Penumbral Eclipse
Taken by Helio de Carvalho Vital on July 5, 2020 @
Rio de Janeiro, Brazil
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Details:
The time-lapse video compares two images, one taken at 3:07 UTC, a few seconds before the beginning of the penumbral eclipse, with another, taken at 04:30 UTC (greatest eclipse). It makes the subtle darkening of the northern limb of the Moon (not visible to the naked eye) become easily noticeable. The figure included, made using the Cartes du Ciel freeware, shows the position of the penumbra across the Moon at mid-eclipse. In order to get an idea of how much darker the Moon became, I devised a simple way to measure it. I held a digital lux meter in front of the 26-mm eyepiece of my C8 (see photo) after centering the focused eclipsed moon in the field of view (occupying some 90% of it). Then I recorded 17 brightness readings, starting at mid-eclipse until the end of the eclipse. Consistently, the measured light flux went up from 91 to 95 lux, while the altitude of the Moon lowered from 68° to 50°. The measurements (95/91) indicated a difference in brightness of only 0.035 magnitude. In addition, accounting for a 0.07-magnitude correction due to differential atmospheric extinction, one finds that at greatest eclipse the full Moon had lost a mere 0.1 magnitude, amazingly. A thought experiment helps us understand why. The magnitude of the eclipse being 0.35, tells us that approximately 70% of the lunar surface remained unobscured. So it did not lose any brightness at all, while the eclipsed area underwent solar illumination losses ranging from 0 to 35% (because the diameter of the Moon approximately matches the width of the penumbral annulus where the solar obscuration goes from 0 to 100%). Then estimating as approximately 25% the mean brightness loss in the eclipsed area and remembering it only occupies 30% of the Moon`s surface leads to 0.25x0.30 = 0.075 or 7.5% loss in brightness that corresponds to approximately 0.1 magnitude, as we wanted to demonstrate. Indeed a very small difference in brightness.
The photos were taken by using the "Moon mode" of the Nikon CoolPix P900 camera. Photoscape freeware was used to produce the animation.
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