<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Larry Burford</i>
<br />Joe,
...How often do we see occultations like this?
...What about the predicted values?
...What sort of chatter is there in the astronomy community, when a deviation like this happens?
...And of course, do you have any personal thoughts about it?
<b>[Joe Keller] "... the 3rd contact, which is the start of emersion, is difficult to time as accurately with manual methods, because one is not looking exactly at the relevant point, when it happens"</b>
Now days I assume these events are recorded and analyzed after the fact...
LB
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Luna can be ~ 5 degrees from the ecliptic but Luna's center must be within ~ 0.25deg of the ecliptic, to occult a planet on the ecliptic. So Luna would occult Jupiter about one month in twenty, or more accurately 0.05radian/(pi/2 radian) = one month in thirty, as measured from Observatory X. Less than half the time, this would be at acceptable altitude in the sky, at Observatory X, and sometimes Jupiter would appear too near the Sun. So, we should see useful Jupiter occultations, at Observatory X, almost once in five years, but the fashion might come and go, for publishing such observations.
As for measuring an occultation somewhere on Earth, not necessarily observatory X, that would happen 4+1 = 5 times as often, and would always be above the horizon somewhere, so the answer is, 5*2 = 10 times as often as seeing it from Observatory X, i.e., almost twice a year. This is because Earth's diameter is four times Luna's.
I'm confident that my predicted values are consistent with the JPL ephemeris on which they are based; one test, was that my result for the Lick observation was the same, for my earlier pocket-calculator back-of-an-envelope effort, and for my computer program which I wrote later from scratch. I'm considering Jupiter's oblateness and illumination, though neither of those corrections is big enough to change the results fundamentally. Maybe the JPL ephemeris is much less accurate than widely believed, for 19th century positions; but, as I mentioned above, that wouldn't explain the difference in discrepancy, for the 1st vs. the 2nd contact. Those two discrepancies should be practically the same, if the error is only in the ephemeris position of Luna and/or Jupiter.
It seems that originally, occultation timing discrepancies were attributed to observation error, not an unlikely explanation, considering the discrepancy between, say, the Princeton and U. of Virginia data in 1889: those observatories are less than 4deg longitude apart. Later, automatic records of stellar occultation found favor, but this isn't the same experiment as a planetary occultation. A planet is here in our own solar system: as emphasized by Bart Laplae in his recent personal communication to me, this implies possible trouble with local ether drift due to the planet's orbital velocity, not only due to Earth's orbital velocity. There have been some automated 20th century observations of planetary occultations, but these were not my first choice to study, for sociological reasons:
Even an automated record involves subjective "interpretation of the squiggles on the chart". Whereas a 19th century journal editor might publish an anomalous occultation timing that he could comfortably ascribe to "error", a 20th or 21st century journal editor would be inclined to reject such a report (not anymore attributable to error) as heretical, not even sending it for peer review. The report that would be published, would be the report that happened not to be anomalous, or whose authors kept silent about the timing anomaly and wrote about other aspects, or happened to misinterpret the squiggles in such a way that it seemed not anomalous, or whose authors faked the data willfully ("publish or perish").
Today, my own best speculation (the thought occurred to me yesterday morning) is that the timing anomaly is due to an ether drift or suchlike effect caused by the rotation of the occulted planet and the rotation of Earth. This could give grossly different timing anomalies at 1st & 2nd contact (edge of planet rotating away from & toward Earth, resp.) and at different observation sites on Earth (where not only would Earth's rotation angle be different, but also Luna would be apparently displaced so that the planet's latitude of contact would be much different).
