CALWELL LUNAR OBSERVATORYCALWELL LUNAR OBSERVATORYIntroduction
Video records of the complete 1996 April 3-4 total lunar eclipse were received from Rui Marques in Caxias-Portugal observing with a Takumar 400 mm f/5.6 telescope reduced to f/16, and Joaquim Garcia also in Portugal observing with a 270 mm objective closed down to f/32.
These records have now been analysed using image analysis techniques to determine the change in geometry of the umbra during the eclipse. Two of the captured frames are shown which generally illustrate the image analysis technique used.
An illustration showing the imaged Moon during the eclipse from the video by Marques. The two best fit circles calibrate the semi-diameter of the moon and measure the umbral semi-diameter.
A second illustration showing the imaged moon during the eclipse from the video by Garcia. The two best fit circles calibrate the semi-diameter of the Moon and measure the umbral semi-diameter.
A third video record has been recently received from Jean Bourgeois at Reux Astronomical Station, Le Pavillan de Reux, Ciney in Belgium. His observation commenced at 22h 45m UT and ended at 02h 10m. Details of his observing site, instrumentation (Newtonian 254 mm f/4.7 and CCD Phillips camera NXA/1011/03), and excellent timing were included on his video. Unfortunately the image focus was poor, but good estimates of three of the primary contacts were taken from his record. Second contact, was clearly seen at 23h 26m 45s, third contact at 0h 52m 45s and a subjective fourth contact at 1h 58m 25s. All three times were as predicted by the author's Improved Lunar Eclipse Ephemerides (ILEE). Even though these contact times were only +8.6, -5.5 and -8.5 seconds different to the Geoid predictions, but confidence in the ILEE was evident.
The umbral measurements are analysed in the report below:
Introduction
Video records of lunar eclipses have been measured by image analyst techniques to determine the geometry of the umbra. As these images are topocentric, a procedure has been developed to compute the expected topocentric umbral size for the observing site from geocentric data. Such values have been compared with the video umbral measures to detect change in the size of the umbra during the recent total lunar eclipse of 1996 April 3-4 as well as a re-evaluation of the data from the partial lunar eclipse of 1995 April 15.
Procedure
In Figure 1 below the moon of radius Sc is shown in eclipse by the umbra of radius F2 with the measured topocentric shadow shown as radius Fi. By applying the cosine rule to triangle OMC, a computer program has been prepared to find the expected topocentric semi-diameter (Fi) of the umbra at the time of each captured video frame recorded at each observer's site, from:
Measurement Comparison
The total lunar eclipse of 1996 April 3-4 was observed by two astronomers in Portugal and one in Belgium, with each observer providing a video recording for analysis. Captured frames from these video records have been digitised and measured using commercial software, Image Analyst Version 8.0, running on a Macintosh IIci computer.
The analysis of the measurements of captured frames before and after totality taken from the video record of the astronomer Marques, are shown in Figures 3 and 4 below. Here the geocentric semi-diameter of the umbra (F2) and the computed topocentric semi-diameter of the umbra are compared with the video frame measurements:
Figure 3: Comparison of geocentric, computed topocentric and umbral measurement from first to second contact after the video record by Marques, Portugal. There is a slight spread between the image measurements and the computed topocentric semi-diameter. However, the departure of the measured semi-diameter from the expected topocentric umbral semi-diameter, shows a decrease in size during the eclipse of around 0.15 of a degree.
Figure 4: Comparison of geocentric, computed topocentric and umbral measurement from third to fourth contact after the video record by Marques, Portugal. There is a slight spread in the image measurements and the computed topocentric semi-diameter. The departure of the measured semi-diameter from the expected umbral semi-diameter, shows an increase in size during the eclipse of around 0.1 of a degree.
Partial Lunar Eclipse of 1995 April 15
Several observers in Australia provided video records of the partial lunar eclipse of 1995 April 15. These are re-analysed in a similar manner to the above, with all measured umbral radii (Ru) from the image frames plotted with their topocentric corrected values (Ft), found from other measures of each image (CH, AH, etc as shown in Figure 1) for comparison with the geocentric theoretical value (F2) and the computed topocentric values (Fi). The results from Bennett and Sture for this eclipse are shown in Figures 5 and 6 below:
Figure 5: Comparison of geocentric, topocentric and umbral measurements (Ru) which are corrected to topocentric (Ft) from first to fourth contact from the video record by John Bennett. The scattered measured values, when corrected to topocentric (Ft) from other measures of the image, are now much nearer to the computed topocentric semi-diameter (Fi). There is a general decrease in umbral size (Fi - Ft) around 0.25 of a degree.
Figure 6: Comparison of geocentric, topocentric and umbral measurements (Ru) which are corrected to topocentric (Ft), from first to fourth contact from the video record by Brian Sture and Harry Moller. The scattered measured values, when corrected to topocentric (Ft) from other measures of the image, are now much nearer to the computed topocentric semi-diameter (Fi). Again there appears to be a decrease in umbral size (Fi - Ft) around 0.2 of a degree.
Change in Umbral Size
By comparing the departure of the measured semi-diameters with the computed topocentric values, dynamic change in umbral geometry is detected. The geometric departures for one observer in Portugal for 1996 April 3-4 are shown in Figures 3 and 4 above, where a nominal decrease before totality and a similar increase after, are both near 0.1 of a degree. This suggests a small but dynamic change in umbral geometry occurred during this lunar eclipse due to variation in transparency and/or cloud cover on the Earth.
A similar departure of umbral measurements from the topocentric umbral semi-diameter (Fi) was found from the video record by Garcia, whom also observed in Portugal. Even though there is considerable spread, the nominal increase before totality is near that shown in Figure 3. This observation also tentatively supports that an increase in umbral geometry occurred before totality during this lunar eclipse.
Other similar but scattered results for the partial lunar eclipse of 1995 April 15 from the observers Bennett and Sture, Australia also showed a general decrease in umbral semi-diameter of around 0.2 of a degree, as shown in Figures 5 and 6 above.
Conclusions
The puzzling result of consistently low values of measured umbral semi-diameters taken from video records near first and fourth contact has now been resolved. Computation of the expected topocentric value at the time of each image frame has been included in the analysis presented here. Most umbral measurements, and those corrected to topocentric values (Ft), are much closer to the expected topocentric semi-diameter (Fi). The previously large difference from the geocentric values of F2 are now adequately explained.
In addition, even though there is some scatter, the 1995 April 15 partial lunar eclipse measurements corrected to topocentric values (Ft) from other measures of the image, now compare well with the computed topocentric values (Fi) as shown in Figures 5 and 6. The dilemma expressed with the large departures from the theoretical (Geiod) value of F2 in earlier publications (Soulsby 1995 and 1994), are now adequately resolved.
There appears to be a measurable, but slow dynamic change in umbral semi-diameter during both lunar eclipses, although the scatter suggests that improved accuracy in image measurement should be persued.
One problem with the image analysis technique is associated with the high magnification used by most observers. A preferred value (around 84 X) is that which provides a complete image of the moon during all stages of the eclipse, as this greatly assists image calibration and measurement. Most images tend to be overexposed, due in part to both high magnification and aperture. An improvement in image contrast (by reducing the effective aperture, but not by filtering) would also assist the measurement analysis of future lunar eclipses.
The video imaging and analysis technique requires improvement, but it is the only known technique where change in the upper atmosphere of the Earth can be detected during lunar eclipses. By improved refinement, it may also be possible to measure any dynamic change umbral oblateness as well as the change in size as presented here.
Acknowledgments
The author would like to express his thanks to the astronomers whom have contributed to this research by providing video records. In particularly to overseas observers whom provide records of lunar eclipses not visible in Australia.
References
Soulsby, B.W., 1995, Analysis of the 1995 April 15 partial lunar eclipse, Aust. J. Astr., 6(2):33-52.
Soulsby, B.W., 1994, Change during lunar eclipses, Proc. 16th Natn. Aust. Conv. amat. Astrer., Canberra, pp 101-114.
Meeus, J., 1991. Astronomical Algorithms, pp 263. Willmann-Bell Inc. 429 pp.
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To send e-mail to the author: minnah@netspeed.com.au