The 10.4-mag. star TAC +31 deg. 1855 will be occulted by the 111-km asteroid 23 Thalia for about 4 seconds along a path across Ontario and central New England, two path-widths south of that shown on the map on p. 86 of the February issue of Sky and Telescope, and on S&T's occultation page at their Web site at http://www.skypub.com. This path starts in central Saskatchewan in strong twilight (2:08.4 UT, Sun alt. -4 deg.), crosses central Lake Winnipeg (near Berens River, 2:08.7 UT, Sun alt. -10 deg.), passes north of Sudbury (2:09.4) and over Ottawa (2:09.7 UT), Ontario, over northernmost New York, over northern Vermont (Burlington and Montpelier, 2:09.8), and central New Hampshire and southernmost Maine (2:09.9 UT). Since no recent observations have been made (the last were on March 5th), and there are rather large changes with time of the residuals from the available orbit (which has not been updated recently), the prediction is a little more uncertain than for some other recent events, I believe + or - 2 path-widths or +/-250 km on the ground. The correction to the time is small, so the event should occur shortly before 9:10 pm EST Sat. April 4th (2:10 U.T. of Apr. 5); it is necessary to observe only from 2:08 to 2:12 U.T. An 8-inch or larger telescope is recommended to observe the event. The star is at J2000 position RA 6h 03m 11s, Dec. +31 deg. 30' 46" in Aurigae about 7 deg. south of theta Aurigae, 0.7 deg. southwest of 5th-mag. SAO 58657, and only 10' north of a 7th-mag. star. A finder chart is on p. 94 of the April issue of Sky and Telescope, and on their Web site; a more detailed IOTA chart can be found under upcoming events at the IOTA asteroidal occultation site at http://www.anomalies.com
More information about the occultation, and path details, are given below. A straight average of the ACT-relative observations made from late last November to March 5 indicates a much more southerly path, 7.4 path-widths south of the one shown in S&T, passing over North Dakota, Minneapolis, Milwaukee, southern Michigan, northeastern Ohio, Pittsburgh, and Maryland, so observers that far south may want to try to observe the event. The most recent ACT-relative observations, made at Table Mtn. Obs. in early March, give a path almost 5 path-widths south of that shown in S&T and 2.6 path-widths south of the updated prediction given here. But the trends in the residuals from Thalia's computed ephemeris are strong and, when extrapolated to April 5, show that the path will be the one described above and detailed below. Observers within two path-widths of this path (such as, throughout New England) are strongly encouraged to try to monitor the star for a possible occultation, if the sky is reasonably clear. Good luck with your observations.
OCCULTATION OF 10.56-MAG. TAC +31d 1855, SPECTRAL TYPE F, BY 23 Thalia
ON 1998 APRIL 5 (U.T.; Sat. evening, April 4th local date)
IN VISUAL, MAG. DROP AT DISAPPEARANCE= 0.80
DIAMETER= 111. KM = 0.076 SECONDS OF ARC, CENTRAL OCCN. = 4.146 SECONDS
63-PERCENT-SUNLIT WAXING MOON 31.3 DEGREES AWAY, SUN 75.75 DEGREES AWAY
Thalia 2.017353 A.U. FROM EARTH, PHASE ANGLE= 28.7DEGREES
CENTER PATH IS ABOUT 122. KM WIDE AT THE COORDINATES LISTED BELOW
UNIV. TIME Thalia SUN MOON
LONGITUDE LATITUDE H M S ALT. AZI. ALT. AZI. ALT.
105.72 W 54.02 2 8 23.6 59.5 234.1 -4.2 286.2 52.8
102.81 W 53.48 2 8 30.7 58.4 238.8 -6.1 288.6 53.3
99.97 W 52.90 2 8 37.7 57.2 243.2 -7.9 290.9 53.8
97.19 W 52.27 2 8 44.7 55.9 247.4 -9.7 293.1 54.1
94.48 W 51.60 2 8 51.5 54.5 251.3 -11.5 295.3 54.4
91.83 W 50.90 2 8 58.2 53.1 255.0 -13.3 297.4 54.5
89.25 W 50.16 2 9 4.8 51.7 258.4 -15.1 299.4 54.5
86.73 W 49.39 2 9 11.3 50.2 261.6 -16.9 301.4 54.4
84.27 W 48.59 2 9 17.6 48.7 264.7 -18.7 303.4 54.3
81.88 W 47.76 2 9 23.8 47.1 267.5 -20.5 305.3 54.0
79.54 W 46.91 2 9 29.8 45.5 270.2 -22.3 307.1 53.6
77.26 W 46.03 2 9 35.7 43.9 272.7 -24.1 308.9 53.1
75.03 W 45.13 2 9 41.3 42.3 275.0 -25.9 310.6 52.5
72.86 W 44.20 2 9 46.9 40.6 277.2 -27.7 312.4 51.8
70.73 W 43.26 2 9 52.2 39.0 279.3 -29.5 314.1 51.1
68.65 W 42.31 2 9 57.3 37.3 281.3 -31.2 315.7 50.2
TRACK 0 CENTER FOR 23 Thalia SAO 0 DATE 1998 APR 5
W. LONG. LAT. W. LONG. LAT. W. LONG. LAT. W. LONG. LAT.
114.000 55 14.8 101.000 53 6.8 88.000 49 47.2 75.000 45 6.7
113.000 55 7.3 100.000 52 54.1 87.000 49 28.6 74.000 44 41.6
112.000 54 59.4 99.000 52 41.0 86.000 49 9.5 73.000 44 16.0
111.000 54 51.1 98.000 52 27.4 85.000 48 49.9 72.000 43 49.9
110.000 54 42.5 97.000 52 13.4 84.000 48 29.9 71.000 43 23.2
109.000 54 33.4 96.000 51 59.0 83.000 48 9.3 70.000 42 56.1
108.000 54 24.0 95.000 51 44.1 82.000 47 48.2 69.000 42 28.4
107.000 54 14.2 94.000 51 28.8 81.000 47 26.7 68.000 42 0.2
106.000 54 4.0 93.000 51 13.0 80.000 47 4.6 67.000 41 31.6
105.000 53 53.4 92.000 50 56.8 79.000 46 42.1 66.000 41 2.4
104.000 53 42.3 91.000 50 40.1 78.000 46 19.0 65.000 40 32.8
103.000 53 30.9 90.000 50 22.9 77.000 45 55.4 0.000 0 0.0
102.000 53 19.0 89.000 50 5.3 76.000 45 31.3 0.000 0 0.0
AT U.T. 2 8 30, W. LONG.= 103.098, LAT.= 53.534
AT U.T. 2 9 0, W. LONG.= 91.137, LAT.= 50.707
AT U.T. 2 9 30, W. LONG.= 79.460, LAT.= 46.875
AT U.T. 2 10 0, W. LONG.= 67.551, LAT.= 41.791
Path if shifts 2 path-widths north
TRACK 150 NORTH FOR 23 Thalia SAO 0 DATE 1998 APR 5
W. LONG. LAT. W. LONG. LAT. W. LONG. LAT. W. LONG. LAT.
113.000 57 22.9 100.000 55 14.7 87.000 51 55.8 74.000 47 16.1
112.000 57 15.3 99.000 55 2.0 86.000 51 37.3 73.000 46 51.0
111.000 57 7.4 98.000 54 48.9 85.000 51 18.3 72.000 46 25.4
110.000 56 59.0 97.000 54 35.4 84.000 50 58.8 71.000 45 59.3
109.000 56 50.4 96.000 54 21.5 83.000 50 38.8 70.000 45 32.6
108.000 56 41.3 95.000 54 7.1 82.000 50 18.3 69.000 45 5.5
107.000 56 31.8 94.000 53 52.3 81.000 49 57.3 68.000 44 37.8
106.000 56 22.0 93.000 53 37.0 80.000 49 35.8 67.000 44 9.6
105.000 56 11.8 92.000 53 21.3 79.000 49 13.8 66.000 43 40.9
104.000 56 1.2 91.000 53 5.2 78.000 48 51.3 65.000 43 11.7
103.000 55 50.2 90.000 52 48.5 77.000 48 28.3 0.000 0 0.0
102.000 55 38.7 89.000 52 31.4 76.000 48 4.7 0.000 0 0.0
101.000 55 26.9 88.000 52 13.9 75.000 47 40.7 0.000 0 0.0
Path if shifts 2 path-widths south
TRACK 150 SOUTH FOR 23 Thalia SAO 0 DATE 1998 APR 5
W. LONG. LAT. W. LONG. LAT. W. LONG. LAT. W. LONG. LAT.
112.000 52 46.0 100.000 50 36.3 88.000 47 23.6 76.000 43 1.5
111.000 52 37.5 99.000 50 22.7 87.000 47 4.5 75.000 42 36.4
110.000 52 28.5 98.000 50 8.7 86.000 46 44.9 74.000 42 10.8
109.000 52 19.1 97.000 49 54.3 85.000 46 24.8 73.000 41 44.7
108.000 52 9.4 96.000 49 39.4 84.000 46 4.2 72.000 41 18.1
107.000 51 59.2 95.000 49 24.1 83.000 45 43.2 71.000 40 50.9
106.000 51 48.6 94.000 49 8.3 82.000 45 21.6 70.000 40 23.3
105.000 51 37.6 93.000 48 52.0 81.000 44 59.5 69.000 39 55.2
104.000 51 26.2 92.000 48 35.3 80.000 44 36.9 68.000 39 26.6
103.000 51 14.4 91.000 48 18.1 79.000 44 13.8 67.000 38 57.5
102.000 51 2.1 90.000 48 0.4 78.000 43 50.2 66.000 38 27.9
101.000 50 49.4 89.000 47 42.3 77.000 43 26.1 65.000 37 57.9
Prediction Comparison
MIN. GEOCENTRIC Shift from nominal (S&T, p. 86 of Feb. issue)
U. T. SEP. Path Time,
h m " widths min.
2 7.4 2.00N 0.0 0.0 Nominal (S&T, map on p. 86 of Feb. issue)
2 7.9 1.44N 7.4S +0.5 Average of all obs., 1997 Nov. - 1998 Mar. 5
2 7.5 1.64N 4.7S +0.1 Table Mtn. Obs., 1998 March 5 and 6
2 7.3 1.84N 2.1S -0.1 Extrapolation of trends in residuals (probable)
David Dunham, IOTA, 1998 April 4, 7:30 U.T.