Understanding REA`s Observations of the

Total Lunar Eclipse of February 20-21, 2008

 

 

Did the weather help? Unfortunately, not. A huge weather system prevailing in Southeastern and central regions of Brazil caused about 4 out of 5 potential observers to be clouded out during the eclipse. Consequently, only 47 limb and mid-crater timings; 6 Danjon numbers and 2 sets of magnitude estimates of the Moon have been reported.

 

Was the eclipse as bright as expected? Yes, it was as bright as possible considering its umbral magnitude (1.11) and an aerosol-free stratosphere. Only eclipses with magnitudes closer to one could be even brighter, when the Moon would possibly fade to a magnitude around -4. See what it may have been like as seen from the Moon.

 

How bright was it? It was fairly bright, as REA`s estimates of Danjon Number and magnitude estimates (averaging 3.00.1 and -2.80.3, respectively) indicate, with conspicuous orange and red colorations being reported.

 

How do such results compare to REA`s predictions? Based on correlations derived from dozens of previous eclipses, the author had estimated that the Danjon Number and the magnitude of the Moon at mid-totality would be respectively, 3.00.4 and -2.60.4, thus closely matching the observed values.

 

Any trace of volcanic aerosols? No. The good agreement between predicted and observed eclipse brightness this time is an indication that the stratosphere is as free of volcanic aerosols as it can be. An eclipse significantly darker than predicted would indicate the probable influence of a recent major volcanic eruption.

 

What could be concluded from REA`s crater timings? Based on 47 timings, the mean observed radius of the umbra was (1.890.06)% larger than predicted (considering a 1/298 oblateness for the umbra) with no significant difference between immersions and emersions. That figure corresponds to a (1.370.04)% increase in the Moon`s parallax (equivalent to an altitude of 873 km). Analysis of last year`s March 3-4 eclipse yielded statistically similar results.

 

Could the analysis of REA`s crater timings provide any clue about oblateness? Since most immersions occurred at low or intermediate umbral angles while emersions occurred at high ones, different values of enlargement could result from the use of inadequate values for the oblateness in the calculations. As expected, the umbral enlargement for emersions were much more sensitive to any change in the oblateness of the umbra used in the calculations than the ones for immersions. In addition no significant asymmetry was noticed and the observed times seemed to fit nicely the ones calculated with basis on a 1/298 oblateness. However, this very small set of crater timings is insufficient to provide any significant conclusion about oblateness.

LUNISSOLAR

 

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