Newsgroup: it.scienza.astronomia
Titolo: The Current Value of Lunar Occultation Observations by Dr. Soma

        subject to real world circumstances.
It's true that observers are often overly optimistic about their
achieved
precision, but I think achieving the 0.2 or 0.3 second precision is not
so difficult for a careful observer on the condition that a time signal
is recorded continuously during the events, even if the tape speeds
vary
machine to machine or with battery condition and temperature.  Three
occultation observers at Shimosato station of Japanese Hydrographic
Department made experiments in around 1980 imitating an occultation
observation of a 7th magnitude star at a dark limb of a moon with the
age of 8 or 9 through a 30cm reflector, and got the following results:
   Personal equations measured by an oscillograph
                     Voice recorded            Key-tapping
                 Phen  n  m/sec  s/sec    Phen  n  m/sec  s/sec
      Observer A   D  20   0.36   0.03      D  18   0.32   0.03
                   R  18   0.36   0.04      R  19   0.32   0.05
      Observer B   D  18   0.34   0.05      D  18   0.33   0.04
                   R  20   0.31   0.03      R  18   0.30   0.03
      Observer C   D  19   0.33   0.04      D  20   0.30   0.04
                   R  19   0.32   0.05      R  20   0.31   0.04
where n is the number of experiments, m is the mean value, and s is the
standard deviation.  They also got the times by hearing the recorded
tapes with an accuracy of 0.1sec or 0.05sec, and the differences
(the time they got by hearing the tape minus the time from the
oscillograph) are as follows:
                 Phen  n  m/sec  s/sec
      Observer A   D  20  -0.01   0.05
                   R  20  -0.02   0.06
      Observer B   D  20  +0.01   0.07
                   R  20  +0.02   0.05
      Observer C   D  20  +0.01   0.05
                   R  20  -0.02   0.04
Therefore if one can estimate their personal equations appropriately,
I do not think achieving the 0.2 or 0.3 second precision is very
difficult.
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I have always maintained that timings of grazes to 0.5s accuracy
are "good enough", since the accuracy of grazes comes more from the
position
of the observer (effectively determines the height) and less to the
timing
(determines only the W.A. of the event, mainly to identify the mountain
or
hill).  This means that visual timings, still the most numerous for
grazes,
are fine.  But for total occultations, the accuracy depends directly on
the
time accuracy, so that timings should be accurate to 0.2s or better to
be
useful.  Carefully-made visual timings can achieve that, but of course
video does better and is more consistent.  This is the current
situation, with
the accurate Hipparcos\ACT\TRC positional information for the stars,
and
accurate lunar ephemeris from analysis of laser ranging observations,
but
still rather large errors in the profile (typical error +/-0.2"
corresponding
to +/-0.4s or more of time).  So by keeping the timing accuracies lower
than above, they are significantly less than the profile error.
     But this does not mean that full-accuracy video timings should not
be
obtained.  If they can be obtained without too much difficulty, for
both
totals
and grazes, they should be obtained, even though they give more
accuracy
than we
can currently use.  That will probably not always be the case, for
example,
after the Japanese Selene mission maps the Moon much more accurately
(with
laser ranging from the spacecraft) and extensively than Clementine did,
and
the lunar profile is improved with that data, then the video
observations
will have more of an advantage over careful visual timings.  The Selene
mission
has been approved for a launch in 2003, but of course that could slip.
Late
in the first decade of the 21st century, either the Gaia or SIMS
mission will
measure star positions more accurately (and for many more stars) than
Hipparcos,
and that will also affect the situation.
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Da oggi è disponibile la nuova versione 3.6 presso
http://www.lunar-occultations.com/iota/occult3.htm
grazie al dr Dave Herald e collaboratori.



[...]
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Vi sono alcune imprecisioni e omissioni nel file di Help di Occult 3.6
e qui ve ne segnalo una che torna utile a chi esegue da sè i calcoli
di previsione delle occultazioni di Luna ed eclissi.
Dispiace infatti rilevare come il Dr. Herald nella v 3.6 di Occult, non
abbia ancora provveduto a correggere, nonostante la segnalazione, il
valore K del raggio della Luna, dando adito all'ipotesi (auguro che sia
solo una mia personale congettura) che possa esservi una svista
addirittura negli algoritmi.
Ciò che vi propongo è un semplice estratto del file di Help con
inserita  la mia correzione secondo le più recenti normative della IAU
per la compilazione di effemeridi.
============
Ricordo infine a quanti possedevano una precedente versione di Occult,
che non è più disponibile l'eseguibile che andava ad aggiornare la
precedente versione (occorre prelevare l'intero eseguibile del peso di
circa 14 Mb); inoltre vi invito a controllare la data del file del
Catalogo di Hypparcos: se il vostro è di data antecedente il Maggio
2004 esso presenta un baco e pertanto dovete prelevare la nuova
versione.
http://www.lunar-occultations.com/iota/occult3.htm
============
============
Eliseo
============
============
Technical details
The following are technical derails concerning the operation of OCCULT
- lunar occultation predictions
Method of predicting events:
·	Besselian elements are computed for all conjunctions that occur
between -0h 0m 36s UT and 24h 00m 36s UT - and held as a quadratic
polynomial.
·	For planetary objects, the besselian elements are computed on the
same basis as besselian elements for solar eclipses.
·	If the Site Select flag is set, the latitude limits of the
occultation are computed using expressions given in the Explanatory
Supplement to the Astronomical Ephemeris (1968); also compared against
whether the moon is likely to be up, or sun likely to be up.
·	Occultations are computed using an iterative process, based on
expressions in the Explanatory Supplement (supra).
·	Grazing Occultation predictions are computed using a
double-iterative process using site longitude as the independent
variable - iterating to find the latitude of a point on the graze
path (with each iteration having to find the time of minimum
separation). Normally, the iteration commences with a latitude of 0
deg, and proceeds towards the poles; for those rare grazes where the
path crosses a line of longitude twice, a second iteration is done with
the initial latitude being the relevant pole (i.e. +/- 90 deg).
·	Visibility of events is computed using the expressions of Schaefer,
B. E., Bulder, H. J. J., and Bourgeois, J., Lunar occultation
visibility, Icarus, 100, 60-72 (1992)
·	Multisite predictions assume a 20cm telescope, with no magnitude
correction.
·	Planetary ephemerides are computed using the VSOP expressions.
Aberration is also derived from those expressions.
·	The lunar ephemeris is based on the expressions of Chapront
(ELP-82B), which was fitted to the Lunar ephemeris of DE200.
·	The profile includes the corrections to the Watts charts determined
by Morrison & Appleby MNRAS (1981) 196, 1013.
·	Precession and Nutation are computed using the IAU1976 constants.
Positions referred to the B1950 equinox are converted to J2000 using
the corrections given in the 1984AE.
Maximum number of stars held in memory:	900
Maximum number of sites held in memory:	100
Maximum number of sites in multisit.dat (sites for multi-site
predictions):	>1000? (Unlimited if edited outside of OCCULT)
Maximum number of stars included in GSC special catalogue:	5000
Maximum number of star catalogues that can be used:	20
Maximum number of asteroids that can be included:	>150
Radius of moon = 0.2725026
=======================================================================================================
Radius of moon = 0.2725026 (THAT'S INCORRECT. PSE READ Radius of moon =
0.2725076 INSTEAD OF 0.2725026)
=======================================================================================================
More details may be found under the File Format book in Help Contents
(end of Technical details)
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