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Astronomy and Cosmology
Astronomy and Astrophysics
How Can Planetary Movements Affect Time and Illumination on an Inhabited Planet?
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[QUOTE="snorkack, post: 6825104, member: 436348"] A professional source, but classic and hopefully not too hard for the results: [URL]https://articles.adsabs.harvard.edu//full/1916ApJ...43..103R/0000114.000.html[/URL] Includes phase curve of Moon to 150 degrees. Note how powerful is the opposition surge of full Moon and how dramatically crescent Moon fades. Moon 30 degrees from full shows 14/15 of the disc, yet only 1/2 of the light. At 150 degrees, the crescent is 1/15 the area of the disc, yet about 1/250 the brightness. Jupiter does NOT fade that way. True, Earth comes closer to Jupiter at opposition - if Jupiter is 5 AU from Sun on average, then it is 4 AU at opposition and 6 AU at conjunction, making Jupiter 2,25 times dimmer. But Jupiter at opposition is about 16 000 times dimmer than Moon. Which means that if Jupiter near conjunction fades to twice dimmer than at opposition, and Moon is 250 times dimmer as narrow crescent, ten Jupiter is still over 100 times dimmer than the crescent Moon. Jupiter outshines all fixed stars, but does not illuminate night sky like Moon does. Now note that for an observer on rotating Earth, Sun is above horizon half the time. So is Moon. Moon is plainly visible in blue daytime sky. It stays in place when clouds drift by. Over a month, the Moon varies to being visible no part of night (new Moon) through increasing part of evening (waxing crescent and gibbous), whole night (full Moon) to decreasing part of morning (waning moon). As the monthly average, one half of time at night is moonlight... on average, at least a small part of every night is moonlit (since moon is not exactly in conjunction) and at least a small part is not (moon is not exactly full - indeed, exact full moon is eclipsed). 3/4 of all time, Sun, Moon or both are above horizon and 1/4 time both are below. Since Moon is dim to start with and fades when not full, a night lit by a narrow crescent moon is not very bright. Now consider a planet of binary star. Like α Cen Ab. At its periapse, B comes to 11,2 AU of A. Since Ab is in habitable zone, the A-Ab distance is about 1,2 AU. Because of the stability requirement, we can find compelling reason why the A-Ab orbit should be at a low inclination to A-B orbit - like Moon is within 5 degrees of ecliptic, and Jupiter too. Therefore, α centauri B must come to opposition and to conjunction with A for Ab regularly. But unlike the case with Moon, and like the case with Jupiter, the period of oppositions and conjunctions of α centauri B must be close (not exactly equal) to α centauri A orbital period - the local year. (Which is close to 1,3 Earth years). In case of moonlight, full Moon is around 1000 times brighter than combined light of stars. Full moon used to have drastic effects on activity of people. Social events set at full moon or maybe waxing gibbous moon (You can stay out late if when it the time to get home the night is moonlit rather than just starlight+airglow!). Street lighting not lit when moonlight made it superfluous. Lunacy. Now consider α Cen Ab. At opposition B is at 11,2-1,2=10 AU. B is 2,2 times dimmer than Sun in visible light. So it is 220 times dimmer at the distance of Ab - but since full Moon is 400 000 times dimmer than Sun, it still makes 1800 times brighter than full Moon. Comparable to twilight with Sun around horizon... When B is near "full", then the whole night is going to be in "twilight" range of 50...500 lx. Which is not day - but neither is it "moonlit night", because that is in range of 0,25...0,05 lx. [/QUOTE]
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How Can Planetary Movements Affect Time and Illumination on an Inhabited Planet?
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