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
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 3.0±0.1 and -2.8±0.3,
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, 3.0±0.4 and -2.6±0.4, 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 (1.89±0.06)% larger
than predicted (considering a 1/298 oblateness for
the umbra) with no significant difference between immersions and emersions.
That figure corresponds to a (1.37±0.04)% increase in
the Moon`s parallax (equivalent to an altitude
of 87±
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.