I have read in several books that the sun subtends a angular diameter of about 0.5 degrees. Or about the size of a quarter held at arms length. My question has always been how big would that look from another planet? Given that Jupiter is about 5 times further away would the sun be about 1/5th the diameter on Jupiter? How about Neptune at 48 times greater distance. Other than brightness, which could be quite bright, it would look like any other star would it not?

David Davis
Toledo, OR 97391

Don't worry, I start another project in a couple of weeks and I will be off line again, so the questions will stop.

I've also read that the Sun is 400X the diameter of the Moon and is 400X farther away, explaining why the moon so neatly covers the sun during a total solar eclipse. So, it would seem logical that if Jupiter is 5X farther out than Earth, our Sun would be 5X smaller when observed from that vantage point.

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I'm not a scientist, but I play one on the internet.
http://www.rrac.org

Yes
No. First, Neptune is not 48 times away, only 30. At 1/30 the angular diameter Sun would be a dot, but an impossibly bright one. There are no comparable examples in everyday life, because nothing we see around -- not the welding torch, not the halogen bulb, -- have the same surface brightness as the Sun. Look at halogen bulb from far enough that it looks like a point, and it will not blind you. Look at Sun from Neptune's distance, and it will burn a hole in your retina -- not right away of course, but if you stare at it long enough. You could easily read newspaper print by that light, and a point source providing that much illumination simply does not exist on Earth outside of ohysics labs.

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Fiction has to be plausible. Reality is under no such constraint.

I checked the light levels with an exposure meter. Typical household indoor lighting is about 1/1,000 the level of sunlight, and as such is comparable to what we would get at Neptune.

By the way, a quarter at arm's length is way too big. A pea is more like it.

Hornblower's right. You can obscure the Moon (and thus also the Sun) with the tip of your little finger at arm's length.

Fred

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Ilya's got it right...I'm guessing a "safe" distance to view the Sun from is hundreds of AUs away, and it wouldn't look similar to the other bright stars in the night sky until--incidentally--one were at the distance of the nearest stars.

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"Call me old-fashioned, but I think fire is magic. And it scares me a lot."

--The State

If you could stand on Neptune with a magnifying glass, and if you had an ant, could you fry the ant with the magnifying glass?

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Arthur C. Clarke

The Brain Science Podcast

You must have short arms...

If the glass were 30 times bigger than the one that works on Earth.

One thing I've wondered about in this regard is the blink reflex. It's nicely tuned to protect our eyes from damage if we happen to glance towards the sun. It doesn't protect our eyes fast enough if we look at a brighter light source, as seems to have been demonstrated by people who were looking directly at the atomic bomb flashes over Hiroshima and Nagasaki.

Is the blink triggered by integrated brightness, or by surface brightness? If the former, then one can imagine that, out around Jupiter or Neptune, it would be possible to burn your retina without ever registering the danger: the solar disc would still have a surface brightness sufficient to damage your retina, but would not be bright enough overall to make you blink.

Grant Hutchison

That does seem possible, I don't know. At least we can say that surface brightness is only relevant on scales larger than the resolution of the eye, which is why starlight does not hurt our eyes even if we stare at it, and it doesn't make us blink. If we consider unresolved light sources, then all the light is focused to the fewest possible rods and cones, and integrated brightness will always control the damage. Then as we imagine the star being brought closer until the source size becomes resolvable, from that point on it is the surface brightness that controls damage-- and we always know in that regime that sunlight is going to be damaging. I believe the eye's resolution is about 20 times smaller than the linear size of the Sun, so the area it can resolve is about 400 times smaller. That also means that the distance to Neptune is close to the worst-case scenario for starlight doing damage with minimum integrated brightness. Neptune astronauts might need to be told not to look at the Sun...

I am willing to bet anything that by the time some descendant of plains apes orbits Neptune, he or she will be genetically modified and/or cyborged to survive in vacuum, withstand radiation, and spend years in zero-g with no adverse effects. Protecting eyes from too-bright light will be a no-brainer.

In case it is not clear, I am entirely serious. The way biotechnology is progressing, within 30 years putting a large portion of life-support system INSIDE the astronaut will be an obvious development.

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Fiction has to be plausible. Reality is under no such constraint.

As with almost all the futurism I see, there's probably a pretty good chance you are right-- but the timescale is always radically accelerated. 300 years I might believe-- 30, no way (unless you just mean it will be considered a clearly good objective by then, that's reasonable). And I'll be around to see that I was right, except I won't remember by then, not having a cyborg-enhanced memory just yet.

Yes, that's what I meant -- within 30 years someone will start working on this.

I do not expect people to walk on Mars, let alone Neptune, until about 2100.

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Why do you not expect any people to walk on Mars until about 2100? I thought the US is seriously considering a program of manned exploration, first back to the Moon and then to Mars, with the Mars landing planned for somewhere in the next 30 years.

I don't think it's possible to walk on the surface of Neptune since as a gas giant, or more accurately, ice giant, Neptune does not likely contain any surface. Even if it does, a person would likely be crushed before reaching that surface.

Obviously -- it was a figure of speech.

As for why I do not expect people to walk on Mars until 2100, that requires a long answer. I will post when I have time.

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Fiction has to be plausible. Reality is under no such constraint.

"Considering", yes. "Seriously"... don't count on it.

There are several reasons for my pessimism. The biggest one is this: NASA can not admit that space is dangerous. Normally in any dangerous activity, be it soldiering, firefighting, or test-flying airplanes, the people in charge decide what level of risk is acceptable, and plan their budgets, training, and operations accordingly. The lower is acceptable risk, the less operation can be carried out on a given budget, and vice versa. The risk level decisions are almost never publicized – on your local fire department’s website you won’t find "we expect X deaths and Y injuries over next decade", - but you can be sure fire chief has that information, and brings it up at the next municipal budget hearing. And both fire chief and city council know that the only way to bring X (let alone Y) to zero is not fight fires at all. So fire departments balance expected deaths, expected number of fires and available money, and when someone dies they grieve, do their best to learn from the experience, and carry on.

The quandary of NASA’s Office of Manned Spaceflight is that it is too much in the public eye, yet does not have a clearly defined purpose. A city can not live without a fire department; nothing drastic will happen to USA if Office of Manned Spaceflight closed tomorrow. Mike Griffin knows that space is dangerous and every once in a while people will die – but he wouldn’t last a week if he went before Congress and said "This mission architecture cost X dollars, has Y percent chance of landing on Mars on schedule, and Z percent chance of killing one or more astronauts. Double the X, and Y will increase such and such, and Z will decrease such and such." Even though it would be the truth. Far too many people who for whatever reasons do not want a Mars mission (or even just do not care about it) would seize on Griffin’s words and demand to know "Why are we risking astronauts lives?" Which, BTW, is as legitimate a question as "Why are we risking firefighters lives?" or "Why are we risking test pilots lives?" The difference is that the latter two have clear, generally accepted answers, and the former one does not.

Hence we get slogans such as "Safety first!" and "Failure is not an option", which sound good, but really do not make much sense. If safety really is your first goal, you should not fly at all. If you do fly but claim perfect safety, you are perpetuating a fraud. Without a fixed, admitted level of acceptable risk NASA is forced to minimize risks endlessly – which causes delays and cost overruns, and never ends, and always fails sooner or later. And when it fails (Challenger, Columbia) there is hand wringing, and Congressional investigations, and design changes, and projects put on hold, and ultimately nothing changes because the fundamental philosophy is fraudulent.

So I expect VSE, or whatever Mars mission, to get endlessly redesigned and delayed in the name of safety, because you can never get safe enough without the honest quantitative definition of "enough". Which ain’t gonna happen.

While all this drags on, two other developments will continue. One, improvements in robotics will keep giving more ammunition to proponents of robotic science (who can and do apply realistic risk analysis). Two, private spaceflight will strip NASA astronauts from what’s left of their heroic aura. Both developments will make manned Mars trip harder and harder to justify as time goes by – especially when (as I expect will happen) private operators will begin selling seats at competitive prices and no red tape to researches who need manned presence in space. This will undermine "No Buck Rogers, no bucks" argument of government-sponsored manned spaceflight.

So I expect NASA manned Mars mission to die eventually burdened by all these difficulties. First person to walk on Mars will not be paid for by US (or any other) government. He or she will do so when Mars trip is within scope of private companies. And for all my hopes on private spaceflight, I do not expect it develop fast enough to make a manned Mars trip profitable (big dfference between possible and profitable) before 2100 or so.

Unless either a) life is unambiguously discovered on Mars*, or b) military-related reason to go there comes up. Then all bets are off.

* In fact, if life is unambiguously discovered on Europa, I would expect a manned expedition to Jupiter BEFORE one to Mars.

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^
Hammer. Nail. Head.

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"Call me old-fashioned, but I think fire is magic. And it scares me a lot."

--The State

Bingo. This is why I actually think a landing on Mars by 2100 is very optimistic. It might happen if people feel it is ethically acceptable to send someone with a terminal condition with only a few years at most to live anyway-- and there's no plan to bring them back. A round trip is probably a factor of 10 more difficult and more expensive, and there just isn't a good enough reason for it. Scale it to the Moon-- how much more will it cost, how much more dangerous is it, how much more must the astronauts sacrifice (even in the best case scenario)-- and compare that to how much more impact landing on Mars versus the Moon would have. It just doesn't compute, in my opinion-- I see maybe a factor of 2 in accomplishment but a factor of 10 to 100 in cost, danger, and sacrifice. I predict that it will always seem better to put virtual reality modules on Mars that bring the experience back to humans who never leave Earth. Thus, my guess is we will essentially never put humans on Mars, unless human technological advancement survives a host of more pressing problems in the very long term (as it is with history, though people tend to overlook that by compressing historical timelines).
Sober words, and true.

Yes, that is going to be very significant, especially as the "virtual reality" capability of the robots improves.This might be the way it will actually happen though-- because private spaceflight could put someone up who either doesn't mind dying in the effort (private individuals are always doing that), or doesn't expect to come back. NASA would have a much harder time with either of those issues. Recall Lindbergh's flight-- that was so poorly planned that he really should have died, and no organization like NASA could have ever attempted that, yet he did succeed and the rest is history.

Hopefully sooner than later, in my view. Good money after bad.
I agree, except I think it will be like Lindbergh-- a persuasive individual who is devoted to the idea of going at all costs and who can galvanize wealthy individuals to back such a mission, in the absence of a profit motive. But like Lindbergh's precursor's, the first few will probably die in the effort.