greater albedo (reflecting power). Jupiter reflects
about 43% of the incoming sunlight, the Moon only about 7% (like
coal-dust!). The end result is that Jupiter has only about a fifth of the
surface brightness of the Moon.

and further-
>It can be seen right up to the last moment of the
>occulation using binoculars (or a small telescope).

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) and have much clearer skies (which is almost certainly the
case).

I ask him how well he could make a timing under those circumstances, and
whether for that purpose he would choose an inner-contact or outer-contact
between the Moon's limb and Jupiter's.

George.



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01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
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