OBSERVATIONS OF THE JUNE 15, 2011 TOTAL LUNAR ECLIPSE:
A PRELIMINARY REPORT
By Helio C. Vital
OVERVIEW
As we had anticipated in our observation project, monitoring totality was a challenge for Brazilian observers. Dense clouds prevented most of them from witnessing the Moon rising during totality and even those who had clear skies experienced a great deal of difficulty in locating the extremely pale moon hanging only a few degrees over the eastern horizon. However, a small group managed to get a glimpse of it despite very poor contrast and challenging observing conditions.
VISIBILITY OF THE MOON
The earliest account of a photographic record of the totally eclipsed moon was reported by Alexandre Amorim (in Florianópolis). It was made 20 minutes after moonrise at 20:44 UT. It must be highlighted that the full Moon was then about 13 magnitudes darker, equivalent to some astonishing 155,000 times fainter than usual.
REPORTS AND IMAGES OF THE ECLIPSED MOON
Some REA`s observers who have shared their observations with us were Raquel Shida (in Munich, Germany); Giancarlo Nappi (in Belo Horizonte); Helio Vital (in Rio de Janeiro); Antonio Campos and others from CEAMIG (in Belo Horizonte).
OTHER REPORTS (E-Mailed to REANET)
Additional reports have been posted by Tasso Napoleão (São Paulo), Antonio Coelho (CASB, in Brasília), Guilherme Grassmann (Curitiba), Nelson Falsarella (São José do Rio Preto), Frederico Quintão (CEAMIG, Belo Horizonte), Willian Souza and Moshe Bain (São Paulo).
ECLIPSE BRIGHTNESS
Since the Moon would probe the center of Earth`s shadow, a dark eclipse was expected. How dark? That would depend on the level of volcanic aerosols in the stratosphere along Earth`s limb. According to Keen, some recent events have launched ashes to the top of the troposphere, but has there been a significant injection into the stratosphere? If so, has such material had sufficient time to spread to the point that it could considerably darken the umbra? Estimating the magnitude of the Moon at mid-totality would possibly enable us to answer that. Unfortunately, the Moon had not yet become visible to Brazilian observers at mid-eclipse. In spite of that, four very rough estimates of the Danjon Number (L) were reported by experienced observers. They were made with basis on color distributions and the visibility of features across the disc of the Moon during the end of totality and in the beginning of the second partial phase. Table 1 provides estimates of the Danjon Number.
Table 1. Estimates of Danjon Number
|
Observer |
Danjon Numbe Estimate (L) |
|
Antonio Rosa Campos |
1.5 |
|
Alexandre Amorim |
1.0 |
|
Antonio Coelho |
1.0 |
|
Helio de Carvalho Vital |
1.5 |
|
Mean |
1.25±0.29 |
By using the author`s correlation mag = 4.2 – 3 L + (L/2)2, one gets, from the mean value of L, the approximate magnitude of the Moon as +0.8 ± 0.7. For an aerosol-free eclipse with umbral magnitude equal to 1.7, another correlation predicts the mid-totality magnitude of the Moon as -0.1. Adding a +0.4 magnitude correction to account for some volcanic darkening (educated guess), yielded our prediction of +0.3 for the magnitude of the Moon at mid-eclipse. If confirmed, that 0.8 – (-0.1) = 0.9 magnitude difference would indicate a slight dimming of the Moon due to the presence of stratospheric aerosols. In order to be sure, magnitude estimates at mid-eclipse would probably help.
CONSIDERATIONS ABOUT THE BORDER OF THE UMBRA
It has been widely accepted for a long time now that Earth`s umbra exhibits a radius that exceeds its theoretical value by about 2%. However, the role of our atmosphere in that phenomenon is still unclear. Our feeling is that optical contrast is indeed one of the keys to understand such effect, since the observer uses it to identify the border of the umbra. However, the role of our atmosphere cannot be neglected since it refracts deeply into the shadow cone sunlight that otherwise would add to the outer layers of Earth`s shadow. As a result, it darkens the penumbra-umbra border, defined as where the light gradient is observed to be maximum. In addition, it must be considered that our eyes are limited, not capable of accurately detecting such frontier because it is too dark, below our visible threshold. As a result, observers actually notice it shifted by about 2% into the lighter penumbral shadow. REA`s thousands of lunar eclipse timings, gathered since 1989, have enabled us to determine a mean umbral enlargement factor equal to 1.89±0.14 %. That figure corresponds to an increase of 1.4% in Earth`s radius, equivalent to a height of 89 km. On the other hand, the argument that air layers above the height of our visible atmosphere (~15 km) cannot affect the umbra seriously contradicts observational findings, since Keen has proved that there is indeed a strong correlation between global levels of volcanic aerosols in the stratosphere and the optical density of the umbra. We are then left with an intriguing idea. What if the approximately 2% enlargement in the visible radius of the umbra, primarily determined by an optical illusion associated to the shadow cast by the solid geoid, is also being affected, to a lesser extent, by changes produced by the atmosphere in the amount of light that reaches the penumbra-umbra border and that could possibly explain significant shifts observed in its position from one eclipse to another?
MEASURING THE SIZE OF THE UMBRA
By timing the emersion of craters and limb, we were able to calculate the mean enlargement factor during the second partial phase of this eclipse. Emersions took place at low umbral angles so that this set of data can give us more precise information on the equatorial radius of the umbra rather than on its oblateness. Twenty-three mid-crater emersion timings have been selected, 20 of them contributed by Antonio Rosa Campos (CEAMIG), 2 by Alexandre Amorim e 1 by Helio C. Vital. Table 2 summarizes the data. The enlargement factors have been calculated relatively to the geoid (1/298.26 oblateness). All times are in UT and expressed in hh:mm:ss format.
Table 2. Mid-crater emersion timings by REA observers
|
21:mm:ss |
Feature |
%Enlarg. |
21:mm:ss |
Feature |
% Enlarg. |
|
06:24 |
Grimaldi |
2.31 |
28:15 |
Aristoteles |
1.99 |
|
07:45 |
Reiner |
2.00 |
29:03 |
Eudoxus |
1.42 |
|
08:27 |
Aristarchus |
1.30 |
29:44 |
Tycho |
1.76 |
|
11:51 |
Billy |
2.32 |
36:23 |
Menelaus |
2.03 |
|
12:59 |
Kepler |
1.95 |
36:57 |
Endymion |
2.68 |
|
18:14 |
Pytheas |
1.18 |
38:12 |
Posidonius |
2.04 |
|
19:23 |
Plato |
1.25 |
39:32 |
Dionysius |
2.19 |
|
19:51 |
Copernicus |
1.40 |
40:26 |
Plinius |
2.43 |
|
20:16 |
Pico |
1.55 |
52:13 |
Taruntius |
2.70 |
|
26:18 |
Cassini |
1.96 |
22:02:50 |
U4 |
2.10 |
|
Mean: 1.93 ± 0.09 % , equivalent to 1.42% increase in Earth`s radius (90.3 km) |
|||||
The mean enlargement factor found matches our all-time average of 1.89±0.14%, considering the uncertainties in both figures.
