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GRAZING OCCULTATION OF ZC 1925 (Spica) - 2006 August 28
A spectacular grazing occultation of the magnitude 1.0 multiple star ZC 1925 (Spica) occurs shortly after 9.15 pm NZST on Monday August 28 for observers in the Wellington and Wairarapa. Those situated close to the graze path may be lucky enough to see the star wink out as it disappears behind mountains on the edge of the moon, and then perhaps reappear in the lunar valleys between the mountains. Timing these events from multiple locations with an accuracy of 0.1-0.2 seconds will help us draw up a picture of the edge of the Moon at the point of occultation. Further, the fact that this is a close multiple star will make time-synced video recordings of the event very valuable. However, even though the star is very bright the circumstances of this event do pose some problems:
Finding the star:
The diagram below shows the position and path of the star relative to the moon as would be seen by the naked eye. (In a telescope the view may appear inverted or back-to-front).
Graze Profile:
The profile of the edge of the moon below (drawn with north uppermost) shows the expected shape of the Moon's northern limb near the point of occultation. However the detail of these graze profiles is often incorrect, and it is our aim to obtain much more complete and accurate data.
The profile shows several dotted lines which come from C.B. Watts's charts of the edge of the Moon. They alone suggest that there may be many mountains and valleys on the visible limb. However the solid dark line shows the profile deduced from past grazing occultations and this is almost flat. If this is correct, there may be few occultation events to time.
The "0km" line is known as the "limit line" - i.e. the path the star would follow (entering from the left) if the moon was perfectly smooth and circular. The scale on the sides of the profile shows distances in kilometres (measured perpendicularly) north or south of this "limit line". You will see that for this graze the observed lunar limb profile does not match the predicted profile at all, and if the observed profile is correct, no-one north of about 1 km south of the limit line is likely to see an occultation.
The Path:
The nominal "limit line" of the graze passes directly across Tawa, through Trentham, and then just north of Greytown. This "limit line" is based on the assumption that the moon is circular with a flat limb. However, as we've seen above, occultations of the star are likely to be seen in a region starting from 1 km south of the nominal limit line.
On the first overview map below the limit line has been plotted with lines at 1, 2, 3 and 4 km south of the limit. Use the scale on the profile above to see how the path of the star behind the moon changes as you move further south. The second overview map shows the limit line only in the region of Greytown.
Observing Locations:
Because the Moon will be so low in the west at the time of occultation, places which have a western horizon which rises more than 7 degrees will be out of luck. There are no suitable locations on the west coast and observers close into the western hills in the Hutt Valley will not have a chance. It appears that the most likely places to observe will be as follows:
Observing the Graze:
Spica will be an unmistakable and spectacular sight as it approaches the moon across its dark limb, eventually being occulted at a time dependent on your location. The time of central graze is about 09:24:37, and you can determine when the first occultation at your loaction can be expected by consulting the time markers along the "limit line" on the profile above. E.g. for someone about 4 km south of the limit line, the first occultation can be expected about 2 minutes before the time of central graze, or about 09:22:30.
For more information about how to observe this graze visit this page.
To participate in this graze please contact (as soon as possible):
Path Co-ordinates:
Haritina Mogosanu (Ph: 021-269-2908) or Graham Blow (Ph: 479-2504).
Grazing Occultation of 1925 SB1 Magnitude 1.0v
1925 = Spica = alpha Virginis
Date 2006 August 28 (Monday) Nominal site altitude 0m
E. Longit. Latitude U.T. Sun Moon TanZ PA WA CA
o ' " o ' " h m s Alt Alt Az o o o
170 0 0 -41 49 5 9 23 2 10 264 5.40 23.3 3.13 -0.46N
170 7 30 -41 48 11 9 23 5 10 264 5.46 23.3 3.10 -0.48N
170 15 0 -41 47 18 9 23 8 10 264 5.52 23.3 3.08 -0.51N
170 22 30 -41 46 23 9 23 11 10 264 5.57 23.2 3.06 -0.53N
170 30 0 -41 45 29 9 23 13 10 264 5.63 23.2 3.04 -0.55N
170 37 30 -41 44 34 9 23 16 10 264 5.69 23.2 3.02 -0.57N
170 45 0 -41 43 38 9 23 19 10 264 5.75 23.2 2.99 -0.59N
170 52 30 -41 42 43 9 23 22 10 264 5.81 23.2 2.97 -0.61N
171 0 0 -41 41 46 9 23 25 10 264 5.88 23.1 2.95 -0.64N
171 7 30 -41 40 50 9 23 27 10 263 5.94 23.1 2.93 -0.66N
171 15 0 -41 39 53 9 23 30 9 263 6.01 23.1 2.91 -0.68N
171 22 30 -41 38 56 9 23 33 9 263 6.08 23.1 2.89 -0.70N
171 30 0 -41 37 58 9 23 35 9 263 6.14 23.1 2.87 -0.72N
171 37 30 -41 37 0 9 23 38 9 263 6.21 23.0 2.85 -0.74N
171 45 0 -41 36 1 9 23 40 9 263 6.29 23.0 2.82 -0.76N
171 52 30 -41 35 2 9 23 43 9 263 6.36 23.0 2.80 -0.78N
172 0 0 -41 34 3 9 23 45 9 263 6.44 23.0 2.78 -0.80N
172 7 30 -41 33 3 9 23 48 9 263 6.51 22.9 2.76 -0.83N
172 15 0 -41 32 3 9 23 50 9 263 6.59 22.9 2.74 -0.85N
172 22 30 -41 31 3 9 23 53 9 263 6.67 22.9 2.72 -0.87N
172 30 0 -41 30 2 9 23 55 8 262 6.75 22.9 2.70 -0.89N
172 37 30 -41 29 1 9 23 58 8 262 6.84 22.9 2.68 -0.91N
172 45 0 -41 27 59 9 24 0 8 262 6.93 22.8 2.66 -0.93N
172 52 30 -41 26 57 9 24 3 8 262 7.01 22.8 2.64 -0.95N
173 0 0 -41 25 55 9 24 5 8 262 7.10 22.8 2.62 -0.97N
173 7 30 -41 24 52 9 24 7 8 262 7.20 22.8 2.60 -0.99N
173 15 0 -41 23 49 9 24 9 8 262 7.29 22.8 2.57 -1.01N
173 22 30 -41 22 46 9 24 12 8 262 7.39 22.7 2.55 -1.03N
173 30 0 -41 21 42 9 24 14 8 262 7.49 22.7 2.53 -1.05N
173 37 30 -41 20 38 9 24 16 7 262 7.60 22.7 2.51 -1.07N
173 45 0 -41 19 33 9 24 18 7 262 7.70 22.7 2.49 -1.09N
173 52 30 -41 18 28 9 24 21 7 262 7.81 22.7 2.47 -1.11N
174 0 0 -41 17 23 9 24 23 7 261 7.92 22.6 2.45 -1.13N
174 7 30 -41 16 17 9 24 25 7 261 8.04 22.6 2.43 -1.15N
174 15 0 -41 15 11 9 24 27 7 261 8.16 22.6 2.41 -1.17N
174 22 30 -41 14 4 9 24 29 7 261 8.28 22.6 2.39 -1.19N
174 30 0 -41 12 57 9 24 31 7 261 8.41 22.6 2.37 -1.21N
174 37 30 -41 11 50 9 24 33 7 261 8.54 22.5 2.35 -1.23N
174 45 0 -41 10 43 9 24 35 7 261 8.67 22.5 2.33 -1.25N
174 52 30 -41 9 35 9 24 37 6 261 8.81 22.5 2.31 -1.27N
175 0 0 -41 8 26 9 24 39 6 261 8.95 22.5 2.29 -1.29N
175 7 30 -41 7 18 9 24 41 6 261 9.10 22.5 2.27 -1.31N
175 15 0 -41 6 8 9 24 43 6 261 9.25 22.4 2.25 -1.33N
175 22 30 -41 4 59 9 24 45 6 260 9.41 22.4 2.23 -1.35N
175 30 0 -41 3 49 9 24 47 6 260 9.57 22.4 2.21 -1.37N
175 37 30 -41 2 39 9 24 49 6 260 9.74 22.4 2.20 -1.39N
175 45 0 -41 1 28 9 24 50 6 260 9.91 22.4 2.18 -1.41N
175 52 30 -41 0 18 9 24 52 6 260 10.09 22.3 2.16 -1.43N
176 0 0 -40 59 6 9 24 54 6 260 10.28 22.3 2.14 -1.45N
176 7 30 -40 57 55 9 24 56 5 260 10.47 22.3 2.12 -1.47N
176 15 0 -40 56 43 9 24 58 5 260 10.67 22.3 2.10 -1.49N
176 22 30 -40 55 30 9 24 59 5 260 10.88 22.3 2.08 -1.51N
176 30 0 -40 54 17 9 25 1 5 260 11.10 22.2 2.06 -1.53N
176 37 30 -40 53 4 9 25 3 5 260 11.32 22.2 2.04 -1.55N
176 45 0 -40 51 51 9 25 4 5 259 11.56 22.2 2.02 -1.56N
176 52 30 -40 50 37 9 25 6 5 259 11.80 22.2 2.00 -1.58N
177 0 0 -40 49 23 9 25 7 5 259 12.06 22.2 1.98 -1.60N
Path coordinates are referred to WGS84 (as used by GPS).
1925 is multiple : 1.3 4.5 0.050" 225.0 : 1.3 7.5 0.50" 180.0 : 0.9 12.0 148" 32.0
Graze Path of component #2 0.08km North, and 0.0 secs. earlier cf. primary.
Graze Path of component #3 0.85km North, and 0.4 secs. later cf. primary.
Graze Path of component #4 266.85km South, and 51.4 secs. later cf. primary.
1925 = alf Vir, 0.95 +/- 0.10 V , Type ELL+BCEP, Period 4.014604 days, Phase 0.25
C A S S I N I R E G I O N G R A Z E
Librations Long -4.86 Lat +3.01
P +1.7 D -2.9
Illumination of moon 17%+
Elongation of Moon 49
Vertical Profile Scale 1.92 km/arcsec at mean distance of moon
Horizontal Scale Factor 1.9 deg/min
Limiting Magnitudes for various telescope apertures (in mm)
CA\Tdia 50 100 150 200 250 300 350
-5.6 1.5 2.6 3.0 3.2 3.4 3.4 3.5
-3.6 1.5 2.6 3.0 3.2 3.4 3.4 3.5
-1.6 1.5 2.6 3.0 3.2 3.4 3.4 3.5
0.4 3.9 5.3 6.0 6.4 6.7 6.9 7.0
2.4 4.0 5.3 6.0 6.4 6.7 6.9 7.1
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