1547: "Solar System Questions"

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Re: 1547: "Solar System Questions"

Postby Neil_Boekend » Tue Jul 07, 2015 2:50 pm UTC

Essah wrote:
ebow wrote:
Beavertails wrote:I have what might be 2(ish) easier questions to answer.

1(ish): Why does the ISS orbit so closely? Is there an advantage to orbiting at that altitude?

2: Why don't we have an observational satellite at the Sun–Earth L3 Lagrange Point?


1: Easy(ish) to resupply.

2: (a) It would be complicated to communicate with a satellite at that location, (b) it would be tricky to acheive that orbit, (c) L3 is unstable, (d) I don't expect you'd learn anything new from that location, and (e) the inhabitants of the planet hidden there have spent a lot of money influencing space program missions to ensure we don't send anything their way.


how do you "hide" a planet... I mean, planets are big vast spheres, and there's not that many things to hide behind in space

Check the Langrangian points wiki page. L3 is always behind the sun. The sun is big. You just won't believe how mind-bogglingly big it is. You might think that the earth is big, but that's just peanuts compared to the sun. Thus a planet would be capable of hiding there for a few years. At some point, however, the inherent instability of L3 would require planetary drives to keep it on L3, or it would move along the orbit to crash into the Earth.There have been some stories around it.
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Re: 1547: "Solar System Questions"

Postby JohnTheWysard » Tue Jul 07, 2015 5:54 pm UTC

cellocgw wrote:Some crazy guy suggested null-buoyancy blimps for living in Venus' atmosphere. Why couldn't we do this on Jupiter as well?


Sort of hard to get a lighter than air blimp when the air is already mostly hydrogen. Probably need a hot air blimp and that takes continual energy.

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Re: 1547: "Solar System Questions"

Postby Pfhorrest » Tue Jul 07, 2015 6:00 pm UTC

da Doctah wrote:
RogueCynic wrote:If Mars has a face on one side, does it have an ass on the other?
No, a buffalo.

I think you mean a Buggalo:
Image
It is Mars, after all.
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Re: 1547: "Solar System Questions"

Postby iamspen » Tue Jul 07, 2015 6:17 pm UTC

Spoiler:
Forgive me, I had forgotten that when one wraps a post in a sheet of irony and make claims so ridiculous they can't possibly be believed, some on the internet take that as a challenge. If it wasn't clear enough, I wasn't actually insinuating we refer to human beings as "it." And while we're on the subject of language evolution, apparently English at one time had a pronoun, "ou," specifically for these gender-unknown situations (as in, "Ou will take my posts far more seriously than I had intended."), says the most reliable source ever


Echo244 wrote:Wooo! We now know Pluto is kind of red!


Pluto has the same colors as Pluto!

Image

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Re: 1547: "Solar System Questions"

Postby moops » Tue Jul 07, 2015 7:55 pm UTC

wayne wrote:We wouldn't be able to use buoyant city-balloons in a hydrogen atmosphere, unless we heated it significantly. Or the bubbles somehow held vacuum instead of gas. (Weird concept, "holding" vacuum...)
A helium atmosphere might be able to support a hydrogen balloon, but it would have to be huge.
The more heavier gasses, the easier it would get.
For any decent buoyant device, like a zeppelin, to fly on Jupiter or Saturn, I'd guess (because I'm too lazy to look up the info at the moment) that it would have to fly so low in the atmosphere that the pressure would be unbearable for humans.


For a system in thermal equilibrium, pressure going up means gas density goes up. Ideal gas says P/rho = constant. So, any balloon, with no higher-density objects inside it would sink until the objects "effective density" matches the gas around it. You can adjust your rigid dirigible volume arbitrarily, so you can get any gas pressure you want inside your floating fortress. It is probably better to think of a rigid ocean liner or submarine for the calculations, then afterwards you figure out how strong it has to be and what direction it needs to be strong (tension or compression) to meet your needs.

The real problem for people floating in Jupiter's atmosphere is that you still have to tolerate Jupiter's gravity, which is 2.5 Earth g. You are not orbiting the planet and weightless, you would be in the planet's inertial frame. Think of standing on an airplane. (Don't think like Archer).

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Re: 1547: "Solar System Questions"

Postby Pfhorrest » Tue Jul 07, 2015 8:19 pm UTC

moops wrote:The real problem for people floating in Jupiter's atmosphere is that you still have to tolerate Jupiter's gravity, which is 2.5 Earth g.

At what elevation is that the case? (And what do we measure elevation relative to on a gas giant anyway? The center of the planet?)
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Re: 1547: "Solar System Questions"

Postby moops » Tue Jul 07, 2015 8:44 pm UTC

Pfhorrest wrote:
moops wrote:The real problem for people floating in Jupiter's atmosphere is that you still have to tolerate Jupiter's gravity, which is 2.5 Earth g.

At what elevation is that the case? (And what do we measure elevation relative to on a gas giant anyway? The center of the planet?)


Well, the g forces you feel on Jupiter depend on where you end up "floating" in it's atmosphere. at the gaseous surface we can see you have 2.4g. As you sink into Jupiter's atmosphere there are two countering effects kicking in. The general form of gravity is F = G*m1*m2/(r^2) when you pass the outer boundary and sink your radius is dropping so F will grow as the square, but, Jupiter's effective mass is decreasing, since more of the planet is going behind you and beside you. Initially gravity will grow since most of the mass of Jupiter is below your feet. Then you would reach some maximum g force, then it would start dropping. At the center of the planet you are effectively weightless again. The formula for F looks like F should go infinite, but m1 approaches zero faster than r^2 does.


To compute the actual depth from center and what the g force is there would require calculus using some formula for density as a function of depth, Newton's Shell theorem for gravity. This rising then falling gravity effect would make buoyancy-based computation harder.

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Re: 1547: "Solar System Questions"

Postby DHT » Tue Jul 07, 2015 8:54 pm UTC

A few answers:

Q2: Current working theory, to my knowledge is: A severe meteor impact penetrated the crust and caused the denser, hot magma from the interior to flow outward. This participated in setting the center of mass off the volumetric center and generally towards the impact site's flow area. Because of tidal locking processes, this weightier region naturally came to rest facing the earth, in its minimal energy state.

Q7: Yes, the Oort cloud is real. It is the region where the gravitational pressure of the sun (inward) and the radiative pressure of the sun (outward) roughly equalize. It is a "cloud" of matter that is carried along by the sun, but doesn't fall into the solar system, proper. This debris slowly accumulates and forms comets and deposits on outer planets, occasionally these can fall further inward, but they are generally unremarkable blocks of icy dust.

Q25: Do you mean the slingshot effect? It is due to the energy associated with the relative position change in the out system, and not really a local phenomenon. If the planet was stationary, there would be no slingshot effect, and there really isn't one in relation to the planet. It is in relation to the sun and other planets that the slingshot effect is relevant. It is a consequence of the fact that there is a dynamic relationship happening: the effective positions of the space craft when it entered and existed the planet's local space are different and the spaceship gains a lot of momentum from that energy transfer (the planet loses an immeasurably small amount of momentum relative to its total momentum). A different angle creates a different energy exchange. Having an energy exchange is effectively unavoidable, any attempt to avoid any energy exchange requires a very precise angle indeed.

Q27: Theoretically, but not studied enough to be sure: Titan looses some air naturally as you would expect, but Saturn prevents that air from getting lost to the solar system as a whole. However, Saturn itself is quite buoyant and doesn't hold even it's own atmosphere particularly well, so while the air doesn't leave the Saturn system, it doesn't fall to Saturn particularly fast either. That leaves Titan as the most able body to collect the ambient small particles cast off by itself and it's outer neighbors, as well as Saturn. What we see is the current equilibrium state of these processes. This is in addition to other planetary atmosphere generating processes such as low grade volcanism. Finally the atmospheric constituents play a role: the heavier/reactive hydrocarbons are more difficult to scatter away than the components of the atmospheres we are used to considering.

Q28: Do you mean why does it stop on the inside or outside? I'll answer both, and I'll try not to be too technical: The Kuiper belt's structure appears to be a mix of the effects of several phenomena. The first and primary is that it is the part of the Oort cloud that is roughly in the rotational plane of the sun, so it feels the same heightened attractive effects of rotation seen in other frameworks (ala galactic disks, black hole accretion disks, etc.). There would be a Kuiper belt without any other factors, but it is also possible that Pluto/Charon may be a factor in shaping the Kuiper belt as well. Given their eccentric orbit, the little guys may slowly dig out a more pronounced disk then the Kuiper belt would otherwise have (I find it a bit ironic that the demoted pair may be responsible for the largest macroscopic feature of the solar system). On the other had, the formation of a eccentric object like Pluto/Charon may be inherent to the Kuiper belt's development (akin to tilted axes). Only further analysis can really tell.

Thus the Kuiper belt's outer end is it just fading into the Oort cloud, which itself simply fades into the interstellar medium. It's inner end is Neptune, which eats or scatters anything that moves too far into the solar system, preventing any coherence in the disk within about 50 AUs. In cross section it looks kinda like this (you may want to tilt your head sideways...):

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;;;;;;;/...\(*)/...\;;;;;;;;;;;;;;
;;;;;;;;;;;/........|........\;;;;;;;;;;;
;;;;;;;;;/......................\;;;;;;;;;
;;;;;;;|..........\ | /..........|;;;;;;;
;;;;;;|..........==O==..........|;;;;;;
;;;;;;;|........../ | \..........|;;;;;;;
;;;;;;;;;\....................../;;;;;;;;;
;;;;;;;;;;;\........|......../;;;;;;;;;;;
;;;;;;;;;;;;;;\.../(*)\.../;;;;;;;;;;;;;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

Where the sun is the "O" in the middle, the outer semi-colons represent the Oort cloud, the dashes its inner edge and the periods are the inner solar system; the (*) represent the Kuiper belt regions.

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Re: 1547: "Solar System Questions"

Postby Rombobjörn » Tue Jul 07, 2015 10:27 pm UTC

Mokurai wrote:There was a story including a nude Olympic hundred-meter dash in vacuum on the moon, but I don' t remember the name of the story or the author, and Google isn't helping me on this.

You may be thinking of The Hammer of God by Arthur C. Clarke, but the race mentioned there is a marathon, not just a hundred meters. The protagonist participates in the very first Olympic games on the Moon, running across Sinus Iridium in a lightweight spacesuit that is only usable at night. It is then briefly mentioned that eight years later another runner runs the marathon naked in the vacuum with his lungs filled with an oxygen-saturated liquid.

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Re: 1547: "Solar System Questions"

Postby 2A4701CC » Wed Jul 08, 2015 4:01 am UTC

Regarding Q2 on blotches/dark maria: Below is a link to a quick article about some research at Penn State discussing how, following a collision, the near side of the moon cooled differently based on the proximity with earth and the tidal lock:

earthsky.org/space/dark-side-of-the-moon-mystery-solved

roostyscoot wrote:I made an account because i read a while back a hypothesized answer for Question 2 and i wanted to know if anyone else heard it.

All the blotches are on the near side, according to my vaguely remembered source, because earth briefly had 2 moons, and the smaller one collided with the near one. tidal forces then did something and helped align the moon the way it is today.

i'm looking frantically for my source, i'll edit when i find it! i hope someone else also heard this!

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Re: 1547: "Solar System Questions"

Postby Beavertails » Wed Jul 08, 2015 7:21 pm UTC

solune wrote:
Beavertails wrote:1(ish): Why does the ISS orbit so closely? Is there an advantage to orbiting at that altitude?


That's the maximum altitude from which a soyuz spacecraft may reenter the atmosphere with a standard deorbit procedure. Since even when the shuttle was available, the soyuz was the prefered escape vehicle, it's the limiting factor when they reboost the orbit.


Thank you! That is an entirely reasonable explanation.
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Re: 1547: "Solar System Questions"

Postby exoren22 » Wed Jul 08, 2015 9:14 pm UTC

Essah wrote:
ebow wrote:
Beavertails wrote:I have what might be 2(ish) easier questions to answer.

1(ish): Why does the ISS orbit so closely? Is there an advantage to orbiting at that altitude?

2: Why don't we have an observational satellite at the Sun–Earth L3 Lagrange Point?


1: Easy(ish) to resupply.

2: (a) It would be complicated to communicate with a satellite at that location, (b) it would be tricky to acheive that orbit, (c) L3 is unstable, (d) I don't expect you'd learn anything new from that location, and (e) the inhabitants of the planet hidden there have spent a lot of money influencing space program missions to ensure we don't send anything their way.


how do you "hide" a planet... I mean, planets are big vast spheres, and there's not that many things to hide behind in space


The L3 Lagrangian point is on the other side of the sun from us. There actually IS a pretty big place to hide there.

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Re: 1547: "Solar System Questions"

Postby exoren22 » Wed Jul 08, 2015 9:34 pm UTC

JohnTheWysard wrote:
cellocgw wrote:Some crazy guy suggested null-buoyancy blimps for living in Venus' atmosphere. Why couldn't we do this on Jupiter as well?


Sort of hard to get a lighter than air blimp when the air is already mostly hydrogen. Probably need a hot air blimp and that takes continual energy.


If you're going to do it on top of the atmosphere, you can use solar cells. Or you can harness the energy of the atmosphere itself, or you could collect fuel form the atmosphere and power a generator.

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Re: 1547: "Solar System Questions"

Postby xtifr » Wed Jul 08, 2015 9:36 pm UTC

Mokurai wrote:
There was also a tale about baseball in a double dome on Mars long ago, where somebody hit a home run that broke through the inner dome and landed between the domes.


I'm not entirely sure, but that might be from Geo. Alec Effinger's1 science-fiction/sports2 short story collection, Idle Pleasures.

1 Better known for his Marîd Audran/Budayeen series that starts with When Gravity Fails, and his breakout novel, What Entropy Means to Me.
2 A combination intended to subvert stereotypes of both nerds and jocks--quite successfully, IMO.
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Re: 1547: "Solar System Questions"

Postby The Moomin » Thu Jul 09, 2015 9:33 am UTC

Did Uranus produce the gas giants?

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Re: 1547: "Solar System Questions"

Postby solune » Thu Jul 09, 2015 9:39 am UTC

OT:

Why haven't we built a big inflatable extreme sports complex on the moon ?


I'd think you'd want to build it underground to screen from the solar radiation. With 1/6th gravity you could build a large cave with less risk of a collapse than on earth.
It would be hard to find materials to make concrete though. I wonder what would be the prefered mix when using only lunar ore ...


The Moomin wrote:Did Uranus produce the gas giants?
I apologise. I couldn't help myself.


My prefered pronounciation for this planet is the one that makes people snicker. The other one sounds weird.

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Re: 1547: "Solar System Questions"

Postby orthogon » Thu Jul 09, 2015 5:41 pm UTC

This comic has reminded me of Brian Cox's brilliant Wonders of the Solar System series. That made me realise what a phenomenal amount we now know about the planets, moons, asteroids and so on, and unlike a lot of science programmes, I came out of each one having learned a lot. These places are weird, wonderful, ridiculous and sublime: there's everything from orbiting icebergs to lakes of liquid methane; and yet somehow they feel much more like our neighbourhood, hardly more foreign than places on earth that I haven't been to either like Iceland or Australia.

The follow-up Wonders of the Universe was pretty good too, but was more about cosmology than it was a visitor's guide to astronomical attractions. I guess we don't know anything outside our solar system that's on a remotely human scale.
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Re: 1547: "Solar System Questions"

Postby Echo244 » Thu Jul 09, 2015 6:01 pm UTC

orthogon wrote:This comic has reminded me of Brian Cox's brilliant Wonders of the Solar System series. That made me realise what a phenomenal amount we now know about the planets, moons, asteroids and so on, and unlike a lot of science programmes, I came out of each one having learned a lot. These places are weird, wonderful, ridiculous and sublime: there's everything from orbiting icebergs to lakes of liquid methane; and yet somehow they feel much more like our neighbourhood, hardly more foreign than places on earth that I haven't been to either like Iceland or Australia.

The follow-up Wonders of the Universe was pretty good too, but was more about cosmology than it was a visitor's guide to astronomical attractions. I guess we don't know anything outside our solar system that's on a remotely human scale.


And yet Wonders of the Solar System is just the little we do know about, with some fancy imagery interpolating across the gaps. And there are a lot of gaps. Hence, for example, the Rosetta team's first assumptions, iirc, being based on a vaguely-spherical target, before finding out that they were landing on something much more irregular.

Something else that struck me about this strip: If the black questions are from Randall, and the red answers are in different handwriting... who filled in the few answers we have?
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Re: 1547: "Solar System Questions"

Postby Pfhorrest » Thu Jul 09, 2015 6:05 pm UTC

I assumed the black questions were written by a younger Randall, and the red answered are slowly being filled in by progressively older Randalls.
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Re: 1547: "Solar System Questions"

Postby Echo244 » Thu Jul 09, 2015 6:09 pm UTC

The "Where's Philae, exactly?" entry kind of suggests it can't a significantly younger Randall...
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Re: 1547: "Solar System Questions"

Postby measure » Thu Jul 09, 2015 6:11 pm UTC

Echo244 wrote:If the black questions are from Randall, and the red answers are in different handwriting... who filled in the few answers we have?

I assumed that meant he had compiled the list of questions some time in the past, and has been filling in answers to his own questions as he learns more about the subjects/new discoveries are made.

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Re: 1547: "Solar System Questions"

Postby Kit. » Thu Jul 09, 2015 6:24 pm UTC

Should then we expect the answers to change over time, like in the Rosetta strip?

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Re: 1547: "Solar System Questions"

Postby exoren22 » Thu Jul 09, 2015 6:25 pm UTC

gmalivuk wrote:
Beavertails wrote:I miss when threads stayed on topic
When was that, exactly?


Wasn't that about that time that Waterman guy taught us all about how we don't know how to physics?

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Re: 1547: "Solar System Questions"

Postby orthogon » Thu Jul 09, 2015 8:33 pm UTC

exoren22 wrote:
gmalivuk wrote:
Beavertails wrote:I miss when threads stayed on topic
When was that, exactly?


Wasn't that about that time that Waterman guy taught us all about how we don't know how to physics?

Those were indeed the days. Actually, I confess it was mostly before my time. When I joined the fora, somebody had a sig with a quote of Steve Waterman saying something like "So no, I am not talking about the Galilean". For a long time I assumed this was a (non-)reference to Jesus Christ.
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Re: 1547: "Solar System Questions"

Postby SecondTalon » Thu Jul 09, 2015 8:46 pm UTC

exoren22, FrobozzWizard

I don't want to see either of you in Individual XKCD Comic Threads for a week, because neither one of you seem capable of following a simple instruction.

Everyone else - I believe I removed all of the off topic discussion. Some on-topic stuff got lost as well. If I missed something, report it.

Echo224, I'm sorry your incredibly reasonable thought provoked such a shitstorm of terrible.
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Re: 1547: "Solar System Questions"

Postby gmalivuk » Thu Jul 09, 2015 9:33 pm UTC

Echo244 wrote:Wooo! We now know Pluto is kind of red!
And it has a heart on it.
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Re: 1547: "Solar System Questions"

Postby speising » Thu Jul 09, 2015 10:17 pm UTC

gmalivuk wrote:
Echo244 wrote:Wooo! We now know Pluto is kind of red!
And it has a heart on it.

How do they expose those images? I assume it is actually pretty dark out there.

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Re: 1547: "Solar System Questions"

Postby gmalivuk » Fri Jul 10, 2015 12:42 am UTC

The LORRI images are 1/10 sec exposures. It's not super bright, but it's no darker than a typical Earth dusk, which is still perfectly suitable for photography.

(The tag #plutotime on various sites is full of pictures people have taken when their local lighting approximates the brightness of noonday sun on Pluto.)
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Re: 1547: "Solar System Questions"

Postby Mikeski » Fri Jul 10, 2015 3:01 am UTC

gmalivuk wrote:(The tag #plutotime on various sites is full of pictures people have taken when their local lighting approximates the brightness of noonday sun on Pluto.)

That's pretty amazing. I would not have expected that much light way out there. Especially after looking at something like this scale map of the solar system. I guess Pluto is "only" about 40 times as far from the sun as we are, so 1/64,000th as much light; 120,000 lux at noon here, 2 lux at noon there...

I have mentally underestimated the power of the Sun by quite a bit...

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Re: 1547: "Solar System Questions"

Postby orthogon » Fri Jul 10, 2015 7:18 am UTC

Mikeski wrote:
gmalivuk wrote:(The tag #plutotime on various sites is full of pictures people have taken when their local lighting approximates the brightness of noonday sun on Pluto.)

That's pretty amazing. I would not have expected that much light way out there. Especially after looking at something like this scale map of the solar system. I guess Pluto is "only" about 40 times as far from the sun as we are, so 1/64,000th as much light; 120,000 lux at noon here, 2 lux at noon there...

I have mentally underestimated the power of the Sun by quite a bit...

Shouldn't it be inverse square, not inverse cube? Lux is a weighted version of Watts per square metre. That would make it 1/1600 or about 80 75 lux.
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Re: 1547: "Solar System Questions"

Postby gmalivuk » Fri Jul 10, 2015 12:54 pm UTC

Yeah, inverse square. And we tend to underestimate the extent to which our eyes can adjust to different light levels.
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Re: 1547: "Solar System Questions"

Postby Flumble » Fri Jul 10, 2015 1:48 pm UTC

Why is lux still a thing? W/m2 (for objective measurement) and a weighted logarithmic scale (for perception, analogous to dB(A)) looks like a far better choice to me.

I can't make clear from the wikipedia article that apparent magnitude is that unit.

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Re: 1547: "Solar System Questions"

Postby Neil_Boekend » Fri Jul 10, 2015 1:59 pm UTC

Flumble wrote:Why is lux still a thing? W/m2 (for objective measurement) and a weighted logarithmic scale (for perception, analogous to dB(A)) looks like a far better choice to me.

I can't make clear from the wikipedia article that apparent magnitude is that unit.

That is because W/m2 ignores the fact that the sensitivity of the human eye varies with wavelength. Lux and lumen compensate for that. For one lumen in 555 nm (green) you need less power than for one lumen in 455 nm (blue) because the eye is most sensitive to 555 nm. Thus it is a useful metric to how bright something seems to us.
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Re: 1547: "Solar System Questions"

Postby gmalivuk » Fri Jul 10, 2015 2:02 pm UTC

Yeah, use a logarithmic unit if you like, but it should still be based on lux rather than W/m^2 if you're going for "brightness" as we perceive it.
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Re: 1547: "Solar System Questions"

Postby orthogon » Fri Jul 10, 2015 2:10 pm UTC

Neil_Boekend wrote:
Flumble wrote:Why is lux still a thing? W/m2 (for objective measurement) and a weighted logarithmic scale (for perception, analogous to dB(A)) looks like a far better choice to me.

I can't make clear from the wikipedia article that apparent magnitude is that unit.

That is because W/m2 ignores the fact that the sensitivity of the human eye varies with wavelength. Lux and lumen compensate for that. For one lumen in 555 nm (green) you need less power than for one lumen in 455 nm (blue) because the eye is most sensitive to 555 nm. Thus it is a useful metric to how bright something seems to us.

This, and furthermore you can make any unit logarithmic by expressing it in dB relative to 1 of the unit: the resulting unit is called dB<X> where X is the original unit. So you're welcome to talk about dBlux if you like. So we can say that the sunlight on Pluto is 18dBlux, and on Earth it's 51dBlux, and we can also say that the sunlight is about 33dB weaker on Pluto than on Earth. Photography doesn't tend to use dBs as much as audio, perhaps because the most important quantities are lengths, areas and times, which people are used to using linear measurements for. Even so, cameras do work logarithmically, e.g. one f-stop is a 3dB change in iris area and therefore light collected.
(Partially ninja'd.)

ETA: There's another possible reason to avoid dBs in relation to light, particularly in the context of optoelectronic systems: optoelectronic devices generally "convert" photons into electrons, which means that you get currents proportional to the rate of arrival of photons, which in turn is proportional to the incident power. But in electronics, power tends to be associated with the square of currents and voltages, which is what leads to the whole 10log(x) vs 20 log(x) business. In other words, a 10dB increase in light power can lead to a 20dB increase in current. This is a bit vexatious and means you have to be a lot more careful about what you mean if you use dBs.
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Re: 1547: "Solar System Questions"

Postby Flumble » Fri Jul 10, 2015 4:19 pm UTC

Sorry for being unclear: the "weighted" part in "weighted logarithmic scale" was refering to the same weighting that lux has.

...or at least the same weighting for a dB(A)-light scale. There's a different weighting for low light levels (the rods don't respond well to red and violet light) and for different types of color blindness.

orthogon wrote:ETA: There's another possible reason to avoid dBs in relation to light, particularly in the context of optoelectronic systems:

You should probably also avoid using lux in the context of optoeletronic systems, as they don't have eyes and brains like we do. :P (To be clear: I'm only advocating the use of a logarithmic scale in relation to perception... and I'm advocating the non-use of the luminosity function in places where there's no eye involved)

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Re: 1547: "Solar System Questions"

Postby gmalivuk » Fri Jul 10, 2015 4:56 pm UTC

Well in this case, our eyes are involved, so we're good sticking to lux.
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Re: 1547: "Solar System Questions"

Postby keithl » Fri Jul 10, 2015 9:04 pm UTC

DHT wrote:A few answers: ...
Q7: Yes, the Oort cloud is real. It is the region where the gravitational pressure of the sun (inward) and the radiative pressure of the sun (outward) roughly equalize. It is a "cloud" of matter that is carried along by the sun, but doesn't fall into the solar system, proper. This debris slowly accumulates and forms comets and deposits on outer planets, occasionally these can fall further inward, but they are generally unremarkable blocks of icy dust.

Both gravity and radiation pressure from the Sun are inverse-square - they balance at all >1 AU distances. If you get Really Really Close to the sun, the radiation pressure drops a bit because the Sun's disk is large and light is coming in over a broader angle, but that close to the sun solid matter will evaporate. So what is special in the Oort cloud? At 10,000 AU, near the hypothetical "inner boundary" of the Oort cloud, the power density from the Sun (in one precise direction) and power density from the cosmic background (in all directions) are about the same, so a spherical object (with zero optical and IR albedo) will be only 20% warmer (maybe 3.3K) than the cosmic background temperature. That makes observing anything at that distance practically impossible using a much warmer telescope - even if the mirror was gigantic, and we knew precisely where to look. My guess is that we will be able to make surface maps of life-bearing planets (if any) in distant parts of the galaxy before we image objects in the Oort.

The Oort cloud is speculative. We have not observed it, but we can hypothesize about it based on comet aphelions and physics. But as is true of most speculation, the universe usually turns out to be more interesting and complicated than our imaginations. And as is true of most observational challenges, astronomers and space scientists will continue to pull new rabbits out of their capacious hats.

BTW, I speculate that the reason that long period comets tend to "originate" with those distant aphelions is that somewhat shorter period comets have made enough passes through the warm parts of the solar system (inside the observed Kuiper belt) to evaporate over billions of years. The temperature argument, stated a different way.

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Re: 1547: "Solar System Questions"

Postby Neil_Boekend » Sat Jul 11, 2015 7:33 am UTC

keithl wrote:
DHT wrote:A few answers: ...
Q7: Yes, the Oort cloud is real. It is the region where the gravitational pressure of the sun (inward) and the radiative pressure of the sun (outward) roughly equalize. It is a "cloud" of matter that is carried along by the sun, but doesn't fall into the solar system, proper. This debris slowly accumulates and forms comets and deposits on outer planets, occasionally these can fall further inward, but they are generally unremarkable blocks of icy dust.

Both gravity and radiation pressure from the Sun are inverse-square - they balance at all >1 AU distances. If you get Really Really Close to the sun, the radiation pressure drops a bit because the Sun's disk is large and light is coming in over a broader angle, but that close to the sun solid matter will evaporate. So what is special in the Oort cloud? At 10,000 AU, near the hypothetical "inner boundary" of the Oort cloud, the power density from the Sun (in one precise direction) and power density from the cosmic background (in all directions) are about the same, so a spherical object (with zero optical and IR albedo) will be only 20% warmer (maybe 3.3K) than the cosmic background temperature. That makes observing anything at that distance practically impossible using a much warmer telescope - even if the mirror was gigantic, and we knew precisely where to look. My guess is that we will be able to make surface maps of life-bearing planets (if any) in distant parts of the galaxy before we image objects in the Oort.

The Oort cloud is speculative. We have not observed it, but we can hypothesize about it based on comet aphelions and physics. But as is true of most speculation, the universe usually turns out to be more interesting and complicated than our imaginations. And as is true of most observational challenges, astronomers and space scientists will continue to pull new rabbits out of their capacious hats.

BTW, I speculate that the reason that long period comets tend to "originate" with those distant aphelions is that somewhat shorter period comets have made enough passes through the warm parts of the solar system (inside the observed Kuiper belt) to evaporate over billions of years. The temperature argument, stated a different way.

Also: the radiation pressure depends mostly on your surface area while the gravity depends on your mass. Whether they balance out depends on your surface/mass ratio. A solar sail will have winning radiation pressure (on all sane distances) while a uranium sphere will have winning gravity.
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Re: 1547: "Solar System Questions"

Postby rmsgrey » Sun Jul 12, 2015 3:51 pm UTC

Neil_Boekend wrote:Also: the radiation pressure depends mostly on your surface area while the gravity depends on your mass. Whether they balance out depends on your surface/mass ratio. A solar sail will have winning radiation pressure (on all sane distances) while a uranium sphere will have winning gravity.


A uranium needle will have winninger gravity.


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