You do realize you’re going to Thulcandra for that, right?! 
Okay, here’s a question for the real astronomy buffs, which I cannot figure out how to answer:
Given the existing ranges of stars: big bright A stars, small cool M stars, distended red giants, etc., if you have a planet in the “habitable zone” (temperatures -10 to 35 C, liquid water, normal terrestrial atmosphere, etc.) of a stellar system, what is the maximum size of visual disc that its star would have? Could you ever get a star that’s 30 degrees wide in apparent visual diameter from a planet you could theoretically see it from in shirtsleeves? (I realize a Mercury-equivalent close enough to its star to give that effect is possible; what I’m talking about is whether you could have a planet with “Earthlike conditions” in an orbit that would give a large visual diameter in terms of degrees of arc.
I think this is a question that’s been shot back and forth, with no clear answer. Probably the most likely star would actually be a small, dim red dwarf, with a planet being very close to the star to stay in its ecosphere, which would be much closer to the star because the heat output is less. An earth-normal atmosphere would probably be able to screen out any dangerous longer-wave radiation that a red star would be more likely to give off, and chlorophyll would do just fine in strongly red light (maybe better than in yellow light, because it reflects green light back,) so plants could evolve and create an oxygenated atmosphere.
The big problem with this scenario, AFAIK, was that for a planet to be that close to a large gravitational source, it would probably get tidally locked, with one side facing the sun and one side away, which would have problems with creating a shirtsleeve environment. (You could back it off a bit, so that the daylight side is shirtsleeve and the night side is cold, but to never have night probably doesn’t count as ‘earthlike conditions’.)
Asimov did a variant on this scenario in ‘nemesis’, making his earthlike planet and earthlike sattelite, tidally locked to a gas giant, and therefore having day and night as it orbited the giant world. Not sure if there was anything important he overlooked there.
And I’m not sure how big the red dwarf star would appear offhand, sorry.
You want your planet to get (approximately) the same amount of radiant energy as the Earth (hence putting it in the habitable zone), but you want the star to have a larger angular size. That means you need for the star to be cooler. The size of the star wouldn’t actually matter here; all that matters is the temperature (which determines color) and the apparent size. So ten stellar radii (say) from a red giant would be the same as ten stellar radii from a red dwarf (assuming both have the same temperature, which is quite possible). Power received will be proportional to the square of the angular radius and to the fourth power of the temperature, so if you had a star with four times the apparent size, it would have to have half the temperature. The coolest stars do in fact have a temperature about half that of the Sun, so an angular diameter of two degrees would be about the limit.
Tidal locking would be a problem if you just had a single planet orbiting a red dwarf; as Asimov pointed out, you could get around this by making your inhabited world a satellite in a circular orbit around some other world (the circular orbit is necessary so you don’t get tidal heating, like Io does). Another way around this would be to make your star a red giant (as mentioned above, size doesn’t matter here, just angular size). Since it would have the same angular size, but much lower density, its tides would be weaker in the same proportion. Then, though, the problem is that your world wouldn’t stay habitable for long, since the red giant will go supernova or planetary nebula in fairly short order.
Finally, you’d probably need a different mechanism for photosynthesis, since Earthy plants need ultraviolet, which will be greatly suppressed in a cooler star. In general (for plants and machines), light from cooler objects is harder to use efficiently than light from hotter objects (this is a direct consequence of the laws of thermodynamics). You would get more long-wave radiation, but there is no such thing as dangerous long-wave radiation (microwaves do damage by virtue of heating, and the total heating is going to be the same for this star as for the Sun).
Let’s pick a red dwarf and see:
Here’s a habitable zone plot
UV Ceti (L 726-8 B) is a red dwarf with "10 percent of Sol’s mass (Geyer et al, 1988; and RECONS estimate), 14 percent of its diameter (Johnson and Wright, 1983, page 649), and less than 4/100,000th of its luminosity "(from here).
It’s a flare star, but we can ignore that for the purpose of making an estimate.
The system’s habitable zone extends from 0,04 - 0,07 AU (which is in rough agreement with the HZ plot in the first link)
L 726-8 B’s diameter is 1,391,980 km X 0.14 = 194877 km
The habitable zone stars at 5,983,920 km from the star.
Sin[sup]-1[/sup] of 194877/5,983,920 = 1.8° -nowhere near 30 degrees.
That can’t be right. The apparent diameter of the Moon in Earth’s sky is about 30 arc minutes, and one-quarter the Moon’s apparent diameter is clearly well above the threshold between “disk” and “point of light”.
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Not until they get old beardie out of his tomb. 
Alarmingly, there is now an organisation in the UK called N.I.C.E. :eek:
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I don’t think anyone’s posted a link to this NASA simulator
You can set it to show you the view from various places in the solar system to most others.
Well, you could try viewingthis clip of Voyager aroound Saturn.
http://www.solarviews.com/cap/sat/vsaturn1.htm
Watch out for the bright looking star with the lens flare.
While it gives a good idea of what you could expect, this animation uses computer generated images and not actual Voyager ones. Amongst other issues, both Voyager flybys were at the start of a northern hemisphere summer on Saturn.
Voyager 2’s trajectory past the planet was arranged so that various things passed in front of the Sun, but I don’t think I’ve ever seen any photos from any of these occultations and the cameras may even have been pointed elsewhere during them.
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As long as you don’t end up letting the Doper philologists hold you for Ransom!
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Buck that for a game of soldiers!
(Not perfect, but I’m doing my best here.)
Btw, I think “Neruval” is Uranus, from context, but the word’s hapax legomenon in “Perelandra” so I ain’t wholly sure.
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The red dwarf I used above has a surface temperature of 2800°K. By Wien’s law for black bodies ( [symbol]l[/symbol][sub]max (nm)[/sub] = 2898000/T (°K) ) its radiation peaks at 1035 nm. This isn’t as bad for plants as you might think, as photosynthetic response has a peak in the red end of the spectra (650 nm), as well the in the blue (450 nm).
With the right ancillary pigments, some bacteria “efficiently use infrared light for photosynthesis. Infrared is light with wavelengths above 700 nm that cannot be seen by the human eye; some bacterial species can use infrared light with wavelengths of up to 1000 nm.”
Oh, wow, that’s great! It’s just perfect. Thank you!
Hm. IANAB, but would it be fair to say that those bacteria use a different (at least in the details) mechanism for photosynthesis than do true plants? I have no problem with supposing that life (of some sort) could live on a planet around an M star, but I would be very surprised if an azelea or maple could handle the different spectrum.
In any event, thank you for the information!
You are right. I have had that clip and many others from that site sitting on my PC for ages. I had always thought that this particular one was composed of voyager images. I never checked.
This site has both real footage and computer animations amongst its clips.
Sure, bacterial photosynthesis won’t necessarily generate oxygen for example. However the primary light absorbing pigments, chlorophylls, are quite similar in plants and bacteria. The wiki link doesn’t say much about bacterichlorophylls, but bacteriochlorophyll b has an extra acetyl group tacked on to the basic chlorophyll b structure, and ring 2 (top right in the illustration) is reduced. This shifts the absorbance peak to 950-1050 nm.
Your azelea or maple would die under an M type star, as their short wavelength, O[sub]2[/sub] producing system wouldn’t work, but earthly photosynthic bacteria could well thrive. Could higher plants evolve from that chemistry, could they evolve a long wavelength system that produces O[sub]2[/sub]? I don’t know, but I wouldn’t be terribly surprised.