Excellent Asteroid Occultation Friday Morning, May 5, 2006

This Is SKY & TELESCOPE's AstroAlert for Occultations

     We need your help to observe the best asteroidal occultation
ever predicted for the northeastern USA, and potentially the best in
the USA since the 1 Vulpeculae/Pallas event in the southern USA in
1983.  This is the occultation of 5.8-mag. 16 Piscium, in the
Circlet of Pisces, by the 200-km S-class asteroid (7) Iris early
Friday morning, May 5, 2006.  Earlier weather forecasts were pessimistic,
but the recent ones for the time have improved, with Accuweather now
predicting mostly clear skies (scattered cirrus) in eastern Penn.,
New York, and western New England.  More cirrus (47%) are predicted
for central Penn., and areas farther south (Maryland) and west
(Ohio) are predicted to be partly cloudy (56%) with mid-level clouds
that could seriously hamper observation at the relatively low
altitude of this event.  But the weather forecast could change;
we'll have a better idea of the cloud cover for the event when the
first Astro Meteo (Clear Sky Clock) forecast is available early
tomorrow (Wed.) afternoon.

     In any case, please let me know if you plan to try to observe
this good occultation, either from a fixed site or with portable
equipment, so that we can optimize location of several
mobile observers for this bright event, targeting them to parts of
the asteroid that won't be covered by fixed-site observers.  A
comprehensive list of stations listed by distance in km from the
central line is on Derek Breit's Web site at
especially useful since it gives the altitudes of the star and the
Sun for each station, important since the star will be relatively
low in the east with twilight being some factor in most areas.
More about the station list is below.  In central Penn., central
Maryland, and central New York, and places farther west to Ohio and
eastern Kentucky, the event might be seen with binoculars, if it is
clear low in the east.  But small telescopes (& larger ones if you
have them) are recommended, especially to give higher magnification
to see the star longer into the twilight.


     The occultation will occur within half a minute of 9:02.0 UT
(5:02 am EDT) for all locations; you only need to observe for a two-
minute period starting at 9:01.0 UT (5:01.0 am EDT).  Twilight will
be too strong to see the event from the Canadian Maritime Provinces,
and probably also from Maine.  It might be seen at very low altitude
as far west as Illinois and Missouri, but it's hard to locate a
rising star so low, and more practically I think Indiana and western
Kentucky will be as far west as the occultation might really be
seen.  The path is very wide, projected by the low altitude into a
much larger area than the asteroid width.  The southern limit
crosses southernmost W. Va.; central Virginia (near Charlottesville
and Fredericksburg); near Waldorf & Easton, MD; central Delaware;
and southernmost N.J.  The northern limit passes over central
Indiana (n. of Indianapolis), n. Ohio (Cleveland), just s. of Erie,
Penn.; over Syracuse, NY; and northern Vermont & New Hampshire.  The
limits have a 1-sigma uncertainty of about 25 miles, so areas north
and south of these lines could have an occultation, and observers at
least out to the 2-sigma limits have some reasonable chance and are
encouraged to observe.  The central line passes over southern Ohio,
s.w. to n.e. Penn. (just n. of Harrisburg & n. of Allentown),  s.
New York (near Newburgh), n.w. Conn., and over Worcester & just n.
of Boston, Mass., but the probability of an occultation is greater
than 90% for distances up to 100 miles from the center, greater than
96% for distances up to 80 miles, and 100% for distances up to 60
miles from center, so there's no huge advantage in going to the
exact center; it's much better to have observers spread evenly
across the path for measuring Iris' size and shape.  Derek Breit's
Web site given above also has a good map of the wide path showing
major cities and highways; besides the central line, it shows the
northern and southern limits, and those limits in case of a very
possible 1-sigma path shift away from the central line.


The spectral type F5 star is at J2000 R.A. 23h 36m 23.2s,
Dec. +02 deg. 06' 08", in the Circlet of Pisces.  The star is
16 Piscium = ZC 3482 = SAO 128281 = HIP 116495.  The star may be a
close double with separation of about 0.01" (about a tenth of Iris'
angular size of 0.11").  The star is about 1.2 deg. west and a
little north of 4.5-mag. lambda Piscium (the southeastern star of
the Circlet).  To help identify the field, if it's dark enough, 9.0-
mag. SAO 128273 is 8' east and a little south of 16, and 8.4-mag.
SAO 128268 is about 15' east-north-east of 16 Piscium.  Detailed
finder charts can be found on Steve Preston's Web site at
have trouble now printing those maps directly from my Web browser;
only a small part of them print.  Later today, I will put them in a
Power Point file and post them on my Web site at
http://iota.jhuapl.edu ; that allows you to view and print the whole

     If an occultation occurs, there will be a 4.4-mag. drop to the
10.2-mag. of the asteroid lasting 5 seconds for a central event.
The observations might reveal the star to be a close double, in
which case, the disappearance and/or reappearance will occur in
steps.  The 52% sunlit waxing Moon will have set.  The best area for
astronomical conditions is in central Maryland - Washington, DC,
where the altitude of the star will be 15 deg. above the eastern
horizon with the Sun 12 deg. below it, so the sky around the target
will not be too bright there.  Farther west, the star will be lower,
twilight will not be an issue, but you need to find the star at
lower altitude as it's rising.  It might be effective to pre-point
your telescope to a brighter star at the same declination and
altitude earlier in the night; I'll try to figure that out and
distribute this evening.  For observers east of central Maryland and
central Pennsylvania, the target star will rise higher, but twilight
will increasingly interfere by the time of the occultation; in New
England, especialy eastern Massachusetts, the twilight will be very
bright and higher magnifications are recommended to try to see the
target at the time of the occultation.  Observers in those areas
will need to locate the target half an hour or so before the
occultation and follow the star to the event, preferably with clock
drive and rather good polar alignment.  A riskier method for them
might be to also prepoint the telescope to the declination and
altitude that 16 Piscium will have at the time of the occultation.


     A list of dozens of cities and stations within 3 sigma of the
path is at Derek Breit's Web site at
station list, arranged by distance from the predicted central line,
you can see the predicted time of the occultation for your location,
as well as the altitudes of the star and Sun, and the probability
for an occultation by Iris.  I'll distribute a list of planned
sites where observations might be attempted in a day or so, after
I've received some messages from observers about their observing
plans.  You can use your Web browser "find" function to find your
name or city in the list.  If your station is not in the list, send
either your coordinates or the postal address of your observing
location to me at dunham@starpower.net and to Derek Breit at
breit_ideas@hotmail.com .  With this, you can see the event time and
circumstances at your location, and we can know that you'll try the
event to help plan our coverage.  A few of us will be mobile and
want to fill in gaps of the fixed-sites coverage; we want to avoid
the duplications and near-duplications that we had at the recent
Turandot occultation.  Mobile observers can be given a distance from
the central line to aim for, and they can input that distance in the
"offset" box on Derek Breit's interactive google maps site at
to view the path, and their offset line, in great detail.

     I plan an expedition from the DC area, mainly to optimize
weather and to cover a more central part of the path.  For now, it
looks like I'll go north (I-83) or northeast, possibly quite far to
get on the northern side of center, but it will depend not only on
the weather, but also on the distribution of fixed-site observers
who say they will try the event and for whom the weather forecast is
good closer to the time of the event.

PS:  As long as you're up that morning, you might get up a little
earlier and also look at the pieces of Comet Schwassman-Wachmann-3
that will be near optimum visibility before Moonlight starts to
interfere.  The Accuweather forecast remains poor for the Sat.
evening Atala occultation in the Washington, DC region.

David Dunham, IOTA   2006 May 2, noon EDT
home dunham@starpower.net 301-474-4722 cell 301-526-5590
office david.dunham@jhuapl.edu 240-228-5609
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