quinta-feira, 24 de setembro de 2015

Circunstances for the 2015 September 27-28 Total Lunar Eclipse

Helio C. Vital
REA/Brasil - Lunissolar


On September 27-28, 2015 a total lunar eclipse will occur in favorable conditions for observers in the Americas. During the event, the Moon will act as a very sensitive screen that will probe the South of Earth`s inner shadow (umbra).  The following table lists predicted times for primary (limb) contacts. The calculations (and all others in this article) were performed by using extensively tested programs developed by the author.  The value used for Delta T in the calculations was ∆T = +67.95 s.


Crater Contacts

Analyses of observed contact times can be used to determine the size of the umbra, which varies significantly from one eclipse to another and sometimes even during the eclipse itself. As craters enter the umbra, regions at the top of Earth`s mesosphere over a region at mid-southern latitudes (-42±17°), centered around longitude (60±8)°E (Indian Ocean, just East of Africa) will be probed. Likewise, as craters exit the umbra, mesospheric regions over an extense region over the equator (-6±17°), centered around longitude (150±8)°W (Pacific Ocean, West of Southern America) will be mostly casting light patterns onto the lunar screen.  By recording the time when the border of Earth`s shadow crosses the center of the most prominent features on the Moon, you can help us measure the umbra. The dynamics of periodic and random wave disturbances at the top of the mesosphere (mesopause) cause variations in the size of umbra. Based on findings from the eclipse on April 15, 2014 and also on results from analyses of images of the April 4, 2015 event, Earth`s radius was increased by 1.27% during immersions and 1.23% during emersions in the calculation of contact times. Such extrapolated figures slightly exceed the minima in our statistics of nearly 30 lunar eclipses. In addition, contact times for the most conspicuous lunar features are provided below so that you can plan your observations. The mean difference between such predictions and timings made by experienced observers has been determined as ± (0.3±0.2) min.


The visual magnitude of the totally eclipsed Moon has been seen to vary from almost invisibility +4.1 to -3.7, that would rival Venus, and is mostly dependent on how deep it goes into the umbra and also on global levels of volcanic aerosols in the stratosphere. On April 22, 2015 there was a powerful explosion of Mount Calbuco and observations made by the author of ash plumes that passed over Rio de Janeiro four days later indicated that a small (though significant) fraction of those aerosols had indeed reached the stratosphere. Thus a small but noticeable decrease in the brightness of the eclipse is expected. If the stratosphere was free of volcanic aerosols, we could use our empirical correlation, that relates the predicted visual magnitude of the Moon (m) to the greatest magnitude of the umbral eclipse (Umag) and also to the apparent semidiameter of the Moon at greatest eclipse (SD, expressed in arc minutes):

m = -1.9 + 4.3 Umag - 5 log(SD)   (Vital)

However, the eruption of Calbuco may have darkened Earth`s umbra moderately. An educated guess would be to assume a possible 1.3±0.8 magnitude decrease in the visual magnitude of the Moon at mid-totality. In order to assist you in selecting comparison stars when attempting to estimate the visual magnitude of the Moon, the following table lists our predictions for the umbral eclipse magnitude and the corresponding visual magnitude of the Moon as a function of time (UTC). Use of the perfected reverse-binoculars method is recommended.  An eclipse with prevailing color between deep red and brick red coloration and of intermediate brightness is expected with the northern  half of the Moon`s disk much darker than the southern. 

Please, send us your contact timings and visual magnitude estimates, briefly describing your observing conditions and equipment. Danjon Number (L) estimates can be made with basis on the color of the Moon at mid-eclipse and will also be welcomed. An overall value of L=2.2±0.4 (1σ) at mid-eclipse would be our prediction. You can use the Danjonmeter (a color form available in: http://www.geocities.ws/costeira1/img/20140415_073978tu.png) as a guide.


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