New data shows NO shift from S&T path for the May 13th Phocaea occultation
The prediction distributed yesterday included only a few (on one night) of the accurate observations that are now available for the occultation of (25) Phocaea on Tuesday evening, May 12th (local; May 13th UT). When all of the data are considered, as M. Federspiel and J. Manek have done as detailed in their messages below, the new path is now the same as the one shown on p. 86 of the February issue of Sky and Telescope (also available in the occultation section of www.skypub.com) crossing southern Arizona (Phoenix and Tucson are near the northern limit) and northern Mexico with an estimated accuracy of +/-1.5 path-widths for the
78-km-wide asteroid. The path is also shown as the "ACT" line on the world view and finder chart that were published in the 1998 Planetary Occultation Supplement to Occultation Newsletter for North American Observers distributed to IOTA members and ON subscribers in early January. That chart will soon be placed on the IOTA Web site at http://www.sky.net/~robinson/iotandx.htm and later on Sunday at http://www.anomalies.com/iota/splash.htm by clicking on the star in the table of North American events in the "Upcoming events" section. The correction to the time is -0.4 +/-0.3 minutes. So the event should occur at 2:41 UT at the mouth of the Rio de la Plata (Montevideo, Buenos Aires, and Rosario could be in the path); 2:44 UT at Arica at the north end of Chile; 2:47 at Guayaquil, Ecuador; 2:50 along the Pacific coast of Guatemala (Guatemala City could have an occultation); less than a minute
later at Vera Cruz, Mexico; and close to 2:53.0 UT (7:53 pm MST) in southern Arizona. Path coordinates and circumstances are given below. Note that the map published in La Red and in the original E. Goffin chart distributed by J. Stamm is not corrected for the better ACT position for
the star that was used for the path shown on S&T and ON Supplement maps;
the "ACT" (now = the updated path) is 0.3" (3.5 path-widths) southwest of Goffin's original path.
The 8.3-mag. spectral type K2 target star, SAO 139602, is at J2000 position RA 13h 52m 59.2s, Dec -7 deg. 43' 49", about 7 deg. northeast of Spica and 1.5 deg. southeast of 88 Virginis. A central occultation is expected to last 7 seconds with a 2.5-magnitude drop.
David Dunham, IOTA, 1998 May 6, 23:30 UT
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UPDATE FOR THE OCCULTATION OF SAO 139602 BY (25) PHOCAEA 1998 MAY 13
This update is based on a new orbit calculated exclusively from 29 positions obtained by Ron Stone (UNSO/Flagstaff) 1998 Jan 24th - May 5 and from 4 positions supplied by Bill Owen (Table Mtn. Observatory) 1998 May 1. Except one USNO observation all data are of superb quality, the rms residuals being 0.07" in RA and 0.09" in Dec, respectively. The derived orbit is very close to the MPC 25716 orbit given in the EMP for 1998. Despite the short arc covered by the observations this orbit is perfectly suited to calculate the event of May 13.
Ron Stone's position of the star (mean of two measurements) is 0.07" in RA and 0.05" in Dec from the ACT catalogue position. With the latter and my orbit for Phocaea I get the central line given below which essentially confirms the line given in Sky & Telescope and does *not* support the shift stated by Jan Manek who analyzed the Table Mountain observations alone (to derive a differential correction). However, the event seems to occur 0.5 minutes earlier than assumed in S&T. I assume the remaining uncertainty (error in the position of the star and uncertainty of the orbit) to be 0.13" or 1.5 path widths.
Central line for an occultation by (25) Phocaea
(25) Phocaea occults SAO 139602 (RA 13 52 59.244 Dec - 7 43 48.49)
orbit from USNO and Tbl. Mtn. obs. only (1/98-5/98), star ACT
Minimum geocentric distance:
date: 1998 5 13 time: 2 45 48.UT; d=1.29", PA=236.72 deg, pi=7.15",
mu=48.14"/h
substellar point on earth (lat., long.), PA of least distance:
-7.72 -63.88 236.72
East
Lat. Long. hh:mm:ss UT StarAlt StarAzi SunAlt SunAzi
-42.31 -50.52 2:40:48. ! 53.8 339.4 -64.7 201.3
-40.42 -52.34 2:41: 5. ! 56.0 341.4 -65.9 206.7
-38.57 -54.04 2:41:23. ! 58.2 343.4 -66.9 212.4
-36.74 -55.64 2:41:41. ! 60.3 345.5 -67.7 218.2
-34.95 -57.15 2:41:59. ! 62.3 347.6 -68.3 224.1
-33.17 -58.59 2:42:17. ! 64.2 349.9 -68.6 230.1
-31.42 -59.97 2:42:35. ! 66.1 352.4 -68.8 236.1
-29.69 -61.30 2:42:52. ! 68.0 355.1 -68.8 242.0
-27.98 -62.58 2:43:10. ! 69.7 358.2 -68.5 247.7
-26.28 -63.83 2:43:28. ! 71.4 1.6 -68.1 253.2
-24.60 -65.04 2:43:46. ! 73.1 5.7 -67.6 258.3
-22.92 -66.22 2:44: 4. ! 74.6 10.4 -66.8 263.0
-21.26 -67.38 2:44:21. ! 76.0 15.9 -66.0 267.4
-19.61 -68.52 2:44:39. ! 77.2 22.6 -65.0 271.4
-17.97 -69.65 2:44:57. ! 78.2 30.3 -63.9 275.1
-16.33 -70.76 2:45:15. ! 79.0 39.4 -62.7 278.4
-14.69 -71.86 2:45:33. ! 79.5 49.4 -61.5 281.4
-13.06 -72.96 2:45:51. ! 79.6 60.0 -60.1 284.1
-11.43 -74.05 2:46: 8. ! 79.4 70.4 -58.7 286.5
-9.80 -75.15 2:46:26. ! 78.8 80.1 -57.3 288.7
-8.17 -76.24 2:46:44. ! 78.0 88.7 -55.8 290.7
-6.54 -77.35 2:47: 2. ! 76.9 96.0 -54.2 292.4
-4.91 -78.46 2:47:20. ! 75.6 102.0 -52.6 294.0
-3.27 -79.58 2:47:38. ! 74.2 107.0 -51.0 295.4
-1.62 -80.71 2:47:55. ! 72.7 111.1 -49.3 296.7
.04 -81.87 2:48:13. ! 71.0 114.5 -47.6 297.8
1.70 -83.04 2:48:31. ! 69.3 117.3 -45.9 298.8
3.38 -84.24 2:48:49. ! 67.5 119.6 -44.1 299.7
5.07 -85.48 2:49: 7. ! 65.7 121.6 -42.2 300.4
10.24 -89.40 2:50: 0. ! 59.7 125.5 -36.4 302.0
13.80 -92.28 2:50:36. ! 55.5 127.0 -32.2 302.5
15.62 -93.83 2:50:54. ! 53.3 127.4 -30.0 302.6
17.48 -95.46 2:51:11. ! 50.9 127.7 -27.8 302.6
19.37 -97.19 2:51:29. ! 48.5 127.8 -25.4 302.4
21.31 -99.05 2:51:47. ! 46.0 127.7 -23.0 302.1
23.29 -101.05 2:52: 5. ! 43.4 127.5 -20.4 301.7
25.34 -103.24 2:52:23. ! 40.6 127.0 -17.7 301.1
27.45 -105.65 2:52:40. ! 37.7 126.3 -14.8 300.3
29.65 -108.35 2:52:58. ! 34.6 125.4 -11.8 299.2
31.95 -111.45 2:53:16. ! 31.1 124.1 -8.4 297.8
34.38 -115.09 2:53:34. ! 27.3 122.4 -4.6 295.9
37.00 -119.58 2:53:52. ! 22.9 120.0 -.3 293.4
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Martin Federspiel ******** E-mail:
martin@astro.unibas.ch
Astronomisches Institut ******** oder:
federspi@astro.unibas.ch
der Universitaet Basel **** oder:
federspielm@ubaclu.unibas.ch
Venusstr. 7 Telefon: (CH)-61/2055413
CH-4102 Binningen (Schweiz) Fax: (CH)-61/2055455
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