Indoor Illumination During Solar Eclipse
Taken by Helio C. Vital on February 26, 2017 @ Rio de Janeiro, Brazil
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Maximum magnitude and obscuration during the solar eclipse today were predicted as 0.535 and 0.430 respectively, for observers in Rio de Janeiro and would occur at 14:41 UTC. However, a multilayered blanket of clouds completely covered Rio`s sky throughout the eclipse. Then under such circumstances, the only thing left to do would be to report a negative observation, right? Maybe not. Since the Sun is our primary source of indoor illumination, due to multiple scattering of its light, then if one assumes the cloud cover not to be greatly heterogeneous, we can come up with a very simple way to measure coarse changes in indoor illumination. Thus, during the second half of today`s solar eclipse, I used an android application called Lux Light Meter to measure the intensity of light in my living room. I had placed my cell phone one meter away from a window, facing upwards. The application accessed the CT 406 light sensor of the camera and provided 1.0-lux resolution data. Starting 15 minutes before maximum eclipse, readings were obtained (5/min to yield a mean value of light intensity each minute) so that 105 data points had been recorded at the end of the eclipse. The data were then smoothed by averaging 15-minute periods in order to reduce short-term fluctuations due to nonuniformities in clouds. The results were then plotted in the graph shown, in which the data points were fitted to a quadratic polynomial (R²=0.997). About 53% of the expected increase in illumination, equal to 23%, could be directly measured (starting as 0.77 at maximum eclipse to 1.00 at the end) compared to the 43% maximum obscuration. However, correcting for the decrease in the Sun`s altitude [delta cos(z)=1.04] (with a very small correction due to differential atmospheric attenuation neglected for the sake of simplification) would then account for a 28% rise in indoor illumination, in better agreement. Possible reasons for the remaining discrepancy could be: a possible increase in cloud thickness during the second half of the eclipse, data smoothing that tends to flatten the figures at the limits of the set, and the limited sensitivity of the light sensor and application used.
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Excelente esclarecimento!
Posted by marcia cris 2017-02-26 20:30:28
25 thumbs up
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