Possible occultation by Pluto - 1-2 July, 2002
Our most distant planet, Pluto, is set to make a rare and fascinating
brush with a brighter star. At a few seconds after 23h UT on Monday
night, the First of July (i.e. just after midnight on Tuesday
morning local BST time), Pluto at magnitude +13.9 will pass close
to the star 5651-1553 in the Tycho Catalogue. An occultation was
considered possible. More recent predictions suggest that the
star will actually pass through the gap between the planet and its moon Charon! Whatever happens, it is a great
challenge for members with moderate sized telescopes to find the
star and watch what happens.
The Epoch 2000.0 co-ordinates for locating the star are as follows:
R.A. 17h 01m 52.3s. Declination –12d 38m 56s. At magnitude +11.7,
it is the brightest star, so far, that the smallest planet in
the solar system has ever been predicted to occult! The southern
part of Pluto’s “shadow” was expected to cross most of Europe,
including the U.K., from east to west, according to early predictions.
The centre of the shadow misses Earth completely passing just
to the north of it in that scenario, pictured below.
As can be imagined, the prospect of an occultation involving Pluto,
has caused some excitement amongst astronomers. By careful observation
of the drop in magnitude of the starlight, much could be learned
about Pluto’s tenuous methane atmosphere. Such an event would
also give a rare opportunity for amateur observers to actually
see the remote little world, helped by its close proximity to
TYC 5651-1553, just two magnitudes brighter.
Updated projection of the position of the "shadows" of Pluto and
Charon in relation
to the Earth, based on Flagstaff astrometry in mid June. A near
miss looks likely
Even if direct observation of the planet is still too difficult
for some people, especially those who live in the northern parts
of the U.K. where summer twilight lasts all night, it should still
be possible to watch the star and, perhaps, time the drop in magnitude
of the star - if it occurs.
I must emphasise though, that there may be no occultation at all!
Recent astrometry made at Flagstaff in the U.S.A., suggests that
the star will slip through the 0.9" arc gap between Pluto and
it’s moon Charon towards the south! Predicting events for remote
objects, such as Pluto, which has not yet completed a complete
circuit about the Sun since its discovery in 1930, is notoriously
difficult. But, despite the uncertainty, it is still worth having
a go!
Observers who witness an occultation should expect to see a drop
in the brightness of the star in the order of two magnitudes.
In other words, if we are fortunate enough to witness the event,
the star will blink out, or maybe fade from sight, as Pluto passes
in front of it. The duration of the occultation could be about
two minutes according to some predictions. A map, showing the
predicted path for the planet, may be seen below.
The event, if it takes place, can only attempted from as dark
a site as possible, using a telescope that can resolve stars down
to at least magnitude + 14. Pluto and the star are to be found
in the southern part of the constellation Ophiuchus and will be
below 30 degrees in altitude at an azimuth of about 189 degrees,
at the time, even if you live in the south of England, so a clear,
unobstructed view towards the south is necessary.
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Finder chart 1 (click to enlarge)
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Finder chart 2 (click to enlarge)
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Finder chart 3 (click to enlarge) |
Pluto's track (click to enlarge) |
What do you need, if you want to do some very useful work in this
observational area and do not have sophisticated CCD equipment
to record the event?
The answer is: a stopwatch which has the ability to give a split/lap
time and an accurate time source such as a radio controlled clock
or transmitted time signal. You should also have made sure that
you have been able to find the 11th magnitude star by practicing finding it on several nights beforehand,
using finder charts such as the ones accompanying these notes.
Always check and set up your equipment in good time, at least
half an hour before the event or as soon as the sky is dark enough
to locate the target star. Check also that you know exactly how
to use the split time/lap function on your watch. Practise starting
the watch and obtaining a split time and stopping the watch a
number of times to make certain.
From 22h 50m UT (23h 50m BST) onwards keep watch and be very alert
(no drinking beforehand as this tend to slow your reflexes somewhat),
stopwatch ready!
If the star blinks out START the watch immediately. When it blinks
in again press the split/lap button and record the time interval
during which the star was occulted (The stopwatch should still
be running!)
If you get this far, CONGRATULATIONS - you have seen and perhaps
measured the effect of the distant little world of Pluto as it
passed in front of a star!
The last thing you must do is to STOP the watch against an exact
time signal as shown on your radio controlled clock or given by
an accurate time signal and make a note of the total elapsed time
since you started the watch when the star winked out.
By deducting the total elapsed time from the exact time you stopped
the watch (as shown on the radio controlled clock), you will have
the exact time the occultation started and by adding to this the
split/lap time you recorded (It will be anything up to 120 seconds
I should think!) you will then have the exact time the occultation
came to an end.
If you do get a positive result please send your timings to me
with any description of what you actually saw the star do as it
went out (Was there any unusual effects, fading, uneven fading,
fluctuations in the stars brightness, not due to Earth’s atmospheric
effects, etc.). Even if you see absolutely nothing please DO send
me a report. This is useful too as it will enable me to know where
the occultation was NOT seen in the U.K.
I will forward any results we may obtain to E.A.O.N (European
Asteroidal Occultation Network) as they may be of extreme value
in helping to work out more accurately the shape, dimensions and
even information about the atmosphere of the planet Pluto, which
is so far, the only planet that no probe from earth as yet visited.
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Original predicted passage of Pluto and Charon - but the star
now looks set pass between them (click on images to enlarge)
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Accompanying diagrams and data courtesy of: Jan Manek, Stefanik
Observatory, Czech Republic and by Project Pluto’s GUIDE 8 software.
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