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Preamble:
The updated path for this event crossed much of Victoria as well as the extreme southern tip of New Zealand.
Six observers saw this occultation, although only four made timings that could be used in the analysis (see comments with the observations below) owing to a variety of factors. Fifteen other observers had cloud or a variety of other problems which prevented timings from being recorded.
Observers: 1 J. Blanksby, Wandin, VIC, Australia 2 S. Buda, St Kilda, VIC, Australia 3 M. Justice, Ivanhoe, VIC, Australia 4 G. Murphy, Sale, VIC, Australia 5(C) G. Walton, Bonbeach, VIC, Australia 6(C) B. Tregaskis, Mt Eliza, VIC, Australia 7(C) P. Skilton, Mt Martha, VIC, Australia 8(C) I. Musgrave, Largs Nth, Sth Australia 9(C) C. Pratt, Glen Waverley, VIC, Australia 10(C) D. Witham, Hawthorn, Vic, Australia 11(C) I. Grant, Ringwood E, Vic, Australia 12(C) M. Valimberti, Viewbank,Vic, Australia 13(C) R. Cockman, Elwood, Vic, Australia 14(C) J. Sunderland, Churchill,Vic, Australia 15(C) J. Kviz, Highton, Vic, Australia 16(C) P. Lowe, Langwarrin, Vic, Australia 17(C) J. Bukovsky, Mordialloc, Vic, Australia 18(C) P. Nelson, Churchill, Vic, Australia 19(C) J. Bailey, Clarendon, Vic, Australia 20(C) R. Cowdell, Ocean Grove, Vic, Austral 21(C) R. Alessio, Noble Pk, Vic, Australia (C) = either cloud, or some other factor that precluded accurate timings being made.
Discussion:
Four chords are plotted with disappearances on the left and reappearances on the right. The relative positions of the remaining 17 observers who experienced cloud or other problems are also plotted for reference.
The observed chords are reasonably concordant and suggest that Iris has an elliptical shape. Further, the distribution of the chords show that the northern part of the asteroid was covered, suggesting that the path moved south by perhaps one half of a track width compared to the updated prediction. As such, the occultation would probably have been seen from Launceston, Tasmania, (and possibly even Hobart) and would have passed south of Invercargill, New Zealand.
The plot above assumes an ellipse of 313 x 198 km in PA -20 deg. However the fit coverged to many potential solutions with major axes between about 200 and 340 km, so it is not possible to make any more definitive statement about the major and minor axes of Iris. One chord across the southern part of the asteroid would have been sufficient to derive a more definitive result.
Iris has previously been observed at occultation twice - on 1995 May 26 from Japan (one chord), and on 1997 July 25 when Jim Blanksby recorded an event but approximately 50 other observers from Victoria were clouded or rained out.
(Our thanks to Peter Skilton for co-ordinating the observing effort from Victoria).
Observational Data - Occultation Seen:
Observer's Name : Jim Blanksby
Aperture (cm) : 15
Focal length (cm) : 75
Type (e.g. SCT; Newtonian) : Newtonian
Magnification : 187
Observing site name : -
Longitude (DD MM SS ; East +ve) : +145d 25' 22"
Latitude (DD MM SS ; South -ve) : -37d 46' 53"
Height above Sealevel (metres) : 181
Geodetic Datum (e.g.WGS84,NZ1949): Aust1966
Height Datum (if known) : -
Sky Transparency (Delete two) : Good
Star Image Stability (Delete two): Good
Other Conditions:
(Wind, Clouds, Lights, etc.): Calm, no Moon
Time Source (e.g. WWVH, GPS) : 10MHz WWVH
Recording method (e.g. tape) : Tape
Could you see the Asteroid? : Y
Approx. Limiting Magnitude : 13
| Estimated |
Universal Time | Reaction | Accuracy, Remarks
h m s | Time (sec) |
Started Observing : 16:01:00
Disappearance At : 16:06:26.0 +/- 0.5 sec
Reappearance At : 16:06:33.4 +/- 0.3 sec
Stopped Observing : 16:11:00
Was your reaction time (also known as Personal Equation) subtracted from
any of the above timings? : No
ADDITIONAL COMMENTS: Impression that both disapp. and reapp. were somewhat gradual - perhaps
an effect of MP being visible. Merged around 15:47 to 15:49; separated around 16:20 to 16:23.
Observer's Name : Stefan Buda
Aperture (cm) : 20
Focal length (cm) : 120
Type (e.g. SCT; Newtonian) : Dobsonian
Magnification : 40
Observing site name : Raglan Street Observatory
Longitude (East +ve) : 144:59:38E
Latitude (South -ve) : -37:51:54
Height above Sealevel (metres) : ?
Geodetic Datum (e.g.WGS84,NZ1949): WGS84 (used www.multimap.com)
Height Datum (if known) : ?
Sky Transparency (Delete two) : Good
Star Image Stability (Delete two): Good
Other Conditions:
(Wind, Clouds, Lights, etc.): Perfect
Time Source (e.g. WWV, GPS) : wwv (10MHz)
Recording method (e.g. tape) : tape
Could you see the Asteroid? : yes (only during occultation)
Approx. Limiting Magnitude : 11.3
| Estimated |
Universal Time | Reaction | Accuracy, Remarks
h m s | Time (sec) |
Started Observing : 16:02:00
Disappearance At : 16:06:25.19
Reappearance At : 16:06:33.89
Stopped Observing : 16:10:00
Was your reaction time (also known as Personal Equation) subtracted from
any of the above timings? : NO
ADDITIONAL COMMENTS: The unexpected visibility of the asteroid during the occultation may have
influenced my reaction times.
Observer's Name : Mark Justice
Aperture (cm) : 15
Focal length (cm) : 120
Type (e.g. SCT; Newtonian) : Newtonian
Magnification : 48
Observing site name : Ivanhoe
Longitude (East +ve) : 145 deg 02' 43.3" E
Latitude (South -ve) : -37 deg 46' 14.6"
Height above Sealevel (metres) : 40
Geodetic Datum (e.g.WGS84,NZ1949): WGS84 from GPS
Height Datum (if known) : -
Sky Transparency (Delete two) : Good
Star Image Stability (Delete two): Good
Time Source (e.g. WWV, GPS) : WWV 10MHz
Recording method (e.g. tape) : Camcorder for audio only
Could you see the Asteroid? : Generally no but some hint suspected during occultation
Approx. Limiting Magnitude : 11
| Estimated |
Universal Time | Reaction | Accuracy, Remarks
h m s | Time (sec) |
Started Observing : 16:05:00
Disappearance At : 16:06:25.9 | ??? | Suspected slow response
Reappearance At : 16:06:33.5 | ??? | Confident response
Stopped Observing : 16:07:00
Was your reaction time (also known as Personal Equation) subtracted from
any of the above timings? : No
Observer's Name : Grant Murphy
Aperture (cm) : 15 cm
Focal length (cm) : 104 cm
Type (e.g. SCT; Newtonian) : Newtonian
Magnification : 57 x
Observing site name : Sale - 14C
Longitude (DD MM SS ; East +ve) : +147 05 27
Latitude (DD MM SS ; South -ve) : -38 06 48
Height above Sealevel (metres) : 13
Geodetic Datum (e.g.WGS84,NZ1949): WGS84
Height Datum (if known) : -
Sky Transparency (Delete two) : Good
Star Image Stability (Delete two): Good
Time Source (e.g. WWVH, GPS) : WWVH
Recording method (e.g. tape) : TAPE
Could you see the Asteroid? : Only during the occultation, not when star was visible
Approx. Limiting Magnitude : Didn't estimate
| Estimated |
Universal Time | Reaction | Accuracy, Remarks
h m s | Time (sec) |
Started Observing : 16 04 00
Disappearance At : 16 06 29.3 0.8
Reappearance At : 16 06 34.2 1.0
Stopped Observing : 16 07 10
Was your reaction time (also known as Personal Equation) subtracted from
any of the above timings? : Yes - measured times have PE already subtracted
If YES, state value : As per columns above, 0.8 and 1.0
ADDITIONAL COMMENTS:
Event marks on tape made using an audio beeper activated by push-button switch.
Timings extracted by playing audio into computer and anaylsing using Cool Edit.
Confident with disappearance and with estimate of PE.
After a few seconds I noticed the asteroid and 'beeped' it as the reappearance.
The star actually returned as this erroneous beep concluded. I then beeped the true
reappearance. At the time I estimated my delay in beeping the true reappearance as 1 sec.
The gap between the end of the erroneous beep and start of the true beep correlates with this delay.
Observer's Name : Peter Skilton
Aperture (cm) : 15
Focal length (cm) : 75
Type (e.g. SCT; Newtonian) : Newtonian, equatorial, driven.
Magnification : 101
Observing site name : The Briars Observatory (Mornington Peninsula Astronomical Society),
Mount Martha, Victoria.
Longitude (DD MM SS ; East +ve) : +145d 02m 29.5s (RMS error +/- 0.08s)
Latitude (DD MM SS ; South -ve) : -38d 16m 22.7s (RMS error +/- 0.07s)
Height above Sealevel (metres) : 57 +/- 3 metres
Geodetic Datum (e.g.WGS84,NZ1949): Previously integrated by a GPS unit, so WGS84.
Height Datum (if known) : -
Sky Transparency (Delete two) : Good
Star Image Stability (Delete two): Good
Other Conditions:
(Wind, Clouds, Lights, etc.): No wind, no Moon, temperature 6 Celsius (thermometer),
some light dew on observing table at conclusion of observation,
small patches of cloud in the north and west at the commencement of
observing, though nothing to naked eye in the direction of the
target star.
Time Source (e.g. WWVH, GPS) : 10MHz WWVH, propagation - fair to good.
Recording method (e.g. tape) : Tape
Could you see the Asteroid? : Yes
Approx. Limiting Magnitude : 12 to 12.5 at this altitude of about 20 degrees.
| Estimated |
Universal Time | Reaction | Accuracy, Remarks
h m s | Time (sec) |
Started Observing : 16:01:12
Disappearance At : 16:05:48.6 +/- 3 sec Very slow dimming, no disappearance.
Suspected onset of thin cloud, dew point reached,
or eyepiece misting interference.
Estimated Closest Approach : 15:59, while setting up prior to starting unbroken observing and timing.
Reappearance At : 16:06:23.9 +/-0.4 sec Rebrightened as a step change relative to star field.
Cannot rule out thin cloud or eyepiece misting
interference across the starfield at the time.
Star and Object Separated : 16:10:23
Stopped Observing : 16:12:00
Was your reaction time (also known as Personal Equation) subtracted from
any of the above timings? : Yes
If YES, state value : 0.37 sec as measured on the night.
If you could tell, in which direction did the asteroid pass relative to
the star : Was able to last separate asteroid from the star 7 minutes prior to occultation,
and again first resolved it to the North east of the star at 16:10:23 UT.
ADDITIONAL COMMENTS: The dimming was highly unusual, and very gradual over several seconds, and occurred
much earlier than I had anticipated, taking me by surprise. This lead me to believe at the time that I
had thin cloud interfering at precisely the worst time. However, I didn't register visible cloud movement
in the eyepiece, though the sky did have small patches of thin, wispy cloud in the north and west before
observing began. Fogging of the eyepiece in the cool conditions was possible, as I recall all stars in
the field did briefly disappear within the time range between dimming and rebrightening (but not for the
entire time). There was definitely a time (not captured on the tape though) when I could see the entire
starfield but the target star was much dimmer than what I recalled it prior to the occultation. In summary,
I don't believe I successfully captured the instants of dimming and rebrightening, though I believe the shadow
did pass over my location. The dimming time can be used as a constraining boundary only, and the rebrightening
may have had cloud/misting interference. By the next morning (2000 UT) when I left for work, the sky was
100% clouded over.
Observer's Name : Greg Walton
(Report prepared by Peter Skilton)
Aperture (cm) : 53 (21 inches)
Focal length (cm) : 238
Type (e.g. SCT; Newtonian) : Newtonian, manually guided. "Sky Dancer".
Magnification : 108
Observing site name : Home, front driveway.
Longitude (DD MM SS ; East +ve) : +145d 07m 56s
Latitude (DD MM SS ; South -ve) : -38d 03m 35s
Height above Sealevel (metres) : approximately 10 metres.
Geodetic Datum (e.g.WGS84,NZ1949): GDA94 (Via Victorian Government's LandVic website)
Height Datum (if known) : -
Sky Transparency (Delete two) : Good
Star Image Stability (Delete two): Good
Time Source (e.g. WWVH, GPS) : ABC Radio National 621KHz AM hourly time pips.
Recording method (e.g. tape) : Tape
Could you see the Asteroid? : Yes
Approx. Limiting Magnitude : Starfield was sketched and showed stars fainter than the target star.
Probably less than magnitude 12.
| Estimated |
Universal Time | Reaction | Accuracy, Remarks
h m s | Time (sec) |
Started Observing : 16:00
Disappearance At : -
Reappearance At : 16:08:56sec At this time Greg recorded that he had seen a "dimming of a second
or more, about a minute earlier" than this time on the tape.
Stopped Observing : 16:09
Was your reaction time (also known as Personal Equation) subtracted from
any of the above timings? : No
If you could tell, in which direction did the asteroid pass relative to
the star (Delete three) : Greg Was able to see the asteroid approach the star, merge with it and separate, but the times of these were not recorded on the tape.
ADDITIONAL COMMENTS:
This was Greg's first reported minor planet occultation. He has tried previously but always been beaten by
the weather. Because he was anticipating a complete disappearance, rather than a dimming of the star,
he didn't believe this was the event occurring at the time, and so did not record any thoughts onto the
tape. It was only after the star rebrightened that he realised he had seen the event and missed recording
it on the tape. He was definitely convinced that he had successfully witnessed the occultation as a dimming
lasting "a second or more about a minute earlier", and this was recorded just prior to ending observation.
Due to likely tiredness, and the anticipated shadow duration over his location, this seems to indicate a
shortened time perception in that the expected occultation was longer than a second and longer than a minute
prior to the recorded time on the tape. As such the time above can only be used as a constraint on the
reappearance, but the observation of an occultation at the site was observed.
Observational Data - Occultation Not Seen:
Summary of other Australian Observers prepared by Peter Skilton:
Observer's Name : Brian Loader
Aperture (cm) : 25.4
Focal length (cm) : 250
Type (e.g. SCT; Newtonian) : SCT
Magnification : n/a
Observing site name : Darfield
Longitude (DD MM SS ; East +ve) : +172° 06' 24.4" E
Latitude (DD MM SS ; South -ve) : -43° 28' 52.9" S
Height above Sealevel (metres) : 210
Geodetic Datum (e.g.WGS84,NZ1949): WGS84
Height Datum (if known) : MSL
Sky Transparency (Delete two) : Good
Star Image Stability (Delete two): Good
Time Source (e.g. WWVH, GPS) : GPS/KIWI
Recording method (e.g. tape) : VIDEO
Could you see the Asteroid? : visible on monitor
Approx. Limiting Magnitude : 11.5
| Estimated |
Universal Time | Reaction | Accuracy, Remarks
h m s | Time (sec) |
Started Observing : 16:02
Star and Object Merged : ca 16:03:30
Star and Object Separated : ca 16:10:20
Stopped Observing : 16:11
If you could tell, in which direction did the asteroid pass relative to
the star (Delete three) : SOUTH (as expected)
ADDITIONAL COMMENTS: No event, expected just south of Invercargill
Observer's Name : Paul Rodmell
At 10 pm it cleared so I generated suitable charts and organised positions of the close bright stars.
Then it clouded in again and rained. Still bad 2:45. Alarm at 4 still 100% cloud. Still like that when
I woke this morning. So no occultation visible.
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