Watts' lunar profile data are currently still being used for lunar
occultation analyses, and they have large enough errors so that
occultations timed carefully visually still have value, even with the
improved stellar data from Hipparcos. An analysis of the Clementine
lunar orbiter laser altimeter data should be helpful in refining the
Watts' data by removing systematic errors. But the Clementine altimeter
data are too sparse to replace Watts' data. Nevertheless, when all the
analyses are complete, visual timings of total occultations may become
less useful than photoelectric and video timings. Video timing
accuracies are smaller than the Clementine laser altimeter errors, and
it is for that reason that I am strongly encouraging them; with
easy-to-use sensitive videocameras now available for less than $90,
many more observers can time occultations to an accuracy of 0.03s. The
Clementine laser did not probe the lunar polar regions, so visual
grazing occultation observations, inherently more accurate in any case
due to the grazing geometry (their accuracy depends more on a good
knowledge of the geographical position of the observer than on the event
timings), will continue to be valuable in the foreseeable future.
Dr. Mitsuru Soma, Japanese National Observatory, further explained
the current value of lunar occultation observations in a message that he
wrote on 1977 November 3rd:
Concerning the value of total occultation observations, I agree with
David Dunham that they are still useful in refining the limb corrections.
In addition to that, I think occultations can be used to analyze the
errors of the Hipparcos proper motion system. The Hipparcos team claims
that the Hipparcos reference frame is linked to the ICRS (International
Celestial Reference System, the VLBI reference frame based on the
extragalactic radio sources) with the accuracy of 0.25 mas/year.
But the direct comparison of the proper motions between Hipparcos and
FK5 gave inconsistent results with the precession error of -3 mas/year
of the FK5, which had been independently obtained from VLBI, Lunar laser
ranging, and proper motion analyses (Based on the preliminary results of
the differences of Hipparcos - FK5 by F. Mignard of CERGA in France,
I pointed out this fact in a letter addressed to him, and he admitted it
at the IAU General Assembly held in Kyoto this August). Now that the
lunar positions in the latest JPL planetary and lunar ephemeris
DE405/LE405 have the mas level accuracy with respect to the ICRS, I think
the problem can be resolved by analyzing lunar occultations using the
DE405/LE405 ephemeris and the Hipparcos catalog. [Since this was
written, Dr. Soma's analyses of Aldebaran grazes observed in 1979-1980,
compared with those observed a Saros cycle later in 1997-1998 in the
same part of the lunar profile, have confirmed that the FK5 proper
motion of this star is more accurate than the Hipparcos ICRS proper
motion.]
I understand that Dietmar Buettner (with Reinhold Buechner) in Germany
and I are using them. As you know, the Watts charts of the lunar limb
profiles, which have been used for predictions and analyses of lunar
occultations, have several gaps and many systematic and accidental
errors. Buettner uses total and grazing occultation observations to
improve the lunar limb profile data. His project is named MOONLIMB,
and the profile data are continuously being improved by incorporating
new observations. I have been updating the ACLPPP lunar limb profile
data to be used for the annual IOTA grazing occultation predictions
using all the grazing occultation observations. I also analyzed
total occultation observations observed photoelectrically or with
video since 1955 to obtain the systematic errors in the Hipparcos
proper motion systems and presented the results at an international
conference a couple of years ago, and I am planning to analyze all
of the lunar occultation observations in order to improve the accuracy
of the results. I think all of the lunar occultation observations and
their analysis results are also being stored in Dave Herald's OCCULT
software.
Best regards, Mitsuru Soma
_____________________________________________________________________
Some publications about lunar occultations (by Dr. Soma, 1998 August)
Concerning Warren's question about occultation papers, recent papers
on occultations in professional journals deal more with physical quality
of stars, such as the detection of binary stars (e.g. Astronomy and
Astrophysics [A&A] Vol. 322, p.202, 1997), the angular sizes and circumstellar
dust shell of giant stars (e.g. Monthly Notices of the Royal Astronical
Society, Vol. 287, p.681, 1997; A&A Vol. 319, p.260, 1997), using
professional lunar occultation observations, and it's true that there
are very few papers dealing with amateur occultation observations
recently, but it is because there are very few professional astronomers
who are analyzing occultations made by amateurs, and it does not mean
that doing occultation observations is a waste of time. [Note that the
International Lunar Occultation Centre is collecting all of the lunar
occultation observations, total and grazing, made worldwide and is
putting them into a standard computer-readable form. This database has
been supplied to Dr. Soma and to other occultation investigators around
the world. Dr. Soma has delayed his comprehensive analysis (and
therefore publication) of lunar occultation observations until the
Hipparcos stellar data could be incorporated into the star catalogs that
he uses for the occultation reductions. That catalog was completed
early in 1999, so that the analyses are now in progress, and publication
of them will follow].
Until a few decades ago, lunar occultation observations were used to
investigate the lunar motion, the Earth's rotation (difference between
Ephemeris Time and Universal Time), and positions of isolated islands,
but now they can be obtained much more precisely from modern technique
(lunar laser ranging, satellite laser ranging, VLBI, atomic clocks, etc.).
I analyzed lunar occultations to mainly investigate the acceleration
term in the lunar longitude and errors in the stars' reference system FK5,
and published the results in 1985 (Celestial Mechanics Vol. 35, p.45).
After that, I wanted to reanalyze them using more modern astrometric
catalogs to derive lunar limb profiles and errors of star catalogs. The
new astrometric catalog PPM was released in 1989 (north) and 1992
(south), but then the Hipparcos astrometric satellite was in orbit and
we knew that the Hipparcos catalog, which is much more precise and
almost free from systematic errors, would soon become available. So we
waited for it, and finally we got the Hipparcos catalog last year. I
analyzed the past grazes using it to derive the lunar limb profiles, and
they are being used for this year's graze predictions. David Dunham is
planning to analyze solar eclipses using the limb data derived from
occultations to detect small variations of the solar diameter, and I am
planning to reanalyze lunar occultations in order to derive errors in
the Hipparcos proper motion system (see the article of David Dunham in
the 1997 December issue of Occultation Newsletter for the value of lunar
occultations; a possible error in the Hipparcos proper motion system was
discussed there). In order to do the analyses correctly and
systematically (the present version of the zodiacal catalog XZ94E or
XZ94F has many mis-identifications and many positional errors especially
for double stars), I am now compiling the new zodiacal catalog using the
Hipparcos catalog and the ACT catalog in the Hipparcos system. After
completing it, I will analyze lunar occultations using the new catalog
and will publish the results in a professional journal.
As for the Clementine altimeter data, the data points are so sparse
that it is very hard to get limb profiles from them, but there is still
hope that we can compare them with Watts and occultation data at some
large basins located on the lunar limb. I will also analyze them soon.
Dr. Soma writing about visual timing accuracies
Warren Offutt wrote to Bob Sandy:
> i applaud anyone who can get 0.2 or 0.3 seconds precision from
> a commercial battery powered tape recorder over a span a few or
> several minutes when both temperature and battery condition is
> 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.
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.